Olympiad Test: Physical And Chemical Changes - 1 - Class 7 MCQ

Burning of wood is a chemical change.

Explanation:

When wood is burned, it undergoes a chemical reaction called combustion. During combustion, the wood reacts with oxygen in the air to produce heat, light, and various gases, such as carbon dioxide and water vapor. This chemical reaction changes the composition and structure of the wood, resulting in the formation of new substances.

Here are some key points to consider:

• Chemical change: Burning of wood involves a chemical reaction where the bonds between atoms are broken and new bonds are formed. This results in the formation of new substances with different properties.


• Energy release: Burning wood produces heat and light energy, which is a characteristic of a chemical change.


• Irreversible: Once wood is burned, it cannot be restored to its original state. The chemical changes that occur during burning are irreversible.


• Physical change: Physical changes do not involve a change in the chemical composition of a substance. Burning wood is not a physical change because it involves a chemical reaction.


• Reversible: Reversible changes are those that can be undone and the substance can be restored to its original state. Burning wood is not reversible as the original wood cannot be regenerated.

In conclusion, the burning of wood is a chemical change because it involves a chemical reaction, produces heat and light energy, and is irreversible.

Neutralization change is a chemical change

Neutralization is a chemical reaction that occurs when an acid reacts with a base to form a salt and water. During this process, the reactants undergo a complete molecular rearrangement, resulting in the formation of new substances with different chemical properties. Here are some key points to support the answer:

- Definition: Neutralization is a chemical reaction that involves the combination of an acid and a base to form a salt and water.
- Chemical Reaction: Neutralization reactions involve the breaking and forming of chemical bonds, resulting in the creation of new substances.
- Change in Chemical Properties: The reactants and products of a neutralization reaction have different chemical properties, indicating a chemical change.
- pH Change: Neutralization reactions result in a change in pH, indicating a chemical transformation.
- Energy Change: Neutralization reactions often release or absorb energy, indicating a chemical change.
- Irreversibility: Once a neutralization reaction occurs, it is typically difficult or impossible to reverse the process, further supporting the idea of a chemical change.
Therefore, based on the definition, nature of the reaction, changes in chemical properties, and irreversibility, it can be concluded that neutralization change is a chemical change.

Acid rain occurs mainly because of carbon dioxide, sulphur dioxide and nitrogen dioxide which are all pollutants. 

This is a physical change even though the characteristics of this mixture is different from the original substances can retain their properties and can be get back easily. 

To determine which of the changes is a chemical change, we need to understand the characteristics of chemical changes and physical changes.
Physical changes:
- Physical changes do not alter the chemical composition or nature of the substance.
- They are reversible changes.
- Examples include changes in state (solid to liquid, liquid to gas), changes in shape or size, and changes in color.
Chemical changes:
- Chemical changes result in the formation of new substances with different chemical properties.
- They are usually irreversible changes.
- Examples include burning, rusting, and decomposition.
Now let's analyze the given changes:
(i) A piece of magnesium gives out bright flames when burnt.
- Burning magnesium is a chemical change.
- When magnesium reacts with oxygen in the air, it forms magnesium oxide.
- The bright flames observed during the burning of magnesium are due to the exothermic reaction between magnesium and oxygen.
(ii) A piece of iron glows red when heated strongly.
- The glowing of iron when heated strongly is a physical change.
- It is caused by the increase in temperature, which excites the electrons in the iron atoms and causes them to emit light.
- The iron does not undergo a chemical reaction or change its chemical composition during this process.
Therefore, the answer is (i) only. The burning of magnesium is a chemical change, while the glowing of iron when heated strongly is a physical change.

Question: Which of the following changes is not a chemical change?

