Class 7 chemistry important questions with answers are provided here for Chapter 6 – Physical and Chemical Changes. These important questions are based on the CBSE board curriculum and correspond to the most recent Class 7 chemistry syllabus. By practising these Class 7 important questions, students will be able to quickly review all of the ideas covered in the chapter and prepare for the Class 7 annual examinations.
Download Class 7 Chemistry Chapter 6 – Physical and Chemical Changes Important Questions with Answers PDF by clicking on the button below.
Recommended Video
Physical and Chemical Changes – Mindmap with Explanation
Class 7 Chapter 6 – Physical and Chemical Changes Important Questions with Answers
Multiple Choice Type Questions
Q1. Which of the following is an example of a chemical change?
(a) Formation of cloud
(b) Dropping sodium into water
(c) Glowing of an electric light
(d) All of the above
Answer: (b) Dropping sodium into water
Explanation: A chemical change occurs when two substances interact to form a new substance with different properties. The formation of clouds and the glowing of an electric light do not constitute a new substance. Thus, these are examples of physical change. In contrast, dropping sodium into water is an irreversible process which leads to sodium hydroxide and hydrogen gas formation. Thus, it is categorised as a chemical change.
Na + H2O → NaOH + H2
Hence, option (b) is correct.
Q2. Combustion is a ________ change.
(a) Chemical change
(b) Physical change
(c) Both (a) and (b)
(d) None of the above
Answer: (a) Chemical change
Explanation: Combustion leads to the formation of a new substance, and the reaction that involves the formation of a new substance is a chemical reaction, i.e. chemical change. Hence, option (a) is correct.
Q3. Which of the following is not a characteristic of a chemical change?
(a) Formation of new substances
(b) They are irreversible
(c) Chemical properties of new substances are the same
(d) None of the above
Answer: (c) Chemical properties of new substances are the same
Explanation: A chemical change occurs when two substances interact to form a new substance with different properties. It is irreversible. Hence, option (c) is correct.
Q4. Which of the following are the characteristics of a physical change?
(a) It is reversible
(b) It is temporary
(c) No new substance is formed
(d) All of the above
Answer: (d) All of the above
Explanation: A physical change occurs when the physical properties of the substance change, but no new substances are formed. It is temporary and reversible, i.e. we can recover it by just reversing the conditions. Hence, option (d) is correct.
Q5. Which of the following is an example of a physical change?
(a) Respiration (Inhaling oxygen gas and exhaling carbon dioxide gas)
(b) Evaporation of water
(c) Mixing vinegar and baking soda
(d) All of the above
Answer: (b) Evaporation of water
Explanation: A physical change occurs when the physical properties of the substance change, but no new substances are formed. It is temporary and reversible. In the evaporation of water, only the physical state of water changes from liquid to gas, but not the chemical composition of water. Thus, it is a physical change. Hence, option (b) is correct.
Q6. Which of the following is a permanent change and can not be recovered by just reversing the conditions?
(a) Chemical change
(b) Physical change
(c) Both (a) and (b)
(d) None of the above
Answer: (a) Chemical change
Explanation: A chemical change occurs when two substances interact to form a new substance with different properties. It is irreversible, i.e. we can not recover it by reversing the conditions. Hence, option (a) is correct.
True or False Type Questions
Q1. Cutting of log of wood into pieces is a chemical change.
Answer: False
Explanation: A chemical change occurs when two substances interact to form a new substance with different properties. Cutting of log of wood into pieces does not involve any new substance formation. Thus, it is not a chemical change.
Q2. The formation of manure from leaves is a physical change.
Answer: False
Explanation: A physical change occurs when the physical properties of the substance change, but no new substances are formed. The formation of manure from leaves involves the formation of a new substance. Thus, it is not a physical change.
Q3. Iron pipes coated with zinc do not get rusted easily.
Answer: True
Explanation: Iron pipes are coated with a layer of zinc to prevent contact with air or moisture. The zinc layer acts as a boundary between the iron and air or moisture and prevents it from getting rusted.
Q4. Condensation of steam is not a chemical change.
Answer: True
Explanation: A chemical change occurs when two substances interact to form a new substance with different properties. It is a permanent and irreversible process as during the condensation of steam, only the physical state of water changes from gas to liquid, but not the chemical composition of water. Thus, it is not a chemical change.
Q5. Rust can be crumbled off easily.
Answer: True
Explanation: Rust is an iron oxide formed when iron and oxygen react with water. The iron oxide layer is not firmly attached to the surface of the iron, so it crumbles off.
Fill in the Blanks Type Questions
Q1. Rust forms on ______.
Answer: Iron
Explanation: Rust is a reddish-brown flaking coating of hydrated iron oxide formed on iron by oxidation in the presence of water or moisture.
Q2. Digestion of food is a ______ change.
Answer: Chemical
Explanation: Digestion of food is a chemical process which comprises chewing, saliva action on food, the action of acid in the stomach on food, and the breakdown of food particles.
Q3. The churning of milk to get butter is a ______ change.
Answer: Physical
Explanation: When milk cream is churned, we get butter. The fat droplets in milk cream bind together to form fat clusters (butter). No new substance is formed during butter formation. Hence, it is classified as a physical change.
Q4. The glowing of an electric bulb is a _____ change.
Answer: Physical
Explanation: When electricity is passed through the bulb, the electrons get excited and release energy in the form of light. No new substance is formed during the glowing of an electric bulb. Hence, it is classified as a physical change.
Q5. When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.
Answer: Calcium carbonate.
Explanation: Carbon dioxide reacts with limewater (Ca(OH)2) to form a white precipitate of calcium carbonate (CaCO3). Thus, it turns milky.
CO2 + Ca(OH)2 → CaCO3
However, the milkiness goes away if an excess of carbon dioxide is passed through lime water. It is due to the formation of colourless calcium bicarbonate, which is soluble in water.
CO2 + Ca(OH)2 → CaCO3 → Ca(HCO3)2
Match the Following Type Questions
Q1. Match the items of column A with those of column B.
| Column A | Column B |
| Physical change | Blue |
| Chemical change | Green |
| Burning of magnesium | Physical properties are changed |
| Iron sulphate | Magnesium oxide |
| Copper sulphate | New substances are produced |
Answer:
| Column A | Column B |
| Physical change | Physical properties are changed |
| Chemical change | New substances are produced |
| Burning of magnesium | Magnesium oxide |
| Iron sulphate | Green |
| Copper sulphate | Blue |
Q2. Match the items of column A with those of column B.
| Column A | Column B |
| Burning of paper | Physical change |
| The ringing of an electric bell | Chemical change |
| Curdling of milk | Physical change |
| The electric light is switched on | Chemical change |
| Melting of butter | Physical change |
Answer:
| Column A | Column B |
| Burning of paper | Chemical change |
| The ringing of an electric bell | Physical change |
| Curdling of milk | Chemical change |
| The electric light is switched on | Physical change |
| Melting of butter | Physical change |
Very Short Answer Type Questions
Q1. What is rust?
Answer: The brownish deposit on an iron material left open for some time is called rust
Q2. Name the product formed when a piece of magnesium is burnt in the air.
Answer: Magnesium oxide is formed when a piece of magnesium is burnt in the air.
Q3. What is galvanisation?
Answer: Galvanisation refers to the process of depositing zinc on iron articles. It is used to prevent rusting of iron articles.
Q4. What is crystallisation?
Answer: Crystallisation refers to the process by which an impure compound is converted into a crystal.
Q5. What are the molecular formula and chemical name of rust?
Answer: The molecular formula and chemical name of rust are Fe2O3 and ferric oxide, respectively.
Q6. What is a physical change?
Answer: A physical change occurs when the physical properties of the substance change, but no new substances are formed.
Q7. What is a chemical change?
Answer: A chemical change occurs when two substances interact to produce one or more new substances with different properties.
Q8. Mention one way of preventing rusting of iron.
Answer: Galvanisation can be used to prevent rusting of iron.
Q9. Mention some examples of chemical changes.
Answer: Rusting of iron, fermentation of food, photosynthesis, burning of a candle and digestion of food are some examples of chemical changes.
Q10. How does painting an iron grill prevent it from rusting?
Answer: Painting an iron grill prevents the contact of iron with air and moisture, so there is no interaction. Thus, painting an iron grill prevents it from rusting.
Q11. Write the chemical equation for rusting of iron.
Answer: The chemical equation for rusting iron is
Iron + Oxygen + Water → Rust (Iron Oxide).
4 Fe + 3 O2 + x.H2O → 2 Fe2O3.
Q12. Name a metal other than zinc coated on iron to prevent rusting.
Answer: Chromium is coated on iron to prevent rusting.
Q13. Name a method by which you can obtain a highly pure sample of salt.
Answer: Crystallisation can be used to obtain a highly pure sample of salt.
Short Answer Type Questions
Q1. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What kind of change is it? Explain.
Answer: When baking soda is mixed with lemon juice, Carbon dioxide (CO2) gas is released in the form of bubbles. The chemical composition of the substance changes and a new product is formed after mixing. Thus, it is a chemical change.
Q2. Why do stainless steel utensils not rust?
Answer: Stainless steel is an alloy with a minimum chromium content of 10.5%. The chromium reacts with the oxygen in the air and forms a protective layer that makes stainless steel highly resistant to corrosion and rust.
Q3. Why is fermented food warmer than ordinary food?
Answer: Fermented food is warmer than everyday food because a chemical reaction occurs during fermentation leading to the elimination of carbon dioxide and heat.
Q4. Name the two conditions that are essential for rusting.
Answer: The two conditions necessary for rusting are:
1. Oxygen
2. Water or water vapour
Q6. Why is crystallisation regarded as a physical change?
Answer: Crystallisation is a physical change as it does not result in the formation of a new substance. Moreover, the change is reversible because crystals formed in the process can be reversed back to non-crystalline form by dissolving it in water.
Q6. How would you show that curdling of milk is a chemical change?
Answer: Curdling of milk is a chemical change because a new substance, i.e. lactic acid, is formed. The curd has a different taste than the milk. Also, once the curd is formed, we can not restore milk from it.
Q7. Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Answer: The burning of wood is a chemical change as after burning it, wood changes into ash, and we can not recover it from ash. In contrast, cutting wood into small pieces is a physical change as it involves a difference in the size of the wood.
Q8. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: Rusting of iron objects occurs if iron comes in contact with moist air. It is faster in coastal areas than in deserts because air contains a high percentage of moisture in coastal areas. In contrast, the air is dry and hot in the deserts. Therefore rusting is more prominent in coastal provinces than in deserts.
Long Answer Type Questions
Q1. What is a chemical change? List some characteristics of chemical changes.
Answer: A chemical change occurs when two substances interact to produce one or more new substances with different properties.
Some of the characteristics of chemical changes are mentioned below.
1. Formation of new substance
2. Changes in the composition of the substance
3. Precipitate (formation of residue)
4. Evolution of a gas (formation of bubbles)
5. Irreversible, i.e. we can not recover it by reversing the conditions.
6. Absorption or evolution of heat and light energy
7. Change of colour
8. Change of smell
9. Production of sound
Q2. What is a physical change? List some characteristics of physical changes.
Answer: A physical change occurs when the physical properties of the substance change, but no new substances are formed.
Some of the characteristics of physical changes are mentioned below.
1. The physical properties of a substance change during a physical change.
For example, when an ice cube melts, water is formed. There is a change in the state of water from solid to liquid.
2. No new substances are formed, and the nature of the substance remains the same during the change.
For example, when an ice cube melts, water is formed. No new substances are formed during the process, and the nature of the substance remains the same.
3. It is temporary and generally reversible though some changes may be irreversible.
For example, when water is heated, water vapours are formed. Once water vapours are cooled, we can obtain water again.
4. Only a small amount of heat is absorbed or given off during the change.
Q3. What is rusting? How can it be prevented?
Answer: Rusting refers to depositing reddish brown material on the iron articles. The formation damages or destroys the iron material.
