Grade 7 Q&A: Chapter 13: Changes - Physical and Chemical

Concept Questions

Q1: What is a change? Give two examples.

Answer: A change is an alteration in a substance or object. It can be a change in its state, shape, size, composition, or properties. Examples include: melting of ice, burning of wood.

Q2: Differentiate between fast and slow changes with examples.

Answer:

• Fast Changes: Occur quickly, in a short duration. Example: Bursting of a balloon, burning of paper.
• Slow Changes: Take a long duration to occur. Example: Rusting of iron, growth of a plant.

Q3: What are reversible changes? Give two examples.

Answer: Reversible changes are those in which the original substance can be recovered or the change can be undone. Examples: Melting of ice (water can be refrozen), stretching of a rubber band.

Q4: Define irreversible changes. Give two examples.

Answer: Irreversible changes are those in which the original substance cannot be recovered, and the change is permanent. Examples: Burning of wood (ash cannot become wood again), cooking of food.

Q5: What is a physical change? List two characteristics.

Answer: A physical change is a temporary change where only the physical properties (like state, shape, size) of a substance are altered, but no new substance is formed. Characteristics: 1. No new substance is formed. 2. It is generally temporary and reversible.

Q6: Provide three examples of physical changes.

Answer: 1. Melting of ice. 2. Dissolving sugar in water. 3. Cutting of paper.

Q7: What is a chemical change? List two characteristics.

Answer: A chemical change (or chemical reaction) is a permanent change in which one or more new substances with entirely different chemical properties are formed. Characteristics: 1. One or more new substances are formed. 2. It is generally permanent and irreversible.

Q8: Provide three examples of chemical changes.

Answer: 1. Burning of wood. 2. Rusting of iron. 3. Cooking of food.

Q9: List three indicators (signs) that suggest a chemical change has occurred.

Answer: 1. Evolution of heat or light (or both). 2. Evolution of gas. 3. Change in color.

Q10: Differentiate between natural and man-made changes with examples.

Answer:

• Natural Changes: Occur on their own in nature without human intervention. Example: Ripening of fruits, change of seasons.
• Man-made Changes: Brought about by human activities. Example: Building a house, burning of fuel.

Q11: Give an example of a beneficial change and a harmful change.

Answer: * Beneficial Change: Ripening of fruit. * Harmful Change: Rusting of iron.

Q12: Is the digestion of food a physical or chemical change? Justify your answer.

Answer: The digestion of food is a chemical change. This is because complex food substances are broken down into simpler, entirely new chemical substances by various enzymes, which cannot be reversed to form the original food.

Q13: Why is the rusting of iron considered a chemical change?

Answer: Rusting of iron is considered a chemical change because iron reacts with oxygen and moisture to form a new substance, iron oxide (rust), which has different properties from iron. This change is permanent and cannot be easily reversed.

Q14: Explain why melting of ice is a physical change.

Answer: Melting of ice is a physical change because only the state of water changes from solid (ice) to liquid (water). The chemical composition (H₂O) remains the same, and no new substance is formed. It is also reversible, as water can be refrozen into ice.

Q15: What happens to the chemical composition of a substance during a physical change?

Answer: During a physical change, the chemical composition of a substance remains unchanged. Only its physical properties like shape, size, or state are altered.

Application-Based Questions

Q16: You leave a peeled apple slice exposed to air, and it turns brown. What type of change is this, and why?

Answer: This is a chemical change. The apple turning brown is due to oxidation, where substances in the apple react with oxygen in the air to form new brown-colored compounds. This change is permanent and cannot be reversed.

Q17: When you mix baking soda with lemon juice, you observe bubbles forming. What does this indicate?

Answer: The formation of bubbles indicates the evolution of a gas, specifically carbon dioxide. This is a clear sign that a chemical change has occurred, as new substances (carbon dioxide, water, and sodium citrate) are being formed.

Q18: Why is baking a cake considered an irreversible change?

Answer: Baking a cake is an irreversible change because the ingredients undergo chemical reactions due to heat, forming new substances with different textures, tastes, and compositions that cannot be converted back into the original raw ingredients (flour, eggs, sugar, etc.).

Q19: A carpenter cuts a large wooden plank into smaller pieces to make furniture. Is this a physical or chemical change? Explain.

Answer: This is a physical change. Cutting the wooden plank only changes its size and shape, but the chemical composition of the wood remains the same. No new substance is formed, and the smaller pieces are still wood.

Q20: You notice that a silver ornament has become dull and blackish over time. What type of change is this, and what caused it?

Answer: This is a chemical change, known as tarnishing. Silver reacts with sulfur compounds in the air (like hydrogen sulfide) to form silver sulfide, a new blackish substance that makes the ornament appear dull.

Q21: Give an example of a change that is both beneficial and man-made.

Answer: Cooking food. It is beneficial because it makes food edible and digestible, and it is man-made as it requires human intervention.

Q22: Classify the bursting of a balloon as a fast/slow, reversible/irreversible, and natural/man-made change.

Answer: * Fast change * Irreversible change (the balloon cannot be easily put back together) * Man-made change (usually initiated by human action)

Q23: When you light a matchstick, you see light and feel heat. What type of change is occurring?

Answer: This is a chemical change. The matchstick's chemicals react with oxygen in the air, producing new substances (ash, gases), light, and heat, which are all indicators of a chemical reaction.

Q24: Explain why the growth of a child is considered a slow and natural change.

Answer: The growth of a child is a slow change because it occurs gradually over many years, involving complex biological processes. It is a natural change because it happens on its own without conscious human manipulation or intervention.

