All Exams  >   Class 9  >   Science Class 9  >   All Questions

All questions of Is Matter Around Us Pure? for Class 9 Exam

A shining thick liquid is often used in glass thermometers. Name it.
  • a)
    Bromine
  • b)
    Mercury
  • c)
    Gallium
  • d)
    Calcium
Correct answer is option 'B'. Can you explain this answer?

Ananya jain answered
Mercury is a thick shining liquid used commonly in glass thermometer. It does not stick with wall and expands uniformly on heating.

Which of the following is not a pure substance?
  • a)
    Mercury
  • b)
    Distilled water
  • c)
    Nitric acid
  • d)
    Tap water
Correct answer is 'D'. Can you explain this answer?

Varun Kumar answered
Tap water is not a pure substance because it is mixed with chemicals that purify it and if it was from the ground it has naturally occurring minerals mixed in it.

Can you explain the answer of this question below:

Which of the following are chemical changes?

  • A:

    Freezing of water

  • B:

    Cooking of vegetables.

  • C:

    Drying of wet clothes in sun light

  • D:

    Melting of ice

The answer is b.

Divya Ahuja answered

If the chemical composition is changing then it is a chemical change and if the chemical composition is remaining the same, then it is a physical change.

Cooking of vegetables is a chemical change because here the chemical composition of the substance changes.

4 g of solute are dissolved in 36 g of water. What is the mass percent of the solution?
  • a)
    20%
  • b)
    100%
  • c)
    10%
  • d)
    5%
Correct answer is option 'C'. Can you explain this answer?

Krishna Iyer answered
Mass percent of solution = (mass of solute / mass of solution)×100%(mass of solute / mass of solution)×100%
    4    ×100%
   4+36
= 10%.

Statement A: Brass is a solution while gun powder is not 
Statement B: Air represents a solution in terms of science 
Which of the two statements is true
  • a)
    Statement B
  • b)
    Statement A
  • c)
    Both the Statement A and B
  • d)
    Neither Statement A nor Statement B.
Correct answer is option 'A'. Can you explain this answer?

Arun Sharma answered
Brass is a homogeneous mixture of copper and Zinc. Gun powder is not a powder is a simple mixture of saltpeter, sulfur and charcoal.
Air is an example of a solution (gaseous solution) because it is a homogeneous mixture of different gases like oxygen, nitrogen, helium, hydrogen, etc.

Which of the following is an example of a pure substance?
  • a)
    Air
  • b)
    Brass
  • c)
    Water
  • d)
    Milk
Correct answer is option 'C'. Can you explain this answer?

Priyanka Barat answered
Air is a mixture of gases (mainly nitrogen and oxygen, plus others).

Brass is an alloy (a mixture of copper and zinc).

Milk is a colloid (a mixture of water, fats, proteins, etc.).

Water (H₂O) is a pure substance because it has a fixed chemical composition.

The answer is Water (C) because it’s made of only one kind of molecule, H₂O. The others are mixtures: air is gases, brass is metals, and milk is a colloid. That’s why water is the only pure substance here.

A compound is a _______ substance made up of _______ .
  • a)
    impure; two or more simpler substances
  • b)
    soft; only one kind of atoms
  • c)
    hard; only one element
  • d)
    pure; two or more elements
Correct answer is option 'D'. Can you explain this answer?

Ananya Sharma answered
A molecule is the smallest particle of a substance that exists independently. Molecules of most elements are made up of only one of atom of that element. Oxygen, along with nitrogen, hydrogen, and chlorine are made up of two atoms.

Match the following with correct response. 
(1) Solute 
(2) Solvent 
(3) Solution 
(4) Suspension 

(A) dissolving component of solution 
(B) A heterogenous mixture 
(C) A homogenous mixture 
(D) dissolved component of solution
  • a)
    1-D, 2-A, 3-C, 4-B
  • b)
    1-A, 2-C, 3-B, 4-D
  • c)
    1-B, 2-D, 3-A, 4-C
  • d)
    1-C, 2-B, 3-D, 4-A
Correct answer is option 'A'. Can you explain this answer?

Amit Kumar answered
Solution is a homogeneous mixture because it is made up two or more substance and chemical composition is uniform throughout.
Suspenison is a heterogeneous mixture because the chemical composition is not uniform and particles are arranged differently in a heterogeneous mixture i.e.random arrangement of particles takes place in a heterogeneous mixture.
Solute is the component of a solution that is to dissolved into the solvent.
Solvent is the medium which is in large quantity and solute is in less quantity as it is the matter which is to be dissolved into solvent let's take an example there is sugar and water. You dissolved sugar in water so water here acts as a solvent and sugar here acts as a solute which is to be dissolved into the solvent i.e. present in large quantity.
Thus, the final answer is 1-D, 2-A, 3-C and 4-B.

Thermometer is an instrument that measuring
  • a)
      Temperature of substance
  • b)
      Heat of substance         
  • c)
      Radiation of substance
  • d)
      Flow energy in a substance.
Correct answer is option 'A'. Can you explain this answer?

