Access solutions for Class 7 Science Physical and Chemical Changes. Clear doubts and understand concepts with step-by-step answers and explanations.
Class 7 Science Physical and Chemical Changes: NCERT Textbook Solutions
Get detailed BOSEM and NCERT solutions for the Class 7 Science chapter on Physical and Chemical Changes. Clear your doubts and understand key concepts easily with these step-by-step answers.
Q1. Classify the changes involved in the following processes as physical or chemical changes:
a). Photosynthesis
b). Dissolving sugar in water
c). Burning of coal.
d) Melting of wax
e). Beating aluminium to make aluminium foil.
f). Digestion of food.
Answer:
| Processes | Type of Change |
|---|---|
| Photosynthesis | Chemical change |
| Dissolving Sugar in water | Physical change |
| Burning of Coal | Chemical Change |
| Melting of wax | Physical Change |
| Beating aluminium to make aluminium foil | Physical change |
| Digestion of food | Chemical change |
Q2. State whether the following statements are true of false. In case a statement is false, write the corrected statement in your notebook.
Answer:
a. Cutting of log of wood into pieces is a chemical change. (False)
b. Formation of manure from leaves is a physical change. (False)
c. Iron pipes coated with zinc do not get rusted easily. (True)
d. Iron and rust are the same substance. (False)
e. Condensation of steam is not a chemical change. (True)
Q3. Fill in the blanks in the following statements:
Answer:
a). When carbon dioxide is passed through lime water, it turns milky due to the
formation of Calcium Carbonate.
b). The chemical name of baking soda is Sodium bicarbonate.
c). Two methods by which rusting of iron can be prevented are painting and galvanisation.
d). Changes in which only physical properties of a substance change are called physical changes.
e). Changes in which new substances are formed are called Chemical changes.
Q4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of changes is it? Explain.
Answer: It is a chemical change. Because mixing baking soda with lime juice produces carbon dioxide gas and other substances which is a completely new substance.
Q5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Answer: The burning of wax is both physical and chemical changes. Melting of wax is a physical change while burning of wax to give carbon dioxide, heat and light is a chemical change.
Another familiar example is cooking gas LPG (Liquefied petroleum gas). Inside the cylinder it exists in a liquid state when comes out, it becomes a gas. This is a physical change. Burning of gas into flame gives out carbon dioxide, heat and light which is a chemical change.
Q6. How would you show that the setting of curd is a chemical change?
Answer: We can say the setting of curd is a chemical change because a new substance is formed with a different taste, smell etc., which is different from the original milk.
Q7. Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Answer: Burning of wood is a chemical change because it forms new substances like charcoal, smoke, ashes etc. while cutting wood into pieces is a physical change because it does not form a new substance. It only changes its shape and size.
Q8. Describe how crystals of copper sulphate are prepared.
Answer: A cupful of water is taken in a beaker and a few drops of sulphuric acid are added to it and start heating. When it starts, boiling, copper sulphate powder is added slowly while stirring continuously till no powder can be dissolved. After that, the solution is filtered and let it cooled. Thus crystal of copper sulphate is prepared.
Q9. Explain how the painting of an iron gate prevents it from rusting.
Answer: Painting prevents iron gates from direct contact with the oxygen present in moist air. As for the rusting of iron, the presence of oxygen and moisture is essential.
Q10. Explain why rusting of iron objects is faster in coastal than in deserts.
Answer: The moisture content in the air is higher in coastal areas than that in deserts. That is why the rusting of iron objects is faster in coastal areas than in deserts.
Q11. The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder, it exits as a liquid.
- When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B)
The following statements pertain to these changes. Choose the correct one.
i. Process – A is a chemical change
ii. Process – B is a chemical change.
iii. Both processes A and B are chemical changes.
iv. None of these processes is a chemical change.
Answer: ii. Process – B is a chemical change.
Q12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
i. Process – A is a chemical change
ii. Process – B is a chemical change.
iii. Both processes A and B are chemical changes.
iv. None of these processes is a chemical change.
Answer: iii. Both processes A and B are chemical changes.
Class 7 Science Physical and chemical changes Extra Questions and Answers
Q1. What is physical change?
Answer: Physical change is a change in which a substance changes in shape, size, colour and state without any change in its properties of substance.
Eg. Cutting wood into pieces.
Q2. Are all physical changes reversible? Give one example and explain.
Answer: No, all physical changes are not reversible.
Example:
Cutting of wood log into pieces is a physical change as it does not change its property as wood but we cannot get the original wood log. It is irreversible.
But in melting ice into water, we can get back the ice by putting the water into the refrigerator. It is reversible.
