HOTS Questions: Is Matter Around Us Pure - Class 9 MCQ

1. Starch solution

Explanation: A colloidal solution consists of tiny particles suspended in a continuous medium. The particle size in a colloid is between that of a true solution and a suspension, meaning the particles do not settle out upon standing.

• Starch solution is a colloid because the starch particles are small enough to remain suspended in water without settling.
• Copper sulphate solution is a true solution, not a colloid.
• Chalk powder in water is a suspension, as the chalk particles settle down when left undisturbed.
• Kerosene oil and water form an immiscible liquid mixture, not a colloid.

Thus, starch solution is the correct example of a colloidal solution.

Colloids
A colloid is a mixture in which small particles of one substance are dispersed evenly throughout another substance. The particles in a colloid are larger than those in a solution but smaller than those in a suspension. Colloids can be found in various forms, including liquids, gels, and solids.
Identifying the Colloid
To determine which of the given options is a colloid, we need to understand the characteristics of a colloid and analyze each option accordingly.
A: Aluminium paint
- Aluminium paint is a mixture of tiny aluminum particles dispersed in a liquid medium, usually oil or water.
- The aluminum particles in the paint are larger than those in a solution but smaller than those in a suspension.
- Therefore, aluminium paint is a colloid.
B: Blood
- Blood is a complex mixture of cells, proteins, and other substances suspended in plasma.
- The cells and proteins in blood are larger than those in a solution but smaller than those in a suspension.
- Therefore, blood is a colloid.
C: Milk
- Milk is a mixture of water, fats, proteins, and other substances.
- The fats and proteins in milk form small particles that are dispersed throughout the liquid.
- These particles are larger than those in a solution but smaller than those in a suspension.
- Therefore, milk is a colloid.
D: All
- Since all three options (aluminium paint, blood, and milk) meet the characteristics of a colloid, the correct answer is D: All.
Conclusion
In summary, all of the given options (aluminium paint, blood, and milk) are colloids. They are all mixtures in which small particles of one substance are dispersed evenly throughout another substance.

Physical Changes:
A physical change is a change in the state or form of matter without any change in the chemical composition or identity of the substance. In other words, the substance remains the same before and after the change.
Examples of Physical Changes:
A. Burning of a Natural Gas:
- This is a clear example of a combustion reaction. Natural gas comprehends methane gas. When methane reacts with oxygen in the atmosphere it produces water and carbon dioxide.
B. Rusting of iron:
- This is a chemical change, not a physical change. Rusting occurs when iron reacts with oxygen in the presence of moisture, forming iron (III) oxide (rust).
C. Freezing of water:
- This is a physical change. When water is cooled below its freezing point, it undergoes a phase change from a liquid to a solid, forming ice. The chemical composition of water (H2O) remains the same.
D. Digestion of food:
- This is a chemical change, not a physical change. Digestion involves the breakdown of complex food molecules into simpler substances through various chemical reactions in the digestive system.
Conclusion:
The freezing of water is the only example mentioned that represents a physical change. The other options involve chemical changes.

When iron and sulphur are heated at high temperature:
- Black coloured FeS is formed: When iron and sulphur are heated at high temperature, they react to form iron sulphide (FeS) which is black in color. This reaction is a synthesis reaction where iron and sulphur combine to form a new compound.
- Mixture of iron and sulphur is obtained: This statement is incorrect. Heating iron and sulphur at high temperature does not result in a simple mixture of iron and sulphur. Instead, a chemical reaction takes place and a new compound, iron sulphide, is formed.
- Yellow coloured iron sulphide is formed: This statement is incorrect. Iron sulphide (FeS) formed from the reaction of iron and sulphur is black in color, not yellow.
- They do not heat: This statement is incorrect. Iron and sulphur can be heated at high temperatures to initiate a chemical reaction between them.
Therefore, the correct answer is option A: Black coloured FeS is formed.

To determine the solubility of each compound at 293 K, we can compare their solubility product constants (Ksp). The higher the Ksp value, the greater the solubility of the compound.
The Ksp values for the given compounds are:
A: KNO3 - 37.0
B: NaCl - 36.0
C: KCl - 74.5
D: NH4Cl - 52.0
From the given options, compound D (NH4Cl) has the highest Ksp value of 52.0. Therefore, NH4Cl has the highest solubility at 293 K among the given compounds.
Key Points:
- Solubility can be determined by comparing the solubility product constants (Ksp) of compounds.
- The higher the Ksp value, the greater the solubility of the compound.
- Compound D (NH4Cl) has the highest Ksp value of 52.0 among the given options.
- Therefore, NH4Cl has the highest solubility at 293 K.

