Samacheer Kalvi 8th Science Solutions Chapter 12 Pdf

Samacheer Kalvi 8th Science Solutions Chapter 12 Pdf

Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 12: Tamilnadu State Board Solution Class 8 Science Chapter 12 – Atomic Structure.

Samacheer Kalvi 8th Science Solutions Chapter 12: Overview

Board

Samacheer Kalvi
Class

8

Subject

Science
Chapter

12

Chapter Name

Atomic Structure


Samacheer Kalvi 8th Science Solutions Chapter 12 Pdf

(1) Choose the best answer.

( 1 ) The same proportion of carbon and oxygen in the carbon dioxide obtained from different sources prove the law of definite proportion.

a ) reciprocal proportion

b) definite proportion

c) multiple proportion

d) conservation of mass

(2) Cathode rays are made up of

neutral particles

b ) positively charged particles

negatively charged particles

d ) None of the above

(3) In water, hydrogen and oxygen are combined in the ratio of1:8  by mass.

a ) 1:8

b) 8:1

c) 2 : 3

d) 1: 3

(4) Which of the following statements made by Dalton has not undergone any change?

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

(5) In all atoms of an element

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 numbersmay vary.

(II) Fill in the blanks.

1 ) Atom is the smallest particle of an element.

2 ) An element is composed of same kind of atoms.

3 ) An atom is made up of proton ,electron and neutron.

4 ) A negatively charged ion is called anion, while positively charged ion is called cation .

5 ) Electron is a negatively charged particle (Electron / Proton )

6) Proton is deflected towards the negatively charged plate ( positively, negatively)

( III ) Match the following .

Law of conservation of mass Sir William Crookes
Law of constant proportion James Chadwick
Cathode rays Joseph Proust
Anode rays Lavoisier
Neutrons Goldstein

Answer

Law of conservation of mass Lavoisier
Law of constant proportion Joseph Proust
Cathode rays Sir William Crookes
Anode rays Goldstein
Neutrons James Chadwick

( IV) Answer briefly.

1) State the law of conservation of mass.

In 1774, a French chemist Lavoisier stated that during any chemical change there is an association of the mass of reactants and the mass of products and this linkage was related by the law of conservation of mass. So, according to the law of conservation of mass, the total mass of products is equivalent to the total mass of the reactants during any chemical change. To put it in another way, the law states that during any chemical reaction, it is neither possible to create the mass nor it is possible to destroy the mass. This Law is also called Law of indestructibility of mass.

2) State the law of constant proportions .

In 1779, the scientist Joseph Proust has proposed the Law of constant proportions. According to him, there is a constant presence of elements in a pure chemical compound and these elements are always in a definite proportions by mass. After observing all the compounds where two or more than two elements were present , he noticed that the same elements were present in each of such compounds and the elements were in same proportions regardless of the origin of the compound or their mode of preparation. In a similar manner, the compositions of the compounds will always remain the same, even if there is a difference in their mode of preparation. It’s ratio will always remain fixed. This law is also called ‘ Law of definite proportions’ .

3) Write the properties of anode rays.

The properties of anode rays are as follows :

  • Anode rays always follow a straight path.
  • Anode rays are composed of material particles.
  • There is a deflection of the anode rays by electric and magnetic fields. They contain positively charged particles because they are deflected in the direction of the negatively charged plate.
  • The properties of anode rays are dependent on the nature of the gas that are contained in the discharge tube.
  • There is an equivalency between the mass of the particles and the atomic mass of the gas that are contained inside the discharge tube.

4) Define valency of an element with respect to hydrogen. 

When there is a combination of the number of hydrogen atoms with one atom of it, then it is known as valency of an elements .

5) Define the term ions or radicals.

The atom remains electrically neutral because there is an equivalency between the number of protons and the number of electrons in an atom. But during chemical reactions, in order to acquire stable electronic configuration , atoms try to lose or gain one or more electrons in accordance with valency. An atom carries negative charge because it has gained more number of electrons. Simultaneously, an atom carries positive charge because by losing an electron, it has retained more number of protons. So, ions refers to these atoms which carry positive or negative charges. Therefore, it is relevant to say that, an atom or a group of atom get’s converted into ions or radicals, the moment they lose or gain electrons

6) What is a chemical equation?

A chemical equation refers to a shorthand representation that is mainly used for the purpose of representing a chemical reaction with the aid of chemical symbols and formulae. Reactants and products are the two main components of a chemical equation. Reactants refers to those substances that participates in a chemical reaction and products refers to those substances that are created in a chemical reaction.