Q3. What is Chemical change? Give examples.
Answer: Chemical change is a change in which a substance changes its properties of a substance and forms a completely new substance.
Example: Formation of curd from milk, ripening of fruit, photosynthesis, digestion of food etc.
Q4. Give one example of a chemical change that produces more than one substance.
Answer: Explosion of a firecracker.
In the explosion of a firecracker, light, heat, sound and unpleasant gases are produces that pollute the air.
Q5. Write three points of difference between physical and chemical change.
Answer:
| Physical change | Chemical change |
|---|---|
| It is a change in shape, size, colour or the state of a substance | It is a change in the properties of a substance and forms a completely new substance. |
| No new substance is formed | One or more new substances are formed |
| It may be reversible or irreversible | It is irreversible |
Q6. What product is given out when magnesium ribbon is burnt in the presence of air?
Answer: When magnesium ribbon is burnt in the presence of air, it gives out magnesium oxide in the form of ash. The following equation can represent the change:
Magnesium(Mg) + Oxygen (O) → Magnesium Oxide (MgO)Q7. Write down the equation for the reaction of Copper sulphate solution and iron.
Answer: The reaction can be represented by the following equation:
Copper sulphate solution(Blue)+Iron → Iron Sulphate solution + Copper(Brown)CuSO4 (aq)+Fe (s) → FeSO4 (aq)+Cu (s)
Q8. Name the substance deposited on the iron after dipping the iron nail in a copper sulphate solution.
Answer: The brown colour deposited on the iron nail is copper.
Q9. What is rust?
Answer: The brownish layer deposited on an iron object after exposure to moist air is called rust.
Rust forms when iron reacts with water and oxygen from the air. This reaction creates a reddish-brown substance called rust, which is a type of iron oxide.
Iron (Fe)+Water (H₂O)+Oxygen (O₂) → Rust (Fe₂O₃.H₂O)Q10. What is rusting?
Answer: The process of depositing a brownish colour layer called rust on an iron object after exposure to moisture for a long time is called rusting.
Q11. What are the essential components of rusting?
Answer: The essential components of rusting are
- Oxygen
- Water or water vapour.
Q12. How can you prevent rusting?
Answer: We can prevent the rusting of iron objects by preventing direct contact with Oxygen or water or both with iron objects.
Another way to prevent rusting is by depositing a layer of metal like Chromium or Zinc on it.
Q13. Why stainless steel does not rust?
Answer: Stainless steel does not get rust because stainless steel is made by mixing iron with carbon and metal like chromium, nickel and manganese.
Q14. Write some advantages and disadvantages of chemical change in our daily life.
Ans: Some advantages of chemical change are
- Digestion of food
- Photosynthesis
- Decomposition of waste to form manure.
- Ripening of fruits etc.
Some disadvantages of chemical change are
- Chemical change spoils food
- Global warming
- Rusting of metal objects
- Formation of acid rain etc.
Q15. What is galvanisation?
Ans: Galvanisation is a process of depositing a layer of Zinc on iron.
Q16. What is Crystallization? Give one example.
Ans: Crystallization is a process by which a solid is formed from a solution.
Example: Formation of salt crystal from seawater.
Class 7 Science Physical And Chemical Change Multiple choice questions (MCQs)
Test your knowledge of Physical and Chemical Changes with these multiple-choice questions (MCQs)
Multiple Choice Questions
- Which of the following is a physical change?
a) Burning of wood
b) Rusting of iron
c) Melting of ice
d) Digestion of food
Answer: c) Melting of ice
Reason: A physical change is a change in the state or appearance of a substance without changing its chemical composition. Melting ice changes from solid to liquid but remains H₂O.
- Which of the following is a chemical change?
a) Tearing of paper
b) Dissolving sugar in water
c) Cooking of food
d) Cutting of a tree
Answer: c) Cooking of food
Reason: Cooking of food involves chemical reactions that produce new substances (e.g., proteins in food being altered by heat).
- Which of the following occurs during a chemical change?
a) Change in shape
b) Change in colour
c) Change in size
d) Change in texture
Answer: b) Change in colour
Reason: Chemical changes often result in a change of colour due to the formation of new substances.
- Rusting of iron is an example of:
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) None of the above
Answer: b) Chemical change
Reason: Rusting is a chemical change because iron reacts with oxygen and water to form iron oxide, a new substance.
- Which of the following is a characteristic of a physical change?
a) New substances are formed
b) It is a permanent change
c) It is reversible
d) Energy is absorbed or released
Answer: c) It is reversible
Reason: Physical changes are typically reversible, like water freezing into ice and then melting back to water.