Prakruti Iyer answered
A Thermometer is a tool that measures temperature - how hot or cold something is. Thermometers are used to see if you have a fever or tell you how cold it is outside.

What happens on adding dilute HCl to a mixture of iron filling and sulphur powder? 
A H2S is formed 
B A colour less and odourless gas is formed 
C A greenish solution appears 
D FeS is formed
  • a)
    (b) and (c) are correct
  • b)
    (a) and (b) are correct
  • c)
    (a), (b) and (c) are correct
  • d)
    All of these
Correct answer is option 'A'. Can you explain this answer?

Hansa Sharma answered
When HCl is added to iron filling then chlorine of HCl reacts with iron by evolving hydrogen gas. The iron dissolves, releasing hydrogen and making an iron chloride solution.
Fe + 2HCl → FeCl₂ + H₂ or
2Fe + 6HCl → 2FeCl₃ + 3H₂
-FeCl₂ is a greenish solution.

How will you bring about the following separation Alcohol from water?
  • a)
    By Centrifugation
  • b)
    By sublimation
  • c)
    By fractional distillation
  • d)
    By separating funnel.
Correct answer is option 'C'. Can you explain this answer?

Shubham Iyer answered
Separation of Alcohol from Water by Fractional Distillation

Fractional distillation is the most appropriate method for separating alcohol from water. It is a process that utilizes the difference in boiling points of the two substances to separate them effectively. Here is a detailed explanation of the process:

1. Introduction to Fractional Distillation:
- Fractional distillation is a technique used to separate a mixture of liquids based on their boiling points.
- It is particularly useful when the liquids have boiling points close to each other, such as alcohol and water.
- The process involves heating the mixture to vaporize the component with the lower boiling point and then condensing it back into liquid form.

2. Boiling Points of Alcohol and Water:
- Alcohol (ethanol) has a boiling point of approximately 78.5°C.
- Water has a boiling point of 100°C.
- These boiling points indicate that alcohol vaporizes at a lower temperature than water.

3. Procedure for Fractional Distillation:
- The mixture of alcohol and water is heated in a distillation flask, which is attached to a fractionating column.
- The fractionating column contains multiple stages or plates, which provide a large surface area for the vapor to condense and revaporize.
- As the mixture is heated, the component with the lower boiling point (alcohol) vaporizes first.
- The vapor rises through the fractionating column, where it starts to condense on the plates.
- The condensed liquid then revaporizes due to the heat from the rising vapor, allowing further separation.
- The vapor continues to rise until it reaches the condenser, where it is cooled and collected as a liquid in a separate container.

4. Collection of Alcohol and Water:
- As the vapor passes through the fractionating column, it undergoes multiple cycles of condensation and revaporization.
- This process allows for the separation of alcohol and water based on their boiling points.
- The alcohol, which has the lower boiling point, will condense and collect at a lower temperature than water.
- The water, which has the higher boiling point, will condense and collect at a higher temperature than alcohol.
- By adjusting the temperature and controlling the flow of vapor, it is possible to separate alcohol and water efficiently.

In conclusion, fractional distillation is the most suitable method for separating alcohol from water due to the difference in their boiling points. This technique allows for the collection of alcohol and water as separate components by utilizing the process of condensation and revaporization in a fractionating column.

Cod-liver oil is an example of
  • a)
    Colloidal solution
  • b)
    Suspension
  • c)
    True solution
  • d)
    Emulsion
Correct answer is option 'D'. Can you explain this answer?

Arvind Singh answered
Water-in-oil (W/O) type emulsions: In water-in-oil type emulsions, water acts as the dispersed phase, whereas oil acts as the dispersion medium. This type of emulsions is also referred to as oil emulsions. Cod liver oil emulsion is a typical example of this type of emulsions in which water is dispersed in cod liver oil. 

Which one of the following pair of gases cannot be separated by diffusion method?
  • a)
    CO2 and NO2
  • b)
    CO2 and N2O2
  • c)
    CO2 and H2
  • d)
    CO2 and N2O
Correct answer is option 'D'. Can you explain this answer?

Ishita Khanna answered
Diffusion Method and Separation of Gases

Diffusion is the process of intermixing of gases. Diffusion of gases occurs due to the random motion of gas molecules from a region of higher concentration to a region of lower concentration. The rate of diffusion of gases depends upon their molecular mass, temperature, and pressure. The diffusion method is used for the separation of gases based on their molecular mass, size, and solubility.

Pair of Gases that cannot be Separated by Diffusion Method

The correct answer is option 'D', i.e., CO2 and N2O. These two gases cannot be separated by the diffusion method because they have almost the same molecular mass and size. The rate of diffusion of gases is directly proportional to their molecular mass. When two gases have the same molecular mass, they diffuse at the same rate and cannot be separated by the diffusion method.

Explanation of Other Options

a) CO2 and NO2: These two gases can be separated by the diffusion method because they have different molecular masses and sizes.

b) CO2 and N2O2: These two gases can be separated by the diffusion method because they have different molecular masses and sizes.

c) CO2 and H2: These two gases can be separated by the diffusion method because they have different molecular masses and sizes.

Conclusion

In conclusion, the diffusion method is used for the separation of gases based on their molecular mass, size, and solubility. CO2 and N2O cannot be separated by the diffusion method because they have almost the same molecular mass and size.

Which of the following are homogeneous in nature?
(i) Ice (ii) Wood
(iii) Soil (iv) Air
  • a)
    (i) and (ii)
  • b)
    (i) and (iv)
  • c)
    (ii) and (iii)
  • d)
    (iii) and (iv)
Correct answer is option 'B'. Can you explain this answer?

Pranab Datta answered
Homogeneous Substances

Homogeneous substances are those that have the same composition and properties throughout their entire volume. In other words, they are uniform in nature and do not have visible differences or variations. Let's analyze each option to determine which ones are homogeneous.

(i) Ice
Ice is a solid form of water, and it is considered a homogeneous substance. It is composed of water molecules arranged in a rigid lattice structure. The composition and properties of ice are the same throughout its entire volume, making it a homogeneous substance.

(ii) Wood
Wood is a complex material composed of various organic compounds, such as cellulose, lignin, and hemicellulose. It is a heterogeneous substance rather than homogeneous. Wood exhibits visible differences and variations in its composition and properties, such as the presence of growth rings, knots, and different colors. These variations make wood a heterogeneous substance.

(iii) Soil
Soil is a mixture of organic and inorganic materials, such as minerals, water, air, and organic matter. It is a heterogeneous substance due to the presence of different-sized particles, variations in composition, and the mixture of various components. Soil can have different textures, colors, and compositions in different areas or layers, making it heterogeneous.

(iv) Air
Air is a mixture of gases, primarily nitrogen, oxygen, carbon dioxide, and traces of other gases. Although air appears to be uniform and transparent, it is actually a mixture of different gases. The composition of air can vary depending on the location and environmental conditions. Therefore, air is a heterogeneous substance.

Conclusion

From the given options, only ice (i) can be considered a homogeneous substance. It has the same composition and properties throughout its entire volume. Wood (ii), soil (iii), and air (iv) are all heterogeneous substances due to visible differences and variations in their composition and properties.

What is true about homogeneous mixture?
  • a)
    Homogeneous mixture is the mixture of two or more than two components.
  • b)
    In homogeneous mixture the composition and properties are uniform throughout the mixture
  • c)
    both (a) and (b) are true
  • d)
    none of the above
Correct answer is option 'C'. Can you explain this answer?

Asha Mukherjee answered
Understanding Homogeneous Mixtures
A homogeneous mixture is a combination of two or more substances that are uniformly distributed. The correct answer to the question is option 'C', as both statements (a) and (b) are true. Here’s a detailed explanation:
Definition of Homogeneous Mixture
- A homogeneous mixture consists of two or more components that blend together completely.
- The individual components cannot be distinguished from one another.
Uniform Composition
- In a homogeneous mixture, the composition is consistent throughout the entire mixture.
- This means that any sample taken from the mixture will have the same proportions of components as any other sample.
Examples of Homogeneous Mixtures
- Saltwater: When salt is dissolved in water, it forms a homogeneous mixture where the salt is uniformly distributed.
- Air: The air we breathe is a homogeneous mixture of gases (mainly nitrogen and oxygen), which are evenly mixed.
Properties of Homogeneous Mixtures
- The physical properties such as color, density, and concentration remain constant throughout the mixture.
- This uniformity makes them different from heterogeneous mixtures, where the components can be seen and separated.
Conclusion
In summary, both statements (a) and (b) accurately describe the characteristics of homogeneous mixtures. Therefore, option 'C' is indeed the correct choice, as it encompasses the essential features of homogeneous mixtures.

Which of the following is a heterogeneous mixture?
  • a)
    Milk
  • b)
    Salt solution
     
  • c)
    Air
  • d)
    Distilled water
Correct answer is option 'A'. Can you explain this answer?

Kritika Bose answered
Understanding Heterogeneous Mixtures
A heterogeneous mixture is one that does not have a uniform composition throughout. The individual components can be distinctly seen and separated.
Examples of Mixtures
- Milk:
- Milk is considered a heterogeneous mixture because it contains fat globules that do not dissolve completely in the liquid.
- When left to sit, milk will separate into layers, showing the distinct components—cream and liquid.
- Salt Solution:
- A salt solution is a homogeneous mixture because the salt completely dissolves in water, creating a uniform composition that cannot be distinguished visually.
- Air:
- Air is generally considered a homogeneous mixture of gases; however, it may contain pollutants and particulates that can be seen, but the primary components (oxygen, nitrogen) are uniformly distributed.
- Distilled Water:
- Distilled water is a pure substance, not a mixture. It contains only H2O molecules, making it homogeneous.
Conclusion
In summary, the correct answer to the question of which is a heterogeneous mixture is option A: Milk. It demonstrates clear differences in composition and can be visually separated into its components, unlike salt solutions, air, or distilled water, which are homogeneous or pure substances.

Which of the following processes describes the direct conversion of a solid into a gas without passing through the liquid state?
  • a)
    Fusion
  • b)
    Condensation
  • c)
    Solidification
  • d)
    Sublimation
Correct answer is option 'D'. Can you explain this answer?

Gitanjali Kaur answered
Understanding Sublimation
Sublimation is a fascinating process that involves the direct transition of a solid into a gas, bypassing the liquid phase entirely. Here’s a detailed breakdown of this process:
Definition of Sublimation
- Sublimation occurs when certain substances, such as dry ice (solid carbon dioxide) or iodine, transition from a solid state directly to a gaseous state when heated.
Characteristics of Sublimation
- No Liquid Phase: The key feature of sublimation is that there is no intermediate liquid form; the solid transforms directly into gas.
- Endothermic Process: Sublimation requires energy, often in the form of heat, which allows the molecules in the solid to gain enough energy to overcome intermolecular forces and escape into the air as gas.
Examples of Sublimation
- Dry Ice: Dry ice sublimes at room temperature, turning directly into carbon dioxide gas, which is commonly used in fog machines and refrigeration.
- Iodine Crystals: When heated, iodine crystals sublimate, producing a purple vapor without forming any liquid.
Importance of Sublimation
- Applications in Industry: Sublimation is used in freeze-drying food, which preserves its structure and nutrients while removing moisture.
- Scientific Observations: Sublimation is also observed in various natural processes and is essential in understanding phase changes in materials.
In summary, sublimation is the process defined in the question, making option 'D' the correct answer. It highlights the unique ability of certain solids to transition directly into gas, a fascinating aspect of physical chemistry.

If 110 g of copper sulphate is present in 550 g of solution, what will be the concentration of
solution?
  • a)
    11%
  • b)
    20%
  • c)
    55%
  • d)
    22%
Correct answer is option 'B'. Can you explain this answer?

Shubham Iyer answered
Solution:

Given:
Mass of copper sulphate = 110 g
Mass of solution = 550 g

To find:
Concentration of solution

Concentration is defined as the amount of solute (in this case, copper sulphate) present in a given amount of solution.

Concentration of solution = (mass of solute / mass of solution) * 100

Substituting the given values into the formula:

Concentration = (110 g / 550 g) * 100

Simplifying the expression:

Concentration = (1/5) * 100 = 20%

Therefore, the concentration of the solution is 20%.

Explanation:

To calculate the concentration of a solution, we need to determine the ratio of the mass of the solute to the mass of the solution and express it as a percentage.

In this case, the mass of copper sulphate is given as 110 g and the mass of the solution is given as 550 g. To find the concentration, we divide the mass of the solute by the mass of the solution and multiply by 100 to express it as a percentage.

Substituting the given values into the formula, we get (110 g / 550 g) * 100 = (1/5) * 100 = 20%.

Therefore, the concentration of the solution is 20%.

This means that for every 100 g of the solution, there are 20 g of copper sulphate present.

Match the following with correct response. 
  • a)
    1-C, 2-B, 3-D, 4-A
  • b)
    1-A, 2-C, 3-B, 4-D
  • c)
    1-D, 2-A, 3-C, 4-B
  • d)
    1-B, 2-D, 3-A, 4-C
Correct answer is option 'B'. Can you explain this answer?

Saurabh Tanwer answered
1.Which solution have same uniform composition called homogeneous.

2.In which solution having separate boundaries of various components different with each others known as Heterogeneous.

3.When meet two components and make a new formed known as chemical change.

4.When Melting Ice it change it's shape and size and this change are physical changes.

Naphthalene can be separated from sand:
  • a)
    By sublimation
  • b)
    By crystallization
  • c)
    By distillation
  • d)
    Using suitable solvent
Correct answer is option 'A'. Can you explain this answer?

Separation of Naphthalene from Sand
Naphthalene, a polycyclic aromatic hydrocarbon, possesses unique physical properties that allow it to be separated effectively from sand using sublimation.
What is Sublimation?
- Sublimation is the process where a solid transitions directly into a gas without passing through the liquid state.
- Naphthalene has a relatively low boiling point (approximately 218°C) and readily sublimates upon heating.
Why is Sublimation Effective for Separation?
- Physical Properties: When naphthalene is heated, it vaporizes, leaving sand, which does not change phase.
- Recondensation: The vapor can be cooled, often on a cold surface, allowing naphthalene to solidify again, effectively collecting it separately from sand.
Steps in the Sublimation Process
- Heating the Mixture: The sand-naphthalene mixture is gently heated.
- Vaporization: Naphthalene vaporizes due to heat.
- Condensation: The vapor is directed onto a cooler surface where it condenses back into solid naphthalene.
- Collection: The solid naphthalene can be collected, leaving sand behind.
Comparison with Other Methods
- Crystallization: This is not suitable since it relies on solubility, while sand is insoluble in naphthalene.
- Distillation: This method separates liquids based on boiling points and is not applicable for a solid-sand mixture.
- Using a Suitable Solvent: This would not work well since sand does not dissolve in solvents typically used for naphthalene.
In conclusion, sublimation is the most efficient method to separate naphthalene from sand due to its unique phase change properties.

Milk of magnesia is
  • a)
    A suspension
  • b)
    A colloid
  • c)
    A true solution
  • d)
    A homogeneous mixture
Correct answer is option 'A'. Can you explain this answer?

Samridhi Patel answered
What is Milk of Magnesia?
Milk of magnesia is a common antacid and laxative, primarily composed of magnesium hydroxide suspended in water.
Classification of Milk of Magnesia
- Suspension: Milk of magnesia is classified as a suspension because it contains fine particles of magnesium hydroxide dispersed in water.
- Characteristics of a Suspension: In a suspension:
- The particles are larger than those in a colloid or solution.
- They are not completely dissolved; instead, they remain suspended and can settle over time if left undisturbed.
- When shaken or stirred, the mixture appears uniform, but it may separate upon standing.
Why Not a Colloid or True Solution?
- Colloid: In a colloid, particles are smaller than those in a suspension and do not settle out. Examples include milk and fog. Milk of magnesia, due to its larger particle size, does not fit this category.
- True Solution: A true solution, like saltwater, has solute particles that are completely dissolved at the molecular level, resulting in a homogeneous mixture. Milk of magnesia does not form a true solution, as the magnesium hydroxide particles are visible and can settle.
Conclusion
In summary, milk of magnesia is best described as a suspension due to its particle size and behavior in liquid. Understanding this distinction is crucial for recognizing different types of mixtures in chemistry.

A solution is prepared by dissolving 80 g of salt in 500 g of water. Find the concentration of the solution
  • a)
    10%
  • b)
    11%
  • c)
    13.8%
  • d)
    1.38%
Correct answer is option 'C'. Can you explain this answer?

Concentration of solution = (mass solute/mass of solution)*100
Mass of solute (salt) = 80 g
Mass of solution = 80 + 500 = 580
=(80/580)*100
=13.793 or 13.8%

When a mixture of iron powder and sulfur powder is heated strongly to form iron sulfide, then heat energy is
  • a)
    Absorbed
  • b)
    Released
  • c)
    First absorbed and then released
  • d)
    Neither absorbed nor released.
Correct answer is option 'C'. Can you explain this answer?

Shail Roy answered

Explanation:

Heating of Iron and Sulfur Mixture:
- When a mixture of iron powder and sulfur powder is heated strongly, a chemical reaction takes place.
- The iron and sulfur react to form iron sulfide.

Energy Change in the Reaction:
- In this reaction, energy is first absorbed to break the bonds in the reactants (iron and sulfur).
- Then, energy is released when new bonds are formed in the product (iron sulfide).
- This means that the reaction absorbs energy in the beginning and releases energy as it progresses.

Overall Energy Change:
- Therefore, the overall energy change in this reaction involves both absorption and release of energy.
- Initially, energy is absorbed to break the bonds in the reactants, and then energy is released when new bonds are formed in the product.

Conclusion:
- In the reaction between iron and sulfur to form iron sulfide, heat energy is first absorbed to break the bonds in the reactants, and then released as new bonds are formed in the product.

Which one of the following is most likely to exhibit tyndall effect?
  • a)
    Chalk powder and water mixture
  • b)
    Potash alum and water mixture
  • c)
    Sugar and water mixture
  • d)
    Potassium permanganate and water mixture
Correct answer is option 'A'. Can you explain this answer?

Advait Gupta answered
The Tyndall effect is the scattering of light by colloidal particles or suspended particles in a medium. When light passes through a medium, it interacts with the particles in the medium. The particle size and composition of the medium determine the extent of scattering and whether the Tyndall effect is observed or not.

Among the given options, the most likely to exhibit the Tyndall effect is option 'A,' which is a mixture of chalk powder and water. Let's understand why this is the correct answer.

Explanation:
1. Chalk powder and water mixture:
- Chalk powder consists of fine particles of calcium carbonate.
- When mixed with water, the chalk particles disperse throughout the liquid, forming a colloid.
- The chalk particles are larger compared to the other options, making them more effective at scattering light.
- As light passes through the mixture, the particles in the chalk powder scatter the light, making the beam visible.
- This scattering of light is known as the Tyndall effect.

2. Potash alum and water mixture:
- Potash alum is a compound that dissolves in water to form an aqueous solution.
- However, the dissolved particles of potash alum are very small and do not scatter light effectively.
- Therefore, a mixture of potash alum and water is less likely to exhibit the Tyndall effect.

3. Sugar and water mixture:
- Sugar dissolves completely in water, forming a transparent solution.
- The dissolved sugar particles are very small and do not scatter light effectively.
- Hence, a mixture of sugar and water does not exhibit the Tyndall effect.

