SCERT KERALA TEXTBOOKS SOLUTIONS & NOTES: Class 8 Basic Science (English Medium) Chapter 05 Basic Constituents of Matter
Textbooks Solution for Class 8th Basic Science (English Medium) | Text Books Solution Basic Science (English Medium) Chapter 05 Basic Constituents of Matter
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SCERT Solutions for Class 8 Basic Science Chapterwise 
Class 8 Chemistry - Questions and Answers
Chapter: 05 - Basic Constituents of Matter
* Substances are classified into pure substances and mixtures. 
Water, sugar, carbon etc. are examples of pure substances and saline water, air, lemon juice etc. are examples for mixtures.

* Substances can also be classified into compounds and elements.
Compounds:
Pure substances that can be decomposed to other substances through the chemical process are called compounds.
Example.
1. Water: It can be decomposed to hydrogen and oxygen by passing a current through it.
2. Sugar: Sugar can be decomposed to carbon and water by heating it.

Elements.
Pure substances that cannot be split into other substance are called elements.
Hydrogen, Oxygen, Aluminium, Iron etc. are examples for elements.
Compounds are formed by the combination of two or more elements through a chemical reaction.

Origin of names of Elements.
More than 100 stable elements are discovered so far.  Elements are known in the names of continents, countries, planets, satellites,  scientists etc.. A few examples are given in the table. 
Symbols
For convenience symbols of elements are used instead of writing the full name of them. The symbols are selected in different ways.
Method.1: The first letter of the English name of the elements is used as symbols.
Examples: Carbon – C, Hydrogen – H, Oxygen – O

Method.2: Along with the first letter, second letter or any prominent letter is also used as a symbol.
Examples: Calcium – Ca, Chlorine – Cl, Bromine – Br, Chromium – Cr

Method.3: Letter or letters of Latin name of elements are used as a symbol.
Examples: 
Sodium – Natrium – Na.
Potassium – Kalium – K
Copper – Cuprum – Cu
Iron – Ferrum – Fe

Atoms and molecules: Atom is the smallest particle that shows all the characteristic properties of an element. All elemental substances are made of their own atoms.
Many of the elemental atoms cannot exist as single atoms. So they combine with two or more atoms and form molecules. Molecules are the smallest particle that can exist independently. 

Representation of Atoms and molecules.
Symbols of elements are used to represent atoms.
For example H – means one hydrogen atom. 4 Na – means four sodium atoms.

Molecules are represented by symbols of the element with the subscription of the number of atoms present in that molecule.
Example: H₂ – One Hydrogen molecule. (A hydrogen molecule consists of two hydrogen atoms)
O₃ – Ozone molecule. (There are three oxygen atoms are present in one ozone molecule)
P₄  - Phosphorous molecule (One phosphorous molecule consists of four phosphorous atoms.

Monoatomic molecules: Molecules with only one atom are called monoatomic molecules.
Helium (He), Neon (Ne), Argon (Ar) etc. are examples for monoatomic molecules.

Diatomic molecules: Molecules having two atoms in it are called diatomic molecules.
Examples: H₂, O₂, N₂

Polyatomic molecules: Molecules with more than two atoms are polyatomic molecules.
Examples: P₄, S₈, O₃
For representing more than one molecules, the number of molecules is expressed to the left of the representation of the molecule.
Example: 3H₂ – means three hydrogen molecules. It is noted that there are six hydrogen atoms in 3H₂.
5O₃ - means five ozone molecules.
2He – two helium molecules. 

Compounds.
Compound molecules are formed by the combination of different elements through chemical reactions. Millions of various substances are formed by two or more various atoms of a limited number of elements.
Examples:1. Carbon dioxide – carbon + Oxygen.
2. Water – Hydrogen + Oxygen.
3. Sugar – Carbon + Hydrogen + Oxygen.
4. Hydrogen chloride – Hydrogen + Chlorine.
One compound is obtained from different sources. Whichever be the source, the ratio of various
elemental atoms present in the molecule is the same. 
For example, one carbon atom and two oxygen atoms are present in every
carbon dioxide molecule. Hence it is represented CO₂.  This representation is
known as chemical formula. Similarly, the chemical formula of water is written as H₂O. The structure of water, carbon dioxide and methane molecules are shown in the figure.
From the chemical formula of a compound, we can get the information of which of the elements and how
many atoms are present in one molecule.
Example:1. Sulphuric acid (H₃SO₄) – It consists of two hydrogen atoms, one sulphur atom and four oxygen atoms.
2. Ammonia (NH₃) – One nitrogen and three hydrogens.