Answer:

The correct answer is B: boiling of water. Boiling of water is a physical change, not a chemical change. Here's a detailed explanation:
Chemical Change:
A chemical change is a process that results in the formation of new substances with different properties from the original substances. It involves the rearrangement of atoms and the breaking or forming of chemical bonds.
Physical Change:
A physical change is a process that alters the state or form of a substance without changing its chemical composition. It does not involve the formation of new substances.
Now let's analyze each option:
A: Boiling of an egg:
- Boiling an egg involves the denaturation of proteins present in the egg white.
- The heat causes the proteins to unfold and coagulate, resulting in a solid state.
- This is a chemical change because the proteins undergo structural changes.
B: Boiling of water:
- Boiling water involves the conversion of liquid water into water vapor (gas) due to the application of heat.
- The chemical composition of water remains the same, and no new substances are formed.
- This is a physical change as only the state of water changes.
C: Burning of cloth:
- Burning cloth involves a combustion reaction where the cloth reacts with oxygen in the air to produce carbon dioxide, water vapor, and other combustion byproducts.
- The cloth undergoes a chemical change and is converted into different substances.
- This is a chemical change.
D: Burning of paper:
- Burning paper also involves a combustion reaction where the paper reacts with oxygen to produce carbon dioxide, water vapor, and other combustion byproducts.
- The paper undergoes a chemical change and is converted into different substances.
- This is a chemical change.
In summary, the boiling of water (option B) is a physical change, whereas the boiling of an egg, burning of cloth, and burning of paper (options A, C, and D) are all examples of chemical changes.

All chemical changes are irreversible and the original substances can not be obtained. 

Physical Change:
- A physical change is a change in the physical properties of a substance without altering its chemical composition.
- It involves a change in the state, shape, size, or phase of matter.
Options:
A: Cooking of food
- Cooking involves chemical reactions, such as the denaturation of proteins, which is a chemical change.
- It is not a physical change.
B: Melting of wax
- Melting is a physical change because it only involves a change in the state of matter.
- The wax changes from a solid to a liquid without any alteration in its chemical composition.
C: Rusting of iron
- Rusting is a chemical change because it involves the reaction of iron with oxygen to form iron oxide.
- The chemical composition of the iron changes during rusting.
D: Curdling of milk
- Curdling is a chemical change because it involves the coagulation of proteins in milk due to the action of acid or enzymes.
- The chemical composition of the milk changes during curdling.
Conclusion:
The physical change among the given options is the melting of wax. It only involves a change in the state of matter without any alteration in the chemical composition of the wax.

When an iron nail immersed in the solution of copper sulphate than iron displaces copper from the solution of copper sulphate because iron is more reactive than copper. Therefore copper sulphate solution colour changes from blue to pale green.

The gas released when vinegar is mixed with baking soda is CO₂.
- When vinegar (acetic acid, CH₃COOH) is mixed with baking soda (sodium bicarbonate, NaHCO₃), a chemical reaction occurs.
- The reaction between acetic acid and sodium bicarbonate produces carbon dioxide (CO₂), water (H₂O), and sodium acetate (CH₃COONa).
- The balanced chemical equation for this reaction is:
CH₃COOH + NaHCO₃ ⟶ CH₃COONa + H₂O + CO₂
- The gas released during this reaction is carbon dioxide (CO₂), which is responsible for the fizzing and bubbling observed.
- Carbon dioxide is a colorless and odorless gas. It is denser than air and can displace oxygen in closed spaces, so it is important to ensure proper ventilation when conducting this reaction in a confined area.
- The production of carbon dioxide gas is commonly used in baking as it helps dough rise and gives it a light texture.
- This reaction is often used in science experiments or as a safe and non-toxic way to create homemade volcanoes or fizzy bath bombs.

No new substances are formed in all these changes. 

The soil is acidic and should be neutralized by adding slaked lime (calcium hydroxide) which is a base. 

The candle started melting, but since no new substance was formed it is a physical change. 

When magnesium (Mg) is burnt in air, it reacts with oxygen (O₂) to form a product.

Key Point:

The reaction of magnesium with oxygen is an example of a combustion reaction.

Explanation:

The balanced chemical equation for the combustion of magnesium in air is:

2Mg + O₂ → 2MgO

Explanation of the balanced equation:

The equation shows that:

• 2 moles of magnesium (2Mg) react with 1 mole of oxygen (O₂)


• 2 moles of magnesium oxide (2MgO) are formed as the product

Conclusion:

When magnesium is burnt in air, the product formed is magnesium oxide (MgO).

Therefore, the correct answer is option A: MgO.

Rusting takes place in moist air.
Rusting is a chemical process that occurs when iron or steel comes into contact with oxygen and water. The presence of moisture is essential for rusting to occur. Here is a detailed explanation:
Explanation:
- Rusting is an electrochemical process in which iron undergoes oxidation, resulting in the formation of iron oxide (rust).
- Moisture in the air contains water molecules, which provide the necessary medium for the reaction to occur.
- When iron or steel is exposed to moist air, the water molecules react with the iron to form hydrated iron(III) oxide, commonly known as rust.
- The presence of oxygen is also crucial for rusting to occur. Oxygen molecules from the air combine with iron atoms to form iron(III) ions.
- The reaction between iron, oxygen, and water takes place at the surface of the metal, forming a layer of rust.
- This layer of rust is porous and allows further exposure of iron to moisture and oxygen, leading to the continuous corrosion process.
- Rusting can be accelerated in environments with high humidity or in the presence of salts, which increase the conductivity of the moisture.
In conclusion, rusting occurs in moist air because it provides the necessary water molecules for the electrochemical reaction between iron, oxygen, and water to take place.

Magnesium hydroxide or Mg(OH)₂ is an antacid. It is also known as milk of magnesia. It neutralizes excess HCl (acid) present in stomach  

Given: Substance X is being described and we need to determine what it could be.
Analysis: To identify the substance X, we can analyze the characteristics and properties of each option (salt, water, base, and acid) and compare them with the given information.
Characteristics of Salt:
- Salt is a compound that is formed by the combination of a metal and a non-metal.
- It is usually solid at room temperature and can dissolve in water.
- Salt is often used for seasoning food and preserving food.
Characteristics of Water:
- Water is a compound composed of two hydrogen atoms and one oxygen atom.
- It is a liquid at room temperature and is essential for all living organisms.
- Water can dissolve many substances and is known as the universal solvent.
Characteristics of Base:
- Bases are substances that can accept protons or donate pairs of electrons.
- They have a bitter taste, feel slippery, and can turn red litmus paper blue.
- Some common bases include sodium hydroxide (NaOH) and ammonia (NH3).
Characteristics of Acid:
- Acids are substances that can donate protons or accept pairs of electrons.
- They have a sour taste, can corrode metals, and turn blue litmus paper red.
- Examples of acids include hydrochloric acid (HCl) and sulfuric acid (H2SO4).
Conclusion: Based on the given information, we can conclude that Substance X is a base (Option C). This is because the description of a base matches the characteristics provided.
Note: It is important to consider the context and additional information to determine the correct answer.

Substance Y:
- The substance Y can be determined by analyzing the given information and options.
- Let's examine each option to identify the characteristics of substance Y.
A: Acid
- Acids are substances that have a pH less than 7.
- They typically taste sour, can corrode metals, and produce hydrogen ions (H+) when dissolved in water.
- If substance Y is an acid, it would exhibit these properties.
B: Base
- Bases are substances that have a pH greater than 7.
- They usually taste bitter, feel slippery, and produce hydroxide ions (OH-) when dissolved in water.
- If substance Y is a base, it would display these characteristics.
C: Water
- Water is a neutral substance with a pH of 7.
- It does not exhibit acidic or basic properties.
- If substance Y is water, it would be neutral.
D: Salt
- Salts are formed when an acid reacts with a base.
- They can be acidic, basic, or neutral depending on the specific acid and base used.
- Without further information, it is difficult to determine whether substance Y is a salt.
Conclusion:
Based on the given information, substance Y could be an acid (option A) as it aligns with the properties described. However, without additional context or experimental data, it is not possible to definitively determine the substance.