DAV Class 7 Science Chapter 3 Question Answer – Chemical Substances and Processes

The DAV Class 7 Science Book Solutions Pdf and DAV Class 7 Science Chapter 3 Question Answer – Chemical Substances and Processes are essential study tools for DAV public school students in Class 7.

DAV Class 7 Science Ch 3 Question Answer – Chemical Substances and Processes

DAV Class 7 Science Ch 3 Solutions – Chemical Substances and Processes

Something To Know

A. Fill in the blanks.

Question 1.
Materials made up of same kind of particles are called ________
Answer:
Pure substance

Question 2.
The type and number of particles of each kind present in a substance is given by its ________
Answer:
Chemical formula

Question 3.
The chemical formula of water is ________
Answer:
H₂O

Question 4.
Iron gets rusted on coming in contact with ________and ________
Answer:
moisture, air

Question 5.
The process that leads to a chemical change is called a ________
Answer:
Chemical reaction

Question 6.
In a neutralization reaction, ________ and ________ are formed.
Answer:
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 fix ratio.
3. The symbol of element copper is Cu.
4. Formula of sodium chloride is written as CINa.
5. The chemical formula of aluminium sulphate is Al₂S0₄.
6. New substance formed in a chemical reaction is called product.
7. Magnesium hydroxide is an acid.
Answer:
1. All matter is made up of atoms. (True)
2. Compounds are substances consisting of two or more elements chemically combined in a fix ratio. (True)
3. The symbol of element copper is Cu. (True)
4. Formula of sodium chloride is written as CINa. (False)
5. The chemical formula of aluminium sulphate is Al₂S0₄. (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. The chemical symbol Ag represents the element—
(a) Sodium
(b) Aluminium
(c) Silver
(d) Sulphur
Answer:
(c) Silver

2. One molecule of nitric acid is made up of
(a) Two atoms of hydrogen, two atoms of nitrogen and two atoms of oxygen.
(b) One atom of hydrogen, one atom of nitrogen and three atoms of oxygen.
(c) One atom of hydrogen, one atom of nitrogen and two atoms of oxygen.
(d) One atom of hydrogen, two atoms of nitrogen and three atoms of oxygen. Answer:: (b) One atom of hydrogen, one atom of nitrogen and three atoms of oxygen.

3. The chemical formula of magnesium phosphate is
(a) Mg(P0₄)₂
(b) Mg₂(P0₄)₃
(c) Mg₃(P0₄)₂
(d) Mg(PO₄)₃
Answer:
(c) Mg₃(P0₄)₂

4. The following reaction is an example of a
Fe(s) + CuSO₄(aq) → FeSO₄ (aq) + Cu(s)
(a) Combination reaction
(b) Displacement reaction
(c) Decomposition reaction
(d) Neutralization reaction
Answer:
(b) Displacement reaction

5. The chemical formula of quicklime is
(a) CaO
(b) CaCO₃
(c) Ca(OH)₂
(d) CaCl₂
Answer:
(a) CaO

D. Answer the following questions in brief.

Question 1.
What are elements?
Answer:
A pure substance which is made up of atoms of similar kind is called element, e.g. iron, copper, etc.

Question 2.
Give the chemical symbol of iron and chlorine.
Answer:
Fe (Iron) and Cl (Chlorine)

Question 3.
Write the chemical formulae of the following compounds:
(a) Aluminium oxide
(b) Zinc acetate
Answer:
(a) Al₂O₃
(b) (CH₃COO)₂Zn

Question 4.
Balance the following equation:
Ca(OH)₂ + HCl → CaCl₂ + H₂0
Answer:
DAV Class 7 Science Chapter 3 Question Answer - Chemical Substances and Processes 1

Question 5.
What does a chemical equation represent?
Answer:
A chemical equation represents a chemical reaction. A chemical equation shows physical states of reactants and products also.

Question 6.
What are reactants and products in a chemical equation?
Answer:
Substances which are written on LHS of an equation are reactants, while those written on RHS of an equation are products.

Question 7.
Give one example of a combination reaction.
Answer:
2H₂ (g) + O₂ (g) > 2H₂O(l)

E. Answer the following questions.

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

Question 2.
How is a chemical change different from a physical change?
Answer:
A new substance is formed in a chemical change, while no new substance is formed in a physical change. Most of the physical changes are reversible, while chemical changes cannot be reversed by simple physical meAnswer:

Question 3.
‘Neutralisation reaction is a chemical change.’ Justify this statement with the help of an example.
Answer:
After a neutralisation reaction, new substances are formed. Hence, it is true that a neutralisation reaction is a chemical change. .

Question 4.
Define a decomposition reaction and give an example of the same.
Answer:
When a substance gives two or more substances, the reaction is called decomposition reaction. Example:

Water molecule gets broken into hydrogen and oxygen when electric passed through it. This can be shown by following equation.

Question 5.
Classify the following reactions into different types, giving reason.
(a) CaO(s) + SiO₂(s) → CaSiO₃(s)
Answer:
Combination reaction

(b) KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)
Answer:
Neutralisation reaction

Answer:
Decomposition reaction

Value Based Question

The school principal told her students that she would like them to follow a ‘spe¬cial practice’ of the Japanese schools. Their schools do not keep staff members for cleaning as such. The students, themselves, work as a team and take pride in maintaining the cleanliness of their school. She went on to say that she would like them to replace their old habits by new, better habits in a way similar to a ‘displacement reaction’ in which one element replaces another in a compound.

