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Language of Chemistry

Solutions for Chemistry, Class 8, ICSE

Exercise

13 questions

Question 1

Define:

(a) Radical

(b) Valency

(c) Molecular formula

Exercise

Answer:

(a) Radical is an atom of an element or a group of atoms of the same or different elements that behave as a single unit and has positive or negative charge.

(b) Valency is the combining capacity of an atom of an element or of a radical with the atoms of other elements or radicals to form molecules.

(c) A molecular formula of a compound is the symbolic representation of its one molecule which shows the number of atoms of each element present in it.

Question 2

Give the symbols and valencies of the following radicals:

(a) Hydroxide

(b) Chloride

(c) Carbonate

(d) Ammonium

(e) Nitrate

Exercise

Answer:

RadicalSymbolValency
HydroxideOH-1
ChlorideCl-1
CarbonateCO32-2
AmmoniumNH4+1
NitrateNO3-1

Question 3

Write the molecular formulae for the oxides and sulphides of the following elements.

(a) Sodium

(b) Calcium

(c) Hydrogen

Exercise

Answer:

ElementOxideSulphide
SodiumNa2ONa2S
CalciumCaOCaS
HydrogenH2OH2S

Question 4

Write the molecular formulae for the following compounds and name the elements present.

(a) Baking soda

(b) Common salt

(c) Sulphuric acid

(d) Nitric acid

Exercise

Answer:

CompoundsMolecular formulaElements present
Baking sodaNaHCO3Sodium, hydrogen, carbon, