class 7 Q & A of chemistry

PODAR INTERNATIONAL SCHOOL

CLASS : 7                              TOPIC: WRITING CHEMICAL FORMULAE

E. ANSWER THE FOLLOWING QUESTIONS:

Q.1. What is an element? Give e.g.

Ans: An element consists of one kind of atom and cannot be broken down into simpler substances by chemical or physical methods. E.g. an OXYGEN molecule (O₂) is composed of two atoms of oxygen.

Nitrogen molecule (N₂) is composed of two atoms of nitrogen.

Hydrogen molecule (H₂) is composed of two atoms of hydrogen.

Q.2. Is copper an element or compound?

Ans: Copper (Cu) is an element because it consists of only one kind of atom, copper atom.

Q 3. Write the valencies of the following.

ELEMENT	VALENCY	ELEMENT	VALENCY
Hydrogen	1	Sulphur	2
Sodium	1	Oxygen	2
Chlorine	1	Zinc	2
Silver	1	Aluminium	3
Calcium	2	Phosphorus	3
Magnesium	2	Carbon	4

Q. 4. Identify the elements represented by

ELEMENT	SYMBOL	ELEMENT	SYMBOL
Sodium	Na	Potassium	K
Copper	Cu	Gold	Au
Magnesium	Mg	Mercury	Hg
Calcium	Ca	Zinc	Zn
Aluminium	Al	Argon	Ar
Silver	Ag	Chlorine	Cl

Q. 5. Name the compounds having the chemical formula:

AgNO₃  , AgCO₃, AgCl ,  Ca(NO₃)₂    

        a. AgNO₃     - Silver nitrate

b. AgCO_{3       -} silver carbonate

c. AgCl        - silver chloride

d. Ca(NO₃)₂     - calcium nitrate

Q. 6. Are 2N and N₂ the same?

No, N₂ represents two nitrogen atoms are there in one molecule of nitrogen. Nitrogen is diatomic molecule.

Q. 7. Correct the following chemical formulae:

a. AG NO₃ – AgNO₃

b. Mgcl₂     - MgCl₂

c.

d. (Na)O – Na₂O

e. Cu(SO₄) – CuSO₄

9. Differentiate between:

a. Atoms & molecules

ATOMS	MOLECULES
Atoms are the smallest structural unit of any substance	Two or more atoms combine together to form molecule

b. Elements & Compounds

elements	compounds
A substance made up of the same kind of atoms	A substance consists of atoms of two or more elements combine chemically in a definite proportion by weight.
It cannot be broken down into simpler substances by chemical or physical methods.	It can be broken down into simpler substances by chemical means but not by physical methods.
e.g. hydrogen, oxygen, nitrogen, sulphur	e.g. water is a compound made up of Hydrogen & oxygen Ammonia is a compound (NH3) made of nitrogen and hydrogen

c. Chemical symbol & chemical formula

Chemical symbol	Chemical formula
The symbol of an element represents the abbreviated name of the element.	It is the short scientific method of representing one molecule of an element or compound.
The symbol is usually the capitalized first letter of the element, in some elements, their symbols take 2 letters. First capital letter followed by small letter.	It is written by using a combination of elements.

F. ANSWER THE FOLLOWING QUESTIONS IN DETAIL:

1. Define an atom. Give any two examples of atoms.

Ans: Atom is the building block of matter.

Atoms are the smallest structural unit of any substance.

All atoms in an element are similar and unique.

3. What is a mixture? Write an example, and explain why it is a mixture.

Ans: Mixtures are substances in which two or more substances are physically mixed together.

The constituents in a mixture can be separated by simple physical methods.

The constituents in a mixture do not have any fixed proportions.

e.g. salad, air, water

Q. 4. Which element does S represent? Write the name of the compound formed by this element with sodium.

S represents sulphur. With sodium it forms sodium sulphide.

Q. 5. Explain the significance of writing a chemical formula.

Ans: A chemical formula represents the elements present in a compound.

It also represents the no. of atoms of each element present in a compound.

Q. 6. What is valency?

Ans: the combining capacity of an element or a radical with other atoms. Valency of carbon is 4

Q.7. What is atomicity?

Ans: the no. of atoms present in a molecule of an element is known as atomicity. For e.g. one hydrogen molecule has  2 hydrogen atoms. Atomicity of hydrogen is 2.