4. Potassium permanganate and water mixture:
- Potassium permanganate dissolves in water to form a purple-colored solution.
- The dissolved particles of potassium permanganate are very small and do not scatter light effectively.
- Therefore, a mixture of potassium permanganate and water is less likely to exhibit the Tyndall effect.

In conclusion, among the given options, a mixture of chalk powder and water is most likely to exhibit the Tyndall effect due to the larger particle size of the chalk particles, which effectively scatter light.

Which one is physical change?
  • a)
    Burning magnesium in air
  • b)
    Mixing NH3 and HCl
  • c)
    Mixing BaSO4+ NaCl
  • d)
    Adding NaCl to water
Correct answer is option 'D'. Can you explain this answer?

Jaya Khanna answered
Adding of common salt (NaCl) to water is physical change as no new substance is formed and no heat is evolved during addition of salt in water.

Assertion (A): A compound is a form of matter that cannot be broken down by chemical reactions into simpler substances.
Reason (R): A compound is a substance composed of two or more different types of elements, chemically combined in a fixed proportion.
  • a)
    Both (A) and (R) are true, and (R) is the correct explanation of (A).
  • b)
    Both (A) and (R) are true, but (R) is not the correct explanation of (A).
  • c)
    (A) is true, but (R) is false.
  • d)
    (A) is false, but (R) is true.
Correct answer is option 'D'. Can you explain this answer?

Imk Pathshala answered
Correct Answer:
D: (A) is false, but (R) is true.
Explanation:
The assertion is false because a compound can be broken down into simpler substances (its constituent elements) by chemical reactions. The reason is true because it accurately describes the composition of a compound, being made of two or more different types of elements chemically combined in a fixed proportion.

Which of the following is true only for elements but not for compounds?
  • a)
    They are homogeneous.
  • b)
    They are the simplest substances and cannot be broken down further.
  • c)
    They are formed by a chemical combination of two or more substances.
  • d)
    They can exist in different physical states (solid, liquid, gas).
Correct answer is option 'B'. Can you explain this answer?

Tara Singh answered
Understanding Elements vs. Compounds
When distinguishing elements from compounds, it is essential to recognize the fundamental properties that define each category.
Elements: The Simplest Substances
- Elements are pure substances that cannot be broken down into simpler substances by chemical means.
- They consist of only one type of atom; for example, hydrogen (H), oxygen (O), and gold (Au) are all elements.
- This characteristic makes option 'B' uniquely true for elements.
Compounds: Combinations of Elements
- Compounds are formed through the chemical combination of two or more different elements.
- They can be broken down into simpler substances or their constituent elements through chemical reactions. For instance, water (H2O) can be separated into hydrogen and oxygen.
Comparison of Properties
- Homogeneity: Both elements and compounds are homogeneous; thus, option 'A' is not unique.
- Formed by Chemical Combination: Only compounds are formed by the combination of elements, making option 'C' exclusive to compounds.
- Physical States: Both elements and compounds can exist as solids, liquids, or gases, so option 'D' is applicable to both.
Conclusion
In summary, the defining characteristic that distinguishes elements from compounds is that elements are the simplest substances that cannot be decomposed further, making option 'B' the correct answer. Understanding these properties reinforces the significance of elements in the study of chemistry.

One of the following substances is neither a good conductor of electricity nor an insulator. This substances is
  • a)
    Chromium
  • b)
    Gallium
  • c)
    Germanium
  • d)
    Potassium
Correct answer is option 'C'. Can you explain this answer?

Sadhana Patel answered
Explanation:

Introduction:
In order to understand why Germanium (Ge) is neither a good conductor of electricity nor an insulator, we need to have a basic understanding of conductivity and the properties of different substances.

Conductivity:
Conductivity refers to the ability of a substance to conduct electricity. Substances can be classified into three categories based on their conductivity:
1. Conductors: These substances allow the flow of electric current easily. They have a high number of free electrons that can move freely within the material. Examples of conductors include metals like copper, aluminum, and silver.
2. Insulators: These substances do not allow the flow of electric current easily. They have a very low number of free electrons. Examples of insulators include rubber, plastic, and wood.
3. Semiconductors: These substances have properties that lie between conductors and insulators. They have a moderate number of free electrons, which can be influenced by external factors such as temperature or impurities. Examples of semiconductors include silicon (Si) and germanium (Ge).

Properties of Germanium:
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors silicon and tin.

Germanium as a Semiconductor:
Germanium is classified as a semiconductor because it has properties that lie between conductors and insulators. It has a moderate number of free electrons that can move within the material, but it cannot conduct electricity as effectively as metals. Germanium's conductivity can be enhanced by adding impurities or by increasing the temperature.

Not a Good Conductor:
Germanium is not a good conductor of electricity compared to metals like copper or aluminum. This is because germanium has a lower number of free electrons available for conducting electricity. The movement of these free electrons is not as efficient in germanium as it is in metals.

