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DAV Class- 7 Chemical Substances and Processes Question and Answer
Something To Know
A. Fill in the blanks.
1. Materials made up of same kind of particles are called Pure substances.
2. The type and number of particles of each kind present in a substance is given by its Chemical Formula.
3. The chemical formula of water is H2O .
4. Iron gets rusted on coming in contact with oxygen and moisture.
5. The process that leads to a chemical change is called a chemical reaction.
6. In a neutralisation reaction, salt and water are formed.
B. Write True or False for the following statements.
Ans:
| 1. All matter is made up of atoms. | True |
| 2. Compounds are substances consisting of two or more elements chemically combined in a fixed ratio. | True |
| 3. The symbol of element copper is Cu. | True |
| 4. Formula of sodium chloride is written as ClNa. | False |
| 5. The chemical formula of aluminium sulphate is Al2SO4. | False |
| 6. New substance formed in a chemical reaction is called product. | True |
| 7. Magnesium hydroxide is an acid. | False |
C. Tick the correct option.
1. Chemical Symbol of Ag represents the element
Ans 1: silver
2. One molecule of nitric acid is made up of
Ans 2: one atom of hydrogen, one atom of nitrogen, and three atoms of oxygen.
3. The chemical formula of Magnesium Phosphate is
Ans 3: Mg3(PO4)2
4. The following reaction is an example of a
Ans 4: Displacement reaction
5. Chemical formula of Quicklime is
Ans 5: CaO
D. Answer the following questions in brief.
1. What are elements?
Ans 1: The substances which are made up of same kind of atoms are called elements.
2. Give the chemical symbol of iron and chlorine.
Ans 2: Iron – Fe and Chlorine – Cl
3. Write the chemical formulae of the following compounds:
(a) Aluminium oxide (b) Zinc acetate
Ans 3:
(a) Aluminium oxide – Al2O3
(b) Zinc acetate -(CH3COO)2Zn
4. Balance the following equation:
Ca(OH)2 + HCl → CaCl2 + H2O
Ans 4: Ca(OH)2 + 2HCl → CaCl2 + 2H2O
5. What does a chemical equation represent?
Ans 5: chemical equation represents a chemical reaction and formulae of substances involved in the reaction.
6. What are reactants and products in a chemical equation?
Ans 6: In a chemical equation, the substances present on the left side of the arrow sign are called reactants and the substances present in right side of the arrow sign are called products.
7. Give one example of a combination reaction.
Ans 7: The combination of Hydrogen and oxygen gas to form water is an example of a combination reaction.
2H2 + O2 → 2H2O(g)
E. Answer the following questions.
1. Write the steps involved in writing the chemical formula of calcium phosphate.
Ans 1: We know that,
Calcium :
The chemical formula of calcium = Ca
The valence of calcium = 2
Phosphate :
The chemical formula of phosphate = PO₄
Valence of phosphate = 3
We need to write the chemical formula of calcium phosphate Using interchanging valency formula
Hence, The chemical formula of calcium phosphate is 
2. How is a chemical change different from a physical change?
Ans 2: The difference between a physical reaction and a chemical reaction is composition. In a chemical reaction, there is a change in the composition of the substances in question; in a physical change, there is a difference in the appearance, smell, or simple display of a sample of matter without a change in composition.
3. Neutralisation reaction is a chemical change. Justify this statement with the help of an example.
Ans 3: In a neutralization reaction, the 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 react to form sodium chloride (salt) and water.
NaOH + HCl → NaCl + H2O
4. Define a decomposition reaction and give an example of the same.
Ans 4: 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 gas, on passing an electric current is an example of a decomposition reaction.
2H2O(l) → 2H2 (g)+ O2 (g)
5. Classify the following reactions into different types, giving a reason.
Ans 5: (a) It is an example of a Combination reaction because two substances combine to form a single product.
(b) It is an example of a neutralization reaction because a base (potassium hydroxide) reacts with an acid (hydrochloric acid) to form a salt (potassium chloride) and water.
(c) It is an example of a displacement reaction because copper displaces silver from silver nitrate solution to form copper nitrate solution.
(d) It is an example of a decomposition reaction because barium carbonate decomposes to form barium oxide and carbon dioxide.
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