Reactants and products.
It is by combining two or more substances through a chemical reaction, new substances are formed. The substance taking part in a chemical reaction is called reactants and substances so produced are called products.
When Zinc reacts with hydrochloric acid, hydrogen and zinc chlorides are formed.
Here Zinc and Hydrochloric acids are reactants and hydrogen and Zinc chloride are products. 
The chemical reaction can be expressed as an equation given below.
Zn + HCl →H₂ + ZnCl₂
It is known as a chemical equation.
In this equation number of atoms on both sides are not equal. (There are two hydrogen atoms and two
chlorine atoms on the right side, but only one each on the left). So the equation is to be balanced.
The balanced equation is  Zn + 2HCl →H₂ + ZnCl₂
Practice Questions & Answers 
1. What are the elements? Give two examples of the same and their symbols.
Ans. Pure substances that cannot be decomposed through the chemical process are called elements.
Examples. Hydrogen (H), Carbon (C).

2. What are compounds? Give two examples for compounds and write down their constituent elements.
Ans. A substance formed by the combination of more than one element through chemical reaction is compound. Compounds can be decomposed to other substance through a chemical process.
Example:1.Water – Hydrogen and oxygen are the constituent elements of water.
2. Sugar: The components of sugar are Carbon, Hydrogen and Oxygen.

3. Elements were named after countries, continents, planets and satellites. Cite this with examples. 

4. Give two examples for elements whose symbols were derived from their Latin names.
Ans. Iron - Ferrum - Fe, Potassium - Kalium - K

5. Molecules are the smallest particles that can exist independently.
Write down one example each for monoatomic, diatomic and polyatomic molecules.
Ans. Monoatomic molecule: Helium (He)
Diatomic molecule: Oxygen (O₂), Polyatomic molecule: Phosphorous(P₄)

6. Symbols of a few elements are given below. Identify the elements.
Ca, S, Be,Fe,Cr,Cu,H,Cl
Ans. Ca: Calcium, S: Sulphur, Be: Beryllium, Fe: Iron, Cr: Chromium,
Cu: Copper, H: Hydrogen, Cl: Chlorine.

7. Write down the symbols of Sodium, Sulphur, Nitrogen, Potassium, Carbon and Zinc.
Ans. Sodium:Na,Sulphur:S, Nitrogen:N, Potassium:K, Carbon:C, Zinc:Zn

8. Elements exist in solid, liquid and gaseous state. Give one example for each.
Ans.Solid: Carbon,  Liquid: Mercury, Gas: Hydrogen.

9. Find out the relation of the first pair and fill the second accordingly.
a. Na: Natrium; K: .......        
b. Chlorine: Colour; Titanium: ........   
c. H₂: Diatomic molecule; P₄: ........
d. Water: Compound; Carbon: .....    
e. Plutonium: Name of the planet; Francium: ....... 
Ans.a. Kalium 
b. Satellite 
c. Polyatomic molecule.   
d. Element.    
e. Name of country.

10.Complete the following statements properly. 
a. .... and  .... are the constituent elements of water. 
b. .... was the scientist who discovered that water can be split into Hydrogen and Oxygen by passing an electric current through it. 
c.  ...... are the substances formed by combining different elements through a chemical process.
d. Substances that cannot be decomposed through the chemical process are called .....
Ans.a.Hydrogen & Oxygen.   
b.Sir Humphrey Davy.   
c. Compounds 
d. Elements.

11. Hydrogen burns in oxygen to give water. Identify the reactants and products in this process.      
Ans. Reactants: Hydrogen and Oxygen.  Product: Water.