7 ) Write the names of the following compounds.

 a ) CO             b) N2O       c) NO2       d) PCl5

Answer

 a ) CO Carbon monoxide
b) N2O Nitrous oxide
c)NO2 Nitrogen dioxide
d) PCl5

 

Phosphorus pentachloride

(V) Answer the following.

1 ) Find the valency of the element which is underlined in the following formula.

a ) NaCl b) CO2           c) Al ( PO4 )

d ) Ba(NO3)2     e) CaCl2

Answer

Valency of the underlined element
a ) NaCl           1
b) CO2           4
c ) Al ( PO4 )           3
d ) Ba (NO3 )2           2
e ) CaCl2           2

2 ) Write the chemical formula for the following compounds.

a ) Aluminium sulphate          b) Silver nitrate

c ) Magnesium oxide              d) Barium chloride

Answer

Chemical formula of the compounds
a ) Aluminium sulphate Al2 ( SO4)3
b ) Silver nitrate AgNO3
c ) Magnesium oxide MgO
d ) Barium chloride BaCl2

3) Write the skeleton equation for the following word equation and then balance them.

a ) Carbon + Oxygen –> Carbon dioxide

b ) Phosphorus + Chlorine –> Phosphorus pentachloride

c ) Sulphur + Oxygen –> Sulphur dioxide

d ) Magnesium + hydrogen chloride –> Magnesium chloride + Hydrogen

Answer

a ) Carbon + Oxygen à Carbon dioxide

Skeleton equation Balanced equation
C + O2 –> CO2 C + O2 –> CO2

b )  Phosphorus + Chlorine à Phosphorus  pentachloride

Skeleton equation Balanced equation
P + Cl2 –> PCl5 P4 + 10Cl2 –> 4 PCl5

c ) Sulphur + Oxygen à Sulphur dioxide

Skeleton equation Balanced equation
S + O2 –> SO2 S + O2 –> SO2

d ) Magnesium + hydrogen chloride à Magnesium chloride + Hydrogen

Skeleton equation Balanced equation
Mg + 2HCl –> MgCl2 + H2 Mg + 2HCl –> MgCl2 + H2

4) Balance the following chemical equation.

a ) Na + O2à Na2O

b ) Ca + N2àCa3N2

c ) N2 + H2àNH3

d ) CaCO3 + HCl àCaCl2 + CO2+ H2O

e ) Pb ( NO3)2àPbO + NO2+ O2

 

a ) Na + O2à Na2O

Ans : 4Na + O2à 2Na2O

b ) Ca + N2à Ca3N2

Ans : 3Ca + N2à Ca3N2

c ) N2 + H2à NH3

Ans : N2 + 3H2à2NH3

d ) CaCO3 + HCl à CaCl2 + CO2 + H2O

Ans : CaCO3 + 2HCl à CaCl2 + CO2 + H2O

e ) Pb( NO3 )2à PbO + NO2  + O2

Ans : 2Pb( NO3 )2à2PbO + 4NO2  + O2

(VI ) Higher Order Thinking Questions.

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

A light paddle wheel begins to rotate, when cathode rays falls on it because cathode rays contains small particles which possess mass and energy and it is this energy which ultimately enables the paddle wheels to rotate.

2) How can we prove that the electrons carry negative charge ?

J.J Thomson noticed that the positively charged plate depicted the tendency to attract the cathode rays and negatively charged plate portrayed the tendency of repulsion. So, from this observation he tried to conclude that, the cathode rays (electrons) contained negatively charged particles. So, in this way we can affirm that the electrons carry negative charge.

3 ) Ruthresh , 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 results showed the ratio of hydrogen to oxygen as 1:8.

a ) What conclusion would you draw from the above experiment?

So, from the above experiment we can conclude that ,water will always contain similar two elements that is hydrogen and oxygen in the ratio of 1:8 by mass , irrespective of the sources from which it is obtained, as for instance, a well, a pond, a river and underground water .

b) Which law of chemical combination does it obey?

It gives adherence to the law of constant proportion.

Explanation :

It adheres the law of constant proportion because the law mainly states that there is a constant presence of elements in a pure chemical compound and these elements are always in a definite proportions by mass. In a similar manner, the compositions of the compounds will always remain the same, even if there is a difference in their mode of preparation. It’s ratio will always remain fixed. This law is also called ‘ Law of definite proportions’ .

Updated: December 1, 2021 — 11:32 am

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