- Which of the following is not a sign of a chemical change?
a) Evolution of gas
b) Change in temperature
c) Change in appearance
d) Change in shape
Answer: d) Change in shape
Reason: While a change in shape can occur in both physical and chemical changes, it does not necessarily indicate a chemical change.
- Which of these changes is an example of a physical change?
a) Burning of paper
b) Dissolving salt in water
c) Cooking an egg
d) Souring of milk
Answer: b) Dissolving salt in water
Reason: Dissolving salt in water is a physical change because no new substance is formed, and the salt can be recovered by evaporating the water.
- The process of cutting a fruit is a:
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) None of the above
Answer: a) Physical change
Reason: Cutting fruit is a physical change because it only changes the shape and size without altering the chemical composition.
- When sugar is heated to form caramel, it undergoes:
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) No change
Answer: b) Chemical change
Reason: Heating sugar to form caramel results in a chemical change because new substances (caramel) are formed.
- Which of the following is not a chemical change?
a) Cooking
b) Burning
c) Melting
d) Fermentation
Answer: c) Melting
Reason: Melting is a physical change because the substance only changes its state from solid to liquid without changing its chemical composition.
- Which of the following is a reversible change?
a) Burning of paper
b) Melting of butter
c) Rusting of iron
d) Cooking of an egg
Answer: b) Melting of butter
Reason: The melting of butter is a reversible physical change, as it can be solidified again by cooling.
- Which of the following is a chemical change that can produce a new gas?
a) Dissolving sugar in water
b) Rusting of iron
c) Burning of wood
d) Freezing of water
Answer: c) Burning of wood
Reason: Burning wood results in the formation of new gases (like carbon dioxide and water vapor) and is a chemical change.
- Which of the following is an example of a chemical change?
a) Boiling of water
b) Freezing of water
c) Burning of a candle
d) Melting of wax
Answer: c) Burning of a candle
Reason: Burning a candle is a chemical change because it involves the combustion of wax, producing heat, light, and new substances like carbon dioxide.
- What happens when a piece of paper is torn?
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) No change
Answer: a) Physical change
Reason: Tearing paper changes its size and shape, but it does not alter its chemical composition.
- Which of the following is a chemical change that occurs in food?
a) Roasting of vegetables
b) Melting of cheese
c) Cutting of fruit
d) Boiling of water
Answer: a) Roasting of vegetables
Reason: Roasting vegetables involves chemical changes, such as caramelization and the breakdown of certain nutrients.
- Which of the following is an example of a physical change?
a) Boiling water to make steam
b) Frying an egg
c) Burning a piece of wood
d) Baking a cake
Answer: a) Boiling water to make steam
Reason: Boiling water is a physical change since it only changes from liquid to gas without any change in the chemical structure of the substance.
- What type of change occurs when a candle is lit?
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) No change
Answer: c) Both physical and chemical change
Reason: The wax melts (physical change), and combustion occurs (chemical change) when the candle is lit.
- Which of the following is an example of a chemical reaction?
a) Ice turning into water
b) Water evaporating
c) Vinegar reacting with baking soda
d) Crushing a stone
Answer: c) Vinegar reacting with baking soda
Reason: The reaction between vinegar and baking soda produces new substances (carbon dioxide gas and other products), indicating a chemical change.
- When a piece of wood burns, which change takes place?
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) No change
Answer: b) Chemical change
Reason: Burning wood is a chemical change because it involves combustion and the formation of new substances, like ash and gases.
- Which of the following is a physical property of a substance?
a) Reactivity with oxygen
b) Flammability
c) Density
d) Ability to rust
Answer: c) Density
Reason: Density is a physical property because it can be measured without changing the chemical identity of the substance.
- Which of the following is not a physical property?
a) Color
b) Hardness
c) Taste
d) Reactivity with water
Answer: d) Reactivity with water
Reason: Reactivity with water is a chemical property, not a physical one, as it involves a chemical reaction.
- Which of the following is a common sign of a chemical change?
a) Change in temperature
b) Change in shape
c) Change in texture
d) Change in size
Answer: a) Change in temperature
Reason: A temperature change, such as heating or cooling, often indicates a chemical reaction is taking place.
- Which of the following is an irreversible change?
a) Freezing water
b) Dissolving salt in water
c) Burning paper
d) Melting wax
Answer: c) Burning paper
Reason: Burning paper is an irreversible chemical change because it produces new substances that cannot be undone.
- The process of photosynthesis is a:
a) Physical change
b) Chemical change
c) Both physical and chemical change
d) No change
Answer: b) Chemical change
Reason: Photosynthesis is a chemical change where plants convert carbon dioxide and water into glucose and oxygen using sunlight.