We can prevent rusting of iron in the following ways:
1. Painting, oiling, greasing, or varnishing its surface.
2. Coating iron with non-corrosive substances like carbon. This process is known as alloying.
3. Galvanisation is another method of protecting iron from rusting by coating iron with a thin layer of zinc.
Q4. Distinguish between physical and chemical changes.
Answer: We can distinguish between physical and chemical changes in the following ways.
| Sl. No. | Physical Change | Chemical Change |
| 1. | A physical change occurs when the physical properties of the substance change, but no new substances are formed. | A chemical change occurs when two substances interact to produce one or more new
substances with different properties. |
| 2. | No new substance is formed. | A new substance is formed. |
| 3. | It is a temporary change. | It is a permanent change. |
| 4. | It affects only physical properties, i.e. shape, size, etc. | It affects the physical and chemical properties of the substance, including its composition. |
| 5. | Minor to no absorption and energy evolution occur. | Absorption and energy evolution occur. |
| 6. | It is reversible, i.e. we can recover the original substance. | It is irreversible, i.e. we can not recover it by reversing the conditions. |
Q5. Classify the following into physical or chemical changes.
(a) Magnetisation of an iron piece
(b) Dissolution of salt in water
(c) Expansion and contraction of metals
Answer: (a) Magnetisation of an iron piece is a physical change as it is a temporary change.
(b) The dissolution of salt in water is the method of dissolving salt in water. It is a physical change as no new substance is formed, and the salt can be recovered by water evaporation.
(c) The expansion and contraction of metal on heating and cooling are physical changes because the expansion and contraction do not cause any chemical changes in metal. Also, contraction and expansion do not produce other elements with different chemical properties.
Q6. What happens when an iron nail is dipped in the copper sulphate solution?
Answer: When an iron nail is immersed in copper sulphate, iron displaces copper from the copper sulphate solution, as iron is more reactive than copper. Therefore, the colour of the copper sulphate solution changes from blue to pale green.
Reaction
CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu (s)
In this reaction, the following two processes take place:
Cu²⁺ + 2 e– = Cu⁰ (reduction process, Cu²⁺ is the oxidising agent)
Fe⁰ – 2 e– = Fe²⁺ (oxidation process, Fe⁰ is the reducing agent)
It is a double displacement and redox reaction (as oxidation and reduction coincide).
Explanation
When an iron nail is dipped in copper sulphate solution, a brown coating of copper is formed on the iron surface, and the copper sulphate solution’s colour changes from blue to pale green. The iron passes into the solution as ferrous, forming the ferrous sulphate solution. The reaction shows iron is more reactive than copper because it displaces copper from the copper sulphate solution.
Q7. What happens if the gas produced by the reaction between vinegar and baking soda is passed through lime water? Justify your answer.
Answer: The reaction between baking soda (sodium bicarbonate) and vinegar (dilute acetic acid) generates carbon dioxide gas and sodium acetate.
NaHCO3 + CH3COOH → CH3COONa + H2O + CO2
When carbon dioxide is passed into the lime water, a white precipitate of calcium carbonate (CaCO3) is formed, making it milky.
CO2 + Ca(OH)2 → CaCO3
However, the milkiness goes away if an excess of carbon dioxide is passed through lime water. It is due to the formation of colourless calcium bicarbonate, which is soluble in water.
CO2 + Ca(OH)2 → CaCO3 → Ca(HCO3)2
Read Also:
- Chemical and Physical Change
- Difference between Physical and Chemical Change
- Chemical and Physical Change Examples
- Physical Change
- Physical Change and Chemical Change
- Chemistry Worksheets Class 7 on Chapter 6 Physical and Chemical Changes with Answers – Set 1
- Chemistry Worksheets Class 7 on Chapter 6 Physical and Chemical Changes with Answers – Set 2
- Chemistry Worksheets Class 7 on Chapter 6 Physical and Chemical Changes with Answers – Set 3
- Chemistry Concept Questions and Answers
- Class 7 Chemistry Chapter 6 Physical and Chemical Changes MCQs
What is Physical and Chemical Change | Physical & Chemical Changes Around Us
What are Physical & Chemical Changes & Its Characteristics
Reversible and Irreversible Changes