12. Find out the number of molecules and the total number of atoms in the following.
O₂, 3N₂, 4Na, CO₂, NaCl, 3H₂O, 4NH₃, C₆H₁₂O₆. 

13. Write down the  chemical formula for Water, Carbon dioxide and Sulphuric acid and identify the constituent elements and find out the total number of atoms present in each molecule. 
Ans. Water:H2O:- Hydrogen & Oxygen. Total number of atoms = 3
Carbon dioxide: CO2:- Carbon & Oxygen. Total number of atoms = 3
Sulphuric acid.: H2SO4:- Hydrogen, Sulphur  & Oxygen. Total number of atoms = 7

14. Briefly explain reactants and products with example.
Ans. New substances are produced through chemical reactions. The substances taking part in a chemical reaction are called reactants and the substances formed as the result of a chemical reaction are called products.   
Example: Zinc chloride and hydrogen are formed by the chemical reaction between Zinc and hydrochloric acid.  Here Zinc & Hydrochloric acid are reactants and Zinc chloride and Hydrogen are products.
15. H is the symbol of Hydrogen. What do   4H2, 3H, H2  indicate?
Ans.4H2: It indicates four hydrogen molecules.    3H: Three hydrogen atoms. 
H2: One hydrogen molecule.

16. Classify the following into elements and compounds.
Sugar,Iron, Carbon, Water, Sodium chloride, Copper sulphate and Sodium.
Ans. Elements:Iron, Carbon, Sodium.
Compounds: Sugar,Water, Sodium chloride, Copper sulphate.

17. Find whether the following statements are 'true' or 'false'.
a. All molecules are formed by the combination of atoms of different elements.
b. Molecules of a compound are formed by combining atoms of different elements. 
c. All molecules contain more than one atom.
d.  The number of  atoms of each element on both sides of a balanced chemical equation should be
equal.
e. The number of molecules  on both sides of a balanced chemical equation may not be equal. 
Ans.a. All molecules are formed by the combination of atoms of different elements – False.
b. Molecules of a compound are formed by combining atoms of different elements.  - True.
c. All molecules contain more than one atom. - False.
d. The number of  atoms of each element on both sides of a balanced chemical equation should be
equal- True
e. The number of molecules on both sides of a balanced chemical equation may not be equal. - True. 

18. In a balanced chemical equation, the number of each elemental atoms on either side will be equal. Check whether the following equations are balanced or not.   
a. H₂ + Cl₂ → 2HCl    
b. Mg +  O₂ → MgO      
c. C + O₂ →CO₂.   
d. N₂ +  H₂ →NH₃. 
e. H2 +O₂ → H₂O + O₂.    
f. NaOH + H₂SO₄ → Na₂SO₄ +  H₂O
g. H₂ +I₂ → 2HI         
h. N₂ +  O₂ →NO.    
i.CH₄ + 2O₂ → CO₂ + 2H₂O
Ans. a,c,g&i are balanced equations.

19. It is given the models of the molecules of Methane, Carbon dioxide and Water.  Identify the elements in each molecule and write down their chemical formula.
Ans.i.Methane: Carbon and Hydrogen - CH4
ii.Carbon dioxide: Carbon and Oxygen - CO2         
iii. Water: Hydrogen and Oxygen. - H2O

20. Match the contents in the columns of A&B.

A

B

Chlorine

Planet

Curium

Colour

Plutonium

Continent

Europium

Scientist
Ans. i. Chlorine - Colour 
ii. Curium - Scientist    
iii. Plutonium - Planet  
iv. Europium – Continent.

21. Figures of the molecules of a few elements are given.
a. Which are the elements seen in the figure? 
b. Classify them into monoatomic, diatomic and polyatomic molecules.
Ans.a. Ne – Neon, O₂ – Oxygen.    S₈ - Sulphur.  P₄ – Phosphorous
He – Helium.
b. Ne, He: Monoatomic molecules,
O₂ – Diatomic molecule, 
S₈, P₄ – Poly atomic molecule. 
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