Q25: Describe a situation where both a physical and a chemical change occur simultaneously or in close sequence.

Answer: A common example is the burning of a candle. * Physical Change: The wax near the flame melts due to heat. This is a physical change because molten wax is still wax, just in a liquid state, and it can solidify again. * Chemical Change: The liquid wax near the wick gets drawn up and burns, reacting with oxygen to produce carbon dioxide, water vapor, light, and heat. This is a chemical change because new substances are formed and energy is released.

Textbook Exercise Solutions

A. Fill in the blanks:

1. Changes that occur quickly are called fast changes.
2. Changes that cannot be reversed are called irreversible changes.
3. A change that takes place due to human intervention is called a man-made change.
4. In a physical change, no new substance is formed.
5. Formation of a new substance is a characteristic of a chemical change.
6. Burning of a candle is an example of a chemical change.

B. State true or false:

1. The growth of a plant is a fast change. False
2. Cooking food is an irreversible change. True
3. Melting of ice is a chemical change. False
4. Rusting of iron is a physical change. False
5. Changes in seasons are natural changes. True
6. A beneficial change is always desirable. True

C. Match the following:

Column A

1. Burning of paper
2. Ripening of fruit
3. Boiling of water
4. Breaking of glass
5. Souring of milk

Column B

1. Physical change
2. Chemical change
3. Slow change
4. Fast change
5. Irreversible change

Answers:

1. Burning of paper - Chemical change, Fast change, Irreversible change
2. Ripening of fruit - Slow change, Chemical change, Irreversible change
3. Boiling of water - Physical change, Reversible change
4. Breaking of glass - Physical change, Fast change
5. Souring of milk - Chemical change, Irreversible change, Slow change

D. Answer the following questions briefly:

Q1: What are fast and slow changes? Give one example of each.

Answer: Fast changes occur quickly (e.g., bursting of a balloon). Slow changes take a long time (e.g., rusting of iron).

Q2: Differentiate between reversible and irreversible changes.

Answer: Reversible changes allow the original substance to be recovered (e.g., melting of ice), while irreversible changes do not, and the change is permanent (e.g., burning of wood).

Q3: What is a physical change?

Answer: A physical change is a temporary change where only the physical properties of a substance are altered (like state, shape, size), and no new substance is formed.

Q4: What is a chemical change?

Answer: A chemical change is a permanent change in which one or more new substances with different chemical properties are formed.

Q5: Give two examples of chemical changes where heat is evolved.

Answer: 1. Burning of wood. 2. Burning of a candle.

E. Answer the following questions in detail:

Q1: Explain physical changes with characteristics and suitable examples.

Answer: A physical change is a temporary alteration in which only the physical properties of a substance, such as its state, shape, size, or appearance, are modified. The chemical composition of the substance remains unchanged, and no new substance is formed. Characteristics of Physical Changes:

• Temporary: The change is not permanent.
• No New Substance Formed: The chemical identity of the substance remains the same.
• Reversible: In most cases, the original substance can be recovered by reversing the conditions.
• Chemical Composition Unchanged: The molecules of the substance are not altered.

• Melting of ice: Ice (solid water) changes to liquid water. Both are H₂O.
• Boiling of water: Liquid water turns into steam (gaseous water). Still H₂O.
• Dissolving sugar in water: Sugar is still sugar, and water is still water.
• Breaking of glass: Glass changes shape but is still glass.

Q2: Explain chemical changes with characteristics and suitable examples.

Answer: A chemical change (or chemical reaction) is a permanent change where one or more new substances with entirely different chemical properties are formed. The original substance loses its identity, and its chemical composition changes. Characteristics of Chemical Changes:

• Permanent: The change is irreversible.
• New Substance(s) Formed: This is the defining feature; new substances with different properties are created.
• Irreversible: The new substances cannot be easily converted back to the original ones by simple methods.
• Change in Chemical Composition: The molecules of the original substance are broken down and rearranged.
• Energy Changes: Often involves the absorption or release of heat, light, or sound.

• Burning of wood: Wood burns to produce ash and gases, which are new substances.
• Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide (rust).
• Cooking of food: Raw ingredients chemically transform into new substances.
• Ripening of fruit: Complex chemical reactions change the fruit's taste, color, and texture.

Q3: Differentiate between physical and chemical changes.

Answer:

Q4: Give reasons for the following:

a) Burning of paper is a chemical change.

Answer: Burning of paper is a chemical change because, during combustion, paper reacts with oxygen to form entirely new substances like ash, carbon dioxide, and water vapor. These new substances have different chemical compositions and properties than the original paper, and the change is permanent and irreversible.

b) Melting of wax is a physical change.

Answer: Melting of wax is a physical change because only the state of the wax changes from solid to liquid due to heat. Its chemical composition remains the same (it is still wax), and no new substance is formed. The molten wax can be solidified back into solid wax upon cooling, making it a reversible change.

c) Cooking of food is an irreversible change.

Answer: Cooking of food is an irreversible change because the application of heat causes chemical reactions within the food ingredients. This results in the formation of new substances with altered chemical compositions, textures, and tastes. These new substances cannot be easily converted back into their original raw forms.

d) Rusting of iron is a slow and undesirable change.

Answer: Rusting of iron is a slow change because the process of iron reacting with oxygen and moisture to form iron oxide (rust) takes a considerable amount of time, often days, weeks, or months. It is an undesirable (harmful) change because rust weakens the iron object, causes damage, and reduces its usability and aesthetic appeal.

References

1. Maharashtra State Board 7th Standard Science Textbook (Based on current curriculum for pages 88-91)
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