Not an Insulator:
Germanium is also not an insulator like rubber or wood. It has a moderate number of free electrons that can conduct electricity to some extent. Insulators, on the other hand, have a very low number of free electrons that do not allow the flow of electric current.

Conclusion:
In conclusion, Germanium is neither a good conductor nor an insulator. It is classified as a semiconductor due to its moderate conductivity properties. Germanium's conductivity can be enhanced by adding impurities or increasing the temperature.

Which of the following does not have a fixed melting point/boiling point?
  • a)
    Ethanol
  • b)
    Air
  • c)
    Oxygen
  • d)
    Gold
Correct answer is option 'B'. Can you explain this answer?

Ananya Yadav answered
Understanding Melting and Boiling Points
Melting points and boiling points are specific temperatures at which a substance changes its state from solid to liquid and from liquid to gas, respectively. However, not all substances exhibit fixed temperatures for these transitions.
Substances with Fixed Melting/Boiling Points
- Ethanol: Ethanol has a defined melting point of around -114.1°C and a boiling point of about 78.37°C, making it a substance with fixed phase transition points.
- Oxygen: Oxygen also has specific melting and boiling points, with melting occurring at -218.79°C and boiling at -182.96°C.
- Gold: Gold, a pure metal, has a fixed melting point of approximately 1064°C and a boiling point of about 2856°C.
Why Air Does Not Have Fixed Melting/Boiling Points
- Composition: Air is a mixture of gases, primarily nitrogen (78%) and oxygen (21%), along with trace amounts of other gases like carbon dioxide and argon.
- Variable Properties: Because air is not a pure substance, its melting and boiling points are not fixed. The transitions depend on the specific composition and pressure conditions.
- Phase Changes: Different gases in air transition at different temperatures, leading to a range of melting and boiling points rather than a single fixed point.
Conclusion
In summary, while substances like ethanol, oxygen, and gold have defined melting and boiling points, air does not possess fixed transitions due to its nature as a mixture of various gases. This characteristic is the reason why air is the correct answer to the question posed.

Which of the following is a true emulsion?
  • a)
    Butter
  • b)
    Jelly
  • c)
    Milk
  • d)
    Whipped cream
Correct answer is option 'C'. Can you explain this answer?

Priya Roy answered
Understanding True Emulsions
Emulsions are mixtures of two immiscible liquids, typically oil and water, where one liquid is dispersed in the other. A true emulsion requires an emulsifying agent to stabilize the mixture.
Examples of Options Given
- a) Butter:
Butter is not a true emulsion; it is a water-in-oil emulsion but primarily consists of fat and water, making it a more complex mixture.
- b) Jelly:
Jelly is a gel-like substance made from fruit juice, sugar, and pectin. It is not an emulsion but rather a colloidal system.
- c) Milk:
Milk is indeed a true emulsion. It consists of fat globules dispersed in water, stabilized by proteins and other emulsifiers. Therefore, it meets the criteria for a true emulsion.
- d) Whipped Cream:
Whipped cream is an example of a foam, which contains air bubbles suspended in a liquid (cream). While it contains emulsified fat, it is primarily classified as a foam.
Conclusion
Thus, the correct answer is option 'C: Milk', as it is the only true emulsion among the given choices. Milk contains fat droplets suspended in water, making it a classic example of a stable emulsion, aided by proteins that prevent separation. Understanding the nature of emulsions helps in various applications, particularly in food science and culinary arts.

Assertion (A): Alloys are considered as mixtures, even though their components cannot be separated by physical methods.
Reason (R): Alloys show the properties of their constituents and can have variable composition.
  • a)
    Both (A) and (R) are true, and (R) is the correct explanation of (A).
  • b)
    Both (A) and (R) are true, but (R) is not the correct explanation of (A).
  • c)
    (A) is true, but (R) is false.
  • d)
    (A) is false, but (R) is true.
Correct answer is option 'A'. Can you explain this answer?

Sankar Bajaj answered
Understanding Alloys as Mixtures
Alloys are indeed classified as mixtures, despite the fact that their components cannot be separated by physical methods. Let's explore the assertion and reason in detail.
Assertion (A) Explained
- Alloys are combinations of two or more metals (or a metal and a non-metal) that are melted together.
- While they form a homogeneous mixture at the atomic level, the individual components retain their unique properties.
- The inability to separate the components using physical methods (like filtration or distillation) is a characteristic of alloys, not a disqualification from being considered a mixture.
Reason (R) Explained
- Alloys exhibit properties derived from their constituent metals, such as strength, ductility, and resistance to corrosion.
- The composition of alloys can vary, meaning that different ratios of components can yield different properties, which is a hallmark of mixtures.
- This variability in composition allows for the customization of alloys to meet specific requirements for various applications.
Relationship Between A and R
- Both statements are true: alloys are classified as mixtures, and they show properties of their constituents while allowing variable composition.
- The reason (R) effectively explains the assertion (A) by emphasizing the nature of mixtures and the behavior of alloys.
Conclusion
Thus, the correct answer is option 'A': both (A) and (R) are true, and (R) is the correct explanation of (A). Understanding this relationship enhances comprehension of material science and the properties of different substances.

