Samacheer Class 8 Science Atomic Structure Book Back Answers & Solutions

1I. Choose the best answer5 questions

Q.1The same proportion of carbon and oxygen in the carbon dioxide obtained from different sources proves the law ofv

A. reciprocal proportion
B. definite proportion
C. multiple proportion
D. conservation of mass

Solution

(b) definite proportion

Answer:

(b) definite proportion

Q.2Cathode rays are made up of –v

A. neutral particles
B. positively charged particles
C. negatively charged particles
D. None of the above

Solution

(c) negatively charged particles

Answer:

(c) negatively charged particles

Q.3In water, hydrogen and oxygen are combined in the ratio of by mass.v

A. 1 : 8
B. 8 : 1
C. 2 : 3
D. 1 : 3

Solution

(a) 1 : 8

Answer:

(a) 1 : 8

Q.4Which of the following statements made by Dalton has not undergone any change?v

A. Atoms cannot be broken.
B. Atoms combine in small, whole numbers to form compounds.
C. Elements are made up of atoms.
D. All atoms of an elements are alike

Solution

(d) All atoms of an elements are alike

Answer:

(d) All atoms of an elements are alike

Q.5In all atoms of an element –v

A. the atomic and the mass number are same.
B. the mass number is same and the atomic number is different.
C. the atomic number is same and the mass number is different
D. both atomic and mass numbers may vary.

Solution

(a) the atomic and the mass number are same.

Answer:

(a) the atomic and the mass number are same.

2II. Fill in the blanks1 questions

Q.G1…………….. is the smallest particle of an element. An element is composed of …………….. atoms. An atom is made up of …………….., …………….. and …………….. A negatively charged ion is called …………….. while positively charged ion is called …………….. is a negatively charged particle (Electron/Proton). Proton is deflected towards the …………….. charged plate (positively, negatively).v

Solution

Atom
same kind of
proton, electron, neutron
anion, cation
Electron
negatively

Answer:

Atom
same kind of
proton, electron, neutron
anion, cation
Electron
negatively

3III. Match the following1 questions

Q.G2Law of Conservation of Mass – Sir William Crookes Law of Constant Proportion – James Chadwick Cathode rays – Joseph Proust Anode rays – Lavoisier Neutrons – Goldsteinv

Solution

Law of Conservation of Mass – Lavoisier
Law of Constant Proportion – Joseph Proust
Cathode rays – Sir William Crookes
Anode rays – Goldstein
Neutrons – James Chadwick

Answer:

Law of Conservation of Mass – Lavoisier
Law of Constant Proportion – Joseph Proust
Cathode rays – Sir William Crookes
Anode rays – Goldstein
Neutrons – James Chadwick

4IV. Answer briefly7 questions

Q.1State the Law of Conservation of Mass.v

Solution

The law states that during any chemical change, the total mass of the products is equal to the total mass of the reactants.

Answer:

The law states that during any chemical change, the total mass of the products is equal to the total mass of the reactants.

Q.2State the Law of Constant Proportions.v

Solution

Law of constant proportions states that in a pure chemical compound the elements are always present in definite proportions by mass.

Answer:

Law of constant proportions states that in a pure chemical compound the elements are always present in definite proportions by mass.

Q.3Write the properties of anode rays.v

Solution

Properties of Anode rays:
Anode rays travel in straight lines.
Anode rays are made up of material particles.
Anode rays are deflected by electric and magnetic fields. Since, they are deflected towards the negatively charged plate, they consist of positively charged particles.

Answer:

Properties of Anode rays:
Anode rays travel in straight lines.
Anode rays are made up of material particles.
Anode rays are deflected by electric and magnetic fields. Since, they are deflected towards the negatively charged plate, they consist of positively charged particles.

Q.4Define valency of an element with respect to hydrogen.v

Solution

Valency of an element is defined as the number of hydrogen atoms which combine with one atom of it.

Answer:

Valency of an element is defined as the number of hydrogen atoms which combine with one atom of it.

Q.5Define the term ions or radicals.v

Solution

An atom or a group of atoms when they either lose or gain electrons, get converted into ions or radicals.

Answer:

An atom or a group of atoms when they either lose or gain electrons, get converted into ions or radicals.

Q.6What is a chemical equation?v

Solution

A chemical equation is a short hand representation of a chemical reaction with the help of chemical symbols and formula.

Answer:

A chemical equation is a short hand representation of a chemical reaction with the help of chemical symbols and formula.