- The size of colloidal particles typically ranges from 10^-9 m to 10^-6 m, which corresponds to option B.
- Colloids are systems where particles are dispersed throughout a continuous medium.
- These particles are larger than molecules but small enough not to settle out under gravity.
- The size range in meters is appropriate for describing colloids, as it captures the scale between true solutions and suspensions.
- This range ensures colloidal stability and distinct optical properties.

Yes, homogenous mixture have a variable composition as the constituents are present in any ratio such as 1:2 or 1:3 etc

To determine which of the given options is a heterogeneous mixture, let's examine each option individually:
A: Air
- Air is a mixture of gases, mainly nitrogen (78%), oxygen (21%), and small amounts of other gases such as carbon dioxide, argon, and trace elements.
- It is a homogeneous mixture because the composition is uniform throughout.
B: Brass
- Brass is an alloy composed of copper and zinc.
- It is a homogeneous mixture because the copper and zinc atoms are evenly distributed throughout the material.
C: Iodised table salt
- Iodised table salt is a mixture of sodium chloride and small amounts of iodine.
- It is a heterogeneous mixture because the iodine is not evenly distributed throughout the salt. The iodine appears as small specks or crystals in the salt.
D: Steel
- Steel is an alloy composed of iron and carbon, along with small amounts of other elements.
- It is a homogeneous mixture because the iron and carbon atoms are evenly distributed throughout the material.
Therefore, the correct answer is C: Iodised table salt as it is a heterogeneous mixture due to the uneven distribution of iodine within the salt.

Size of particles in true solutions:

• True solutions: True solutions are homogeneous mixtures of two or more substances where the solute particles are uniformly dispersed in the solvent. These particles are very small in size.


• Particle size: The size of particles in true solutions is extremely small.


• Range of particle size: The size of particles in true solutions is typically measured in nanometers (nm).


• Particle size range: The particle size of true solutions is generally less than 1 nm.


• Characteristics of true solution particles: The small size of the particles in true solutions allows them to remain dispersed and stable, without settling or separating.


• Examples of true solutions: Examples of true solutions include saltwater, sugar dissolved in water, and alcohol dissolved in water.

Therefore, the correct answer is A: < 1 nm. The size of particles in true solutions is generally less than 1 nm.

Brownian movement arises because of the impact of the molecules of the dispersion medium with the colloidal particles. Since the impacts of the molecules of the dispersion medium on the colloidal particles are unequal (i.e. unbalanced bombardment), the result is zig-zag motion.

Mass of common salt (solute) = 40g
Mass of water (soluted) = 320g
Mass of solution = 320 + 40 = 360g
Concentration of solution = M(mass of solute) / mass of solution = 40/360 x 100 = 11.11%

Lead (Pb) is not a compound. It is an element present in its pure form so it is a single homogeneous identity. Compounds have two or more elements present in the uniform composition in the compound state.

The particles of suspension:
- Can be seen with the naked eye. Suspensions are heterogeneous mixtures in which solid particles are dispersed in a liquid or gas. These particles are usually large enough to be visible to the naked eye.
- Cannot be seen with a powerful microscope. While the particles in a suspension may not be visible under a powerful microscope, they can still be seen without the aid of any magnification.
- Cannot be seen with an electron microscope. Electron microscopes are capable of magnifying objects to a much higher degree than light microscopes, but even they may not be able to visualize the particles in a suspension due to their size and distribution.
- Are larger than the particles in a solution. In contrast to solutions, where the solute particles are evenly distributed and too small to be seen, the particles in a suspension are larger and can settle over time due to gravity.
- Can be separated by filtration or settling. Since the particles in a suspension are larger and more visible, they can be separated from the liquid or gas phase through processes such as filtration or allowing them to settle and then decanting the liquid above.

Statement: Compound is homogeneous and have fixed composition.