At room temperature, a non-metal which is liquid is
  • a)
    Bromine
  • b)
    Fluorine
  • c)
    Oxygen
  • d)
    Sulphur
Correct answer is option 'A'. Can you explain this answer?

Hiral Datta answered
At room temperature, Bromine is present in liquid state. Bromine is a non-metal belonging to family of halogens.

Identify homogeneous mixture from the following
  • a)
    Dust storm
  • b)
    A suspension of chalk in water
  • c)
    A dilute solution of alcohol in water.
  • d)
    Soap bubbles are formed by blowing air into soap solution
Correct answer is option 'C'. Can you explain this answer?

Akash Nambiar answered
A dilute solution of alcohol in water is an example of homogenous mixture. Alcohol and water do not forms a separate boundary if left undisturbed and both are uniformly distributed.

Which one is NOT an element?
  • a)
    Metals
  • b)
    Non-metals
  • c)
    Alloys
  • d)
    Semi-metals
Correct answer is option 'C'. Can you explain this answer?

Rajeev Unni answered
Elements include metals, non-metals and semi-metals (metalloids). Alloys are mixture of two or more elements.

Which of the following are chemical changes?
(i) Decaying of wood (ii) Burning of wood
(iii) Sawing of wood (iv) Hammering of nail into wood
  • a)
    (i) and (ii)
  • b)
    (ii) and (iii)
  • c)
    (i) and (iv)
  • d)
    (ii) and (iv)
Correct answer is option 'A'. Can you explain this answer?

Decaying of wood and burning of wood, because there will be a change of chemical composition and wood cannot be restored to its original form. Sawing of wood and hammering of nails into a piece of wood are physical processes where the chemical composition of compounds is not changed.

Which one of the following is a solid foam
  • a)
    Ruby
  • b)
    Shaving cream
  • c)
    Bread
  • d)
    Butter
Correct answer is option 'C'. Can you explain this answer?

Explanation:

Solid Foam:
- A solid foam is a material that has a cellular structure with pockets of gas trapped within a solid material.
- The most common example of solid foam is bread, which has a spongy texture due to the presence of air pockets within the bread structure.

Ruby:
- Ruby is a precious gemstone that is known for its red color and is not a solid foam.

Shaving Cream:
- Shaving cream is a type of foam that is used for shaving and is not a solid foam.

Butter:
- Butter is a dairy product that is spreadable and not a solid foam.
Therefore, the correct answer is option C) Bread as it is an example of a solid foam due to its cellular structure with trapped air pockets.

Assertion (A): Lemonade is an example of a homogeneous mixture.
Reason (R): In a solution, there is homogeneity at the particle level, where the solute particles are evenly distributed in the solvent.
  • a)
    Both (A) and (R) are true, and (R) is the correct explanation of (A).
  • b)
    Both (A) and (R) are true, but (R) is not the correct explanation of (A).
     
  • c)
    (A) is true, but (R) is false.
  • d)
    (A) is false, but (R) is true.
Correct answer is option 'A'. Can you explain this answer?

Gowri Ahuja answered
Understanding the Assertion (A) and Reason (R)
The assertion states that lemonade is a homogeneous mixture, while the reason explains the nature of solutions and their uniformity at the particle level.
What is a Homogeneous Mixture?
- A homogeneous mixture is one that has a uniform composition throughout.
- In such mixtures, the individual components are not distinguishable from one another.
- Lemonade, when properly mixed, appears uniform; you cannot see separate particles of sugar, water, or lemon juice.
Why Lemonade is Homogeneous?
- When making lemonade, sugar dissolves in water, and the lemon juice integrates well.
- The resulting mixture has consistent taste and appearance, fulfilling the criteria for homogeneity.
Understanding the Reason (R)
- The reason states that in a solution, solute particles are evenly distributed in the solvent.
- This concept applies directly to lemonade, as sugar (solute) is evenly dispersed in water (solvent).
Connection Between (A) and (R)
- Both the assertion and reason are true:
- Lemonade is indeed a homogeneous mixture.
- The particle-level distribution in solutions explains why lemonade is homogeneous.
- Therefore, the reason effectively supports the assertion.
Conclusion
- Based on this analysis, both (A) and (R) are true, with (R) serving as the correct explanation for (A).
- This aligns with option (a): "Both (A) and (R) are true, and (R) is the correct explanation of (A)."

Chapter doubts & questions for Is Matter Around Us Pure? - Science Class 9 2026 is part of Class 9 exam preparation. The chapters have been prepared according to the Class 9 exam syllabus. The Chapter doubts & questions, notes, tests & MCQs are made for Class 9 2026 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests here.

Chapter doubts & questions of Is Matter Around Us Pure? - Science Class 9 in English & Hindi are available as part of Class 9 exam. Download more important topics, notes, lectures and mock test series for Class 9 Exam by signing up for free.

Science Class 9

88 videos|481 docs|61 tests

Top Courses Class 9