Physical and Chemical Changes Class 7 NCERT Solutions For Science Chapter 6

Physical and Chemical Changes Class 7 NCERT Solutions

SimplyAcad has prepared the NCERT Solutions for Class 7 Science Chapter “Physical and Chemical Change” to aid students in their studies. The solutions provided offer deep insights into the theories and ideas discussed in the chapter. To excel in their exams, students should prepare answers and solve all the questions given in the NCERT textbook. Our subject experts have compiled all the crucial details related to the chapter to enhance the learner’s performance. Scroll below to access all the solutions explained in detail.

NCERT Solutions For Class 7 Physical And Chemical Change

Question 1: Classification of Changes

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:

(a) Photosynthesis → Chemical change
(b) Dissolving sugar in water → Physical change
(c) Burning of coal → Chemical change
(d) Melting of wax → Physical change
(e) Beating aluminium to make aluminium foil → Physical change
(f) Digestion of food → Chemical change

---

Question 2: True or False Statements

State whether the following statements are true or false. In case a statement is false, write the corrected statement.

(a) Cutting a log of wood into pieces is a chemical change. (True/False)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)

Answer:

(a) False
Cutting a log of wood into pieces is a physical change.
(b) False
Formation of manure from leaves is a chemical change.
(c) True
(d) False
Iron and rust are different substances. Rust is iron oxide.
(e) True

---

Question 3: Fill in the Blanks

Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of ________.
(b) The chemical name of baking soda is ________.
(c) Two methods by which rusting of iron can be prevented are ________ and ________.
(d) Changes in which only ________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called ________ changes.

Answer:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate (CaCO3).
(b) The chemical name of baking soda is sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are galvanization and painting.
(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.

---

Question 4: Chemical Reaction with Baking Soda

When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Answer:

When baking soda (sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change.

The citric acid in lemon juice reacts with sodium hydrogen carbonate, resulting in the formation of carbon dioxide gas.

Chemical equation:
Lemon juice (citric acid) + Baking soda (sodium hydrogen carbonate) → Carbon dioxide + Other substances

---

Question 5: Candle Burning

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both chemical and physical changes take place.

Answer:

When a candle burns:

• Physical change: Melting of wax
• Chemical change: Burning of wax

Another example where both physical and chemical changes occur simultaneously is eating food:

• Physical change: Breaking down of larger food particles into smaller particles
• Chemical change: Digestion of food

---

Question 6: Setting of Curd

How would you show that the setting of curd is a chemical change?

Answer:

Once curd is formed, milk cannot be re-obtained from it, and milk and curd have different properties. These characteristics indicate a chemical change.

---

Question 7: Wood Changes

Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer:

• Burning of wood: A new substance (coal) is formed, indicating a chemical change.
• Cutting wood: Only the shape and size of the wood change, with no new substance formed, indicating a physical change.

---

Question 8: Preparing Copper Sulphate Crystals

Describe how crystals of copper sulphate are prepared.

Answer:

Crystals of copper sulphate are prepared by the method of crystallization:

1. Take a cupful of water in a beaker.
2. Add a few drops of dilute sulphuric acid.
3. Heat the water until it starts boiling.
4. Add copper sulphate powder while stirring until the solution becomes saturated.
5. Filter the solution into a china dish and allow it to cool undisturbed.
6. Slowly, crystals of copper sulphate will separate out.

---

Question 9: Preventing Rusting

Explain how painting of an iron gate prevents it from rusting.

Answer:

Painting an iron gate prevents contact with air and moisture, which are necessary for rusting. Hence, rusting is prevented.

---

Question 10: Rusting in Coastal Areas

Explain why rusting of iron objects is faster in coastal areas than in deserts.

Answer:

Rusting requires both air (oxygen) and moisture. Coastal areas have higher moisture content in the air compared to desert areas. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

---

Question 11: LPG Changes

The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder, it becomes a gas (Change − A) and 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.

---

Question 12: Biogas Production

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.