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DAV Class – 7 Science Chapter – 3 Solutions

DAV Class – 7 Science Chapter – 3 Solutions: DAV class 7 Science Book Chapter 3 Chemical Substances and Processes Question Answer is given below. Here class 7 Science Question Answer is provided with great explanation.

DAV Class – 7 Science Chapter – 3 Solutions


A. Fill in the blanks.

1. Materials made up of same kind of particles are called ___________.

2. The type and number of particles of each kind present in a substance is given by its ___________.

3. The chemical formula of water is ___________.

4. Iron gets rusted on coming in contact with ___________ and ___________.

5. The process that leads to a chemical change is called a ___________.

6. In a neutralisation reaction, ___________ and ___________ are formed.

Answer: (1) pure substances (2) chemical formula (3) H2O (4) oxygen, moisture (5) chemical reaction (6) salt, water

B. Write True or False for the following statements.

1. All matter is made up of atoms.

2. Compounds are substances consisting of two or more elements chemically combined in a fixed ratio.

3. The symbol of element copper is Cu.

4. Formula of sodium chloride is written as ClNa.

5. The chemical formula of aluminium sulphate is Al2SO4.

6. New substance formed in a chemical reaction is called product.

7. Magnesium hydroxide is an acid.

Answer: (1) True (2) True (3) True (4) False (5) False (6) True (7) False

C. Tick the correct option.


1. silver

2. one atom of hydrogen, one atom of nitrogen, and three atoms of oxygen.

3. Mg3(PO4)2

4. displacement reaction

5. CaO

D. Answer the following questions in brief.

1. What are elements?

Answer: The substances which are made up of same kind of atoms are called elements.

2. Give the chemical symbol of iron and chlorine.

Answer: Iron – Fe and chlorine – Cl

3. Write the chemical formulae of the following compounds:

(a) Aluminium oxide (b) Zinc acetate


(a) Al2O3

(b) (CH3COO)2Zn

4. Balance the following equation:

Ca(OH)2 + HCl → CaCl2 + H2O

Answer: Ca(OH)2 + 2HCl → CaCl2 + 2H2O

5. What does a chemical equation represent?

Answer: chemical equation represents a chemical reaction and formulae of substances involved in the reaction.

6. What are reactants and products in a chemical equation?

DAV Class - 7 Science Chapter - 3 Solutions

7. Give one example of a combination reaction.

Answer: Combination of carbon and oxygen gas to form carbon dioxide is an example of a combination reaction.

C + O2 → CO2

E. Answer the following questions.

1. Write the steps involved in writing the chemical formula of calcium phosphate.


To write the formula of calcium phosphate, we first write the symbols for calcium and phosphate ions along with their charges. 

                            Ca²+ (calcium), (PO4)³ (phosphate)

There are no common factors in the number corresponding to their charges. We now crisscross their charges and write the crisscrossed numbers as subscripts against the two charges.

                              Ca2+ X (PO4)3-  →  Ca3(PO4)2

2. How is a chemical change different from a physical change?


DAV Class - 7 Science Chapter - 3 Solutions

3. Neutralisation reaction is a chemical change. Justify this statement with the help of an example.


In neutralization reaction, acid reacts with a base to form salt and water i.e., new substances are formed. So, it is a chemical change. For example, hydrochloric acid and sodium hydroxide reacts to form sodium chloride (salt) and water.

                              NaOH + HCl → NaCl + H2O

4. Define a decomposition reaction and give an example of the same.


A reaction in which more than one product is obtained from a single reactant, is called a decomposition reaction.

For example: the breakdown of water into hydrogen and oxygen.

                                         2H2O → 2H2 + O2

5. Classify the following reactions into different types, giving reason.


(a) It is a Combination reaction because two substances combine to form single product.

(b) It is a neutralization reaction because a base (potassium hydroxide) reacts with an acid (hydrochloric acid) to form salt (potassium chloride) and water.

(c) It is a displacement reaction because copper displaces silver from silver nitrate solution to form copper nitrate solution.

(d) It is a decomposition reaction because barium carbonate decomposes to form barium oxide and carbon dioxide.

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