Question 1.
State two values displayed by students of Japanese schools.
Answer:
Respect for others’ work and self help

Question 2.
Why did the principal tell her students that the suggested ideas, in a way, similar to what happens in a displacement reaction?
Answer:
In this case, teacher wants the students to replace their old habits by better habits. She has compared this with replacement of a metal from another metal’s salt. Hence, the principal is comparing behaviour change with displacement reaction.

Question 3.
Give one example of a displacement reaction.
Answer:
Cu(s) + 2AgNO₃ (aq) > Cu(NO₃)₂ (aq) + 2Ag(s)

Something To Do

Question 1.
List any five physical and chemical changes that you see around you.
Answer:
Five physical changes around us:
(a) Melting of ice
(b) Melting of wax
(c) Making of dough
(d) Drying of clothes
(e) Folding a paper Five chemical changes around us:

(a) Cooking of rice
(b) Growth of a tree
(c) Germination of seeds
(d) Burning of candle
(e) Burning of paper

Question 2.
Think of a small activity to show that rusting of iron requires both oxygen and water.
Answer:
For this, take three nails and four test tubes. Fill one test tube with water and keep a nail in it. Fill the second test tube with water and keep a nail in it. Then pour a layer of oil over water in the second test tube. The layer of oil would prevent oxygen from entering water. Keep the third test tube empty and keep a nail in it. Close the mouth of the third test tube with cotton wool. You will notice that the nail in the first test tube gets rusted but nails in other test tubes do not get rusted. This shows that both oxygen and water are necessary for rusting of iron.

DAV Class 7 Science Chapter 3 Solutions – Chemical Substances and Processes

I. Fill in the blanks.

Question 1.
A …………. substance is composed of same type of element or molecule.
Answer:
pure

Question 2.
Chemical symbol of oxygen molecule is
Answer:
O₂

Question 3.
________ and ________ react in a neutralisation reaction.
Answer:
Acid, Base

Question 4.
Iron displaces ________ form copper sulphate soulution
Answer:
Copper

Question 5.
Water gets decomposed when ________ is passed through it.
Answer:
Electric current

II. Tick (✓) the correct option.

1. Which of the following is a pure substance?
(a) Normal water
(b) Pure ghee
(c) Sodium chloride
(d) Sand
Answer:
(c) Sodium chloride

2. How many atoms of hydrogen are there in one molecule of water?
(a) One
(b) Two
(c) Three
(d) Four
Answer:
(b) Two

3. How many atoms of oxygen are there in one molecule of oxygen?
(a) One
(b) Two
(c) Three
(d) Four
Answer:
(b) Two

4. What is the colour of copper sulphate solution?
(a) Red
(b) Green
(c) Yellow
(d) Blue
Answer:
(d) Blue

5. What is the colour of iron sulphate solution?
(a) Red
(b) Green
(c) Yellow
(d) Blue
Answer:
(b) Green

6. Which reaction happens when an acid and a base react with each other?
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
(d) Neutralisation reaction
Answer:
(d) Neutralisation reaction

III. Answer the following questions.

Question 1.
What do understand by element?
Answer:
Element: An element is a pure substance which is made up of atoms of the same kind. Example, iron, copper, etc.

Question 2.
What do you understand by a compound?
Answer:
Compound: A compound is a pure substance which is made up of molecules of the same kind. A molecule can be made up of two or more elements. Example, sodium chloride, copper sulphate, etc.

Question 3.
What is a chemical formula?
Answer:
A substance can be shown by a chemical formula which shows the composition of that substance. For example; the chemical formula of water is H₂O.

Question 4.
Show various steps of writing the chemical formula of calcium nitrate.
Answer:

Question 5.
With the help of a suitable example, explain displacement reaction.
Answer:
When one element displaces another element in a compound, the reaction is called displacement reaction. Example:

When iron nail is put into copper sulphate solution, iron displaces copper and forms iron sulphate. As a result, the blue colour of copper sulphate solution fades and the solution changes into green colour. This can be shown by following equation.
DAV Class 7 Science Chapter 3 Question Answer - Chemical Substances and Processes 6

IV. Projects

Question 1.
Make a list of five chemical changes happening around you.
Answer:
Do it yourself.

Question 2.
Make a list of five physical changes happening around you.
Answer:
Do it yourself.

Question 3.
Ask your teacher and write the names of some common acids which are used in science laboratory.
Answer:
Do it yourself.

Question 4.
SO₂(g) + H₂O(l) → H₂SO₃(aq)
Answer:
Combination reaction

Question 5.
Ba(OH)₂(aq) + 2HNO₃(aq) → Ba(NO₃)₂(aq) + 2H₂O(l)
Answer:
Neutralisation reaction

Question 6.
Give two examples of a chemical change where a new substance is formed.
Answer:
(a) When hydrogen and oxygen combine, they form water.
(b) When an acid reacts with a base, a salt and water are formed.

Question 7.
What does a chemical equation represent?
Answer:
A chemical equation represents a chemical reaction. A chemical equation shows physical states of reactants and products also.

Question 8.
What is a combination reaction? Give an example.
Answer:
When two or more substances combine to form a new substance, the reaction is called combination reaction. Example:
Hydrogen and oxygen combine to form water. This can be shown by following equation.
DAV Class 7 Science Chapter 3 Question Answer - Chemical Substances and Processes 7