Class 7 Science Chapter 5 Important Questions - Physical and Chemical Changes

Very Short Answer Type Questions

Q1: What is a physical change? Provide an example.
Ans: A physical change is when a substance alters its physical properties, such as:

• Shape
• Size
• Colour
• State (solid, liquid, gas)

Importantly, no new substance is created during this process. An example of a physical change is the melting of butter.

Q2: What is a chemical change? Provide an example.
Ans: A chemical change occurs when one or more substances react to create a new substance with different properties. Key characteristics include:

• Formation of new substances
• Changes in energy, such as heat or light being released or absorbed
• Possible changes in smell or colour
• Production of gas or sound

For example, the burning of magnesium results in magnesium oxide, a new substance.

Q3: Is a physical change temporary or permanent?
Ans: A physical change is generally considered to be temporary.

• It involves changes in physical properties such as shape, size, or state.
• No new substances are formed during this process.
• Physical changes can often be reversed.

Q4: Is a chemical change permanent or temporary?
Ans: A chemical change is a process that results in the formation of one or more new substances. This type of change is permanent because the original substances cannot be easily recovered.

• Examples include:
• Burning wood
• Rusting of iron
• Cooking food

In contrast, physical changes can often be reversed, but chemical changes lead to new properties and substances.

Q5: What forms of energy are typically emitted or absorbed during a chemical change?
Ans: Forms of energy emitted or absorbed during a chemical change include:

• Heat - Often released or absorbed.
• Light - Can be emitted, such as in combustion.
• Sound - May be produced during reactions.
• Smell - New or changed odours can occur.
• Colour changes - Indicate new substances forming.
• Gas formation - New gases may be produced.

Q6: Give an example of a chemical change.
Ans: An example of a chemical change is the reaction between copper sulfate and iron. This process results in:

• The formation of iron sulfate.
• The production of copper as a brown deposit.

In this reaction, the blue copper sulfate solution changes to green, indicating a new substance has been created.

Q7: What is rust, and how does it form?
Ans: Rust is a brownish substance that forms on iron when it is exposed to oxygen and water. This process is known as rusting, which is a type of chemical change.

• Rust occurs when iron reacts with oxygen in the presence of moisture.
• The chemical reaction produces iron oxide, commonly known as rust.
• Rusting can weaken iron structures over time, leading to significant damage.

Q8: Why is it important to dry wet iron objects in the kitchen?
Ans: Wet iron objects in the kitchen should be dried promptly to prevent rusting. Here are the key reasons:

• Rust Formation: Rust develops when iron reacts with water and oxygen.
• Moisture: High humidity accelerates the rusting process.
• Prevention: Keeping iron items dry stops rust from forming.

By drying wet iron objects, you help maintain their condition and extend their lifespan.

Q9: How can you prevent the rusting of iron objects?
Ans: Rusting of iron objects can be prevented using several methods:

• Apply a coat of paint or grease regularly.
• Use a protective layer of another metal, such as zinc or chromium.
• Keep iron items dry to avoid exposure to moisture.
• Store iron objects in a low-humidity environment.

Q10: What is crystallization, and how does it help obtain pure crystals?
Ans: Crystallization is a method used to obtain pure crystals from a solution. It involves the following steps:

• Heat a concentrated solution until it dissolves completely.
• Allow the solution to cool gradually.
• As it cools, pure crystals begin to form.
• This process separates the pure substance from impurities.

Crystallization is an example of a physical change, as it does not alter the chemical composition of the substance.

Short Answer Type Questions

Q11: Explain the properties of a physical change.
Ans: Properties of a physical change include:

• Temporary: The change does not last forever.
• No new substances: No new materials are created.
• No energy change: There is no emission or absorption of energy.
• Generally reversible: The original state can often be restored.

Q12: Describe the properties of a chemical change.
Ans: Properties of a chemical change include:

• Permanence: The change is usually irreversible.
• New substances: One or more new substances are formed.
• Energy changes: There may be emission or absorption of energy, such as heat or light.
• Observable effects: Changes in colour, smell, or sound may occur.

Q13: Provide an example of a chemical change and explain the new substances formed.
Ans: In the reaction between copper sulfate and iron, the following occurs:

• The blue copper sulfate solution reacts with iron.
• This reaction produces a green iron sulfate solution.
• A brown deposit of copper is also formed.

The new substances created are iron sulfate and copper.

Q14: Why does rusting occur, and what factors are essential for rusting to take place?
Ans: Rusting occurs when iron reacts with oxygen and water, resulting in the formation of iron oxide, commonly known as rust. The essential factors for rusting include:

• Oxygen from the air
• Water or water vapour
• Increased humidity accelerates the rusting process

To prevent rusting:

• Keep iron away from oxygen and water
• Apply a coat of paint or grease regularly
• Use galvanisation, which involves coating iron with a layer of zinc

Q15: How can you prevent the rusting of iron objects, and what methods are commonly used?
Ans: Rusting of iron objects can be prevented using several methods:

• Oiling or greasing the surface to create a barrier.
• Painting the iron to shield it from moisture and air.
• Galvanisation, which involves coating iron with zinc to prevent rust.
• Converting iron into stainless steel, which is resistant to rust.

Regular maintenance of these protective layers is essential to ensure long-lasting protection against rust.

Q16: What is crystallization, and how is it different from the process of evaporation?
Ans: Crystallization is a method used to form large, pure crystals from a solution. It involves:

• Cooling a solution to allow crystals to form.
• Separating the solid from the liquid.

In contrast, evaporation simply removes the solvent, leaving behind the solid solute without forming distinct crystals.

Q17: Explain the steps involved in the crystallization of copper sulfate.
Ans: The steps involved in the crystallisation of copper sulfate are as follows:

• Prepare a concentrated solution of copper sulfate by dissolving the powder in warm water.
• Allow the solution to cool gradually without stirring.
• Observe the formation of copper sulfate crystals as the solution cools.

Q18: Why is crystallization used to obtain pure crystals from an impure solid substance?
Ans: Crystallization is a method used to obtain pure crystals from an impure solid. It works by:

• Allowing the controlled formation of large, well-defined crystals.
• Separating the pure crystals from impurities in the solution.
• Utilising a physical change that enhances the purity of the substance.

This process is effective because it takes advantage of the differences in solubility between the desired substance and the impurities.

Q19: How can you test for the presence of carbon dioxide, and what is the result of this test?
Ans: The presence of carbon dioxide can be tested by:

• Passing the gas through freshly prepared lime water.
• If carbon dioxide is present, the lime water will turn milky.

This milky appearance is due to the formation of calcium carbonate.

Q20: Explain how vinegar and baking soda react and the result of this reaction.
Ans: When vinegar (acetic acid) is mixed with baking soda (sodium hydrogen carbonate), they react to produce carbon dioxide gas. This can be confirmed by passing the gas through lime water, which turns milky due to the formation of calcium carbonate. The chemical reaction is: Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O).