Definition of compound:
- A compound is a substance that is composed of two or more elements chemically combined in a fixed ratio.
- It has its own unique set of properties that are different from the properties of its constituent elements.
Explanation:
- A homogeneous mixture is a mixture that has a uniform composition throughout, meaning that the components are evenly distributed.
- A compound is a type of homogeneous mixture because it has a fixed composition.
- The elements in a compound are combined in a specific ratio, which means that the relative amounts of the elements are always the same.
- This fixed composition gives compounds their distinct properties and makes them different from mixtures.
Key points:
- A compound is homogeneous, meaning it has a uniform composition throughout.
- The composition of a compound is fixed, with elements combined in a specific ratio.
- Compounds have distinct properties that are different from the properties of their constituent elements.
Conclusion:
The correct statement is option A: Compound is homogeneous and has a fixed composition.

e correct answer is c

   The process by which we can separate the particles of a colloidal solution is called Centrifugation.

Explanation:
When a substance undergoes a physical change, its composition remains the same. This means that the atoms or molecules that make up the substance do not rearrange or combine with other atoms or molecules to form a new substance. Instead, the physical properties of the substance may change, such as its shape, size, density, state (solid, liquid, gas), or phase (melting, freezing, boiling, condensation).
Key points:
- Physical change refers to a change in the physical properties of a substance without any change in its chemical composition.
- In a physical change, the substance may change its state (e.g., from solid to liquid or gas) or undergo a change in physical properties (e.g., color, shape, size), but the composition of the substance remains the same.
- The atoms or molecules that make up the substance do not rearrange or combine with other atoms or molecules to form a new substance.
- Examples of physical changes include melting ice, boiling water, dissolving salt in water, and crushing a can.

  The correct answer is a

   Full-fat milk is standardised to 3.5% of fat and semi skimmed milk contains approximately 1.5% fat. Skimmed milk and buttermilk are very low in fat and, on average, contain 0.1 or 0.2% fat respectively.

Answer:
To determine which of the following options is heterogeneous, let's first understand the concept of heterogeneity.
Heterogeneous mixtures are mixtures that have different components or substances that are not uniformly distributed throughout. In other words, the composition of a heterogeneous mixture is not the same in all parts of the mixture.
Now let's analyze each option:
A:

Stainless steel

Stainless steel is an alloy composed of iron, carbon, and other elements such as chromium and nickel. It is a homogeneous mixture as its composition is uniform throughout.
B:

Brass

Brass is an alloy composed mainly of copper and zinc. Similar to stainless steel, it is a homogeneous mixture as its composition is uniform throughout.
C:

Air

Air is a mixture of gases, primarily nitrogen, oxygen, carbon dioxide, and trace amounts of other gases. While it may have variations in the concentration of these gases in different locations, it is considered a homogeneous mixture on a larger scale.
D:

Smoke

Smoke is a complex mixture of solid particles, liquid droplets, and gases. It is considered a heterogeneous mixture as its composition is not uniform throughout. It contains visible particles suspended in the air, giving it a non-uniform appearance.
Therefore, the correct answer is option D: Smoke. Smoke is a heterogeneous mixture.

Separating Kerosene Oil and Water

To separate kerosene oil and water, you can use the following methods:
1. Separating Funnel:
- A separating funnel is a glassware commonly used in laboratories to separate immiscible liquids (liquids that do not mix together).
- You can pour the mixture of kerosene oil and water into the separating funnel and allow it to stand until the two liquids separate into distinct layers based on their densities.
- Carefully open the stopcock at the bottom of the funnel and collect the kerosene oil in a separate container.
2. Distillation:
- Distillation is a process that involves heating a mixture to separate its components based on their boiling points.
- In this case, the mixture of kerosene oil and water is heated, and the kerosene oil, having a lower boiling point, vaporizes and can be collected separately.
- The water, having a higher boiling point, remains in the distillation flask.
3. Centrifugation:
- Centrifugation is a technique used to separate substances of different densities by spinning them at high speeds.
- By placing the kerosene oil and water mixture in a centrifuge and spinning it, the denser water will move to the bottom, while the less dense kerosene oil will accumulate at the top, allowing for separation.
4. Evaporation:
- Evaporation involves heating a mixture to convert the liquid components into vapor, leaving behind the solid or liquid that does not evaporate.
- In this case, the mixture of kerosene oil and water can be heated to evaporate the kerosene oil, which has a lower boiling point.
- The water will remain in the container while the kerosene oil evaporates and can be collected separately.
In conclusion, kerosene oil and water can be separated by using a separating funnel, distillation, centrifugation, or evaporation, depending on the specific requirements and equipment available.