Q.7Write the names of the following compounds. CO N 2 O NO 2 PCl 5v

Solution

Carbon monoxide.
Nitrous oxide
Nitrogen dioxide
Phosphorous pentachloride

Answer:

Carbon monoxide.
Nitrous oxide
Nitrogen dioxide
Phosphorous pentachloride

5V. Answer the following3 questions

Q.1Find the valency of the element which is underlined in the following formula. Na Cl C O 2 Al (PO 2 ) Ba (NO 3 ) 2 Ca Cl 2v

Solution

Na Cl = 1
C O 2 = 4
Al (PO 4 ) = 3
Ba (NO 3 ) 2 = 2
Ca Cl 2 = 2

Answer:

Na Cl = 1
C O 2 = 4
Al (PO 4 ) = 3
Ba (NO 3 ) 2 = 2
Ca Cl 2 = 2

Q.2Write the chemical formula for the following compounds Aluminium sulphate Silver nitrate Magnesium oxide Barium chloridev

Solution

Aluminium sulphate = Al 2 (SO 4 ) 3
Silver nitrate = AgNO 3
Magnesium oxide = MgO
Barium chloride = BaCl 2

Answer:

Aluminium sulphate = Al 2 (SO 4 ) 3
Silver nitrate = AgNO 3
Magnesium oxide = MgO
Barium chloride = BaCl 2

Q.3Write the skeleton equation for the following word equation and then balance them. Carbon + Oxygen → Carbon dioxide Phosphorus + Chlorine → Phosphorus pentachloride. Sulphur + Oxygen → Sulphur dioxide Magnesium + hydrogen chloride → Magnesium chloride + Hydrogenv

Solution

Balanced equation:
C + O 2 → CO 2
P 4 + 10 Cl 2 → 4PCl 5
S + O 2 → SO 2
Mg + 2HCl → MgCl 2 + H 2
Skeleton equation:
C + O 2 → CO 2
P + Cl 2 → PCl 5
S + O 2 → SO 2
Mg + 2HCl → MgCl 2 + H 2

Answer:

Balanced equation:
C + O 2 → CO 2
P 4 + 10 Cl 2 → 4PCl 5
S + O 2 → SO 2
Mg + 2HCl → MgCl 2 + H 2
Skeleton equation:
C + O 2 → CO 2
P + Cl 2 → PCl 5
S + O 2 → SO 2
Mg + 2HCl → MgCl 2 + H 2

6VI. Higher Order Thinking Questions3 questions

Q.1Why does a light paddle wheel placed in the path of cathode rays begin to rotate, when cathode rays fall on it?v

Solution

It is because the small particles of the cathode rays (electrons) have mass and energy. This energy is used in rotating the paddle wheels.

Answer:

It is because the small particles of the cathode rays (electrons) have mass and energy. This energy is used in rotating the paddle wheels.

Q.2How can we prove that the electrons carry negative charge?v

Solution

J.J. Thomson found that cathode rays were attracted by the positively charged plate and repelled by the negatively charged plate. This led him to the conclusion that the cathode rays (electrons) were made of negatively charged particles.

Answer:

J.J. Thomson found that cathode rays were attracted by the positively charged plate and repelled by the negatively charged plate. This led him to the conclusion that the cathode rays (electrons) were made of negatively charged particles.

Q.3Ruthresh, Hari, Kanishka and Thahera collected different samples of water from a well, a pond, a river and underground water. All these samples were sent to a testing laboratory. The test result showed the ratio of hydrogen to oxygen as 1:8. What conclusion would you draw from the above experiment? Which law of chemical combination does it obey?v

Solution

Water obtained from different sources like a well, a pond, a river and underground water will always consist of the same two elements hydrogen and oxygen in the ratio 1 : 8 by mass.
It obeys the law of constant proportion.

Answer:

Water obtained from different sources like a well, a pond, a river and underground water will always consist of the same two elements hydrogen and oxygen in the ratio 1 : 8 by mass.
It obeys the law of constant proportion.

7Activity – 21 questions

Q.1Classify the following ions into monovalent, divalent and trivalent. Ni 2+ , Fe 3+ , Cu 2+ , Ba 2+ , Cs + , Zn 2+ , Cd 2+ , Hg 2+ , Pb 2+ , Mn 2+ , Fe 2+ , CO 2+ , Sr 2+ , Cr 3+ , Li + , Ca 2+ , Al 3+v

Solution

Monovalent ions – Li + , Cs +
Divalent ions – Ni 2+ , Cu 2+ , Ba 2+ , Zn 2+ , Cd 2+ , Hg 2+ , Pb 2+ , Mn 2+ , Fe 2+ , CO 2+ , Ca 2+ , Sr 2+
Trivalent ions – Fe 3+ , Cr 3+ , Al 3+

Answer:

Monovalent ions – Li + , Cs +
Divalent ions – Ni 2+ , Cu 2+ , Ba 2+ , Zn 2+ , Cd 2+ , Hg 2+ , Pb 2+ , Mn 2+ , Fe 2+ , CO 2+ , Ca 2+ , Sr 2+
Trivalent ions – Fe 3+ , Cr 3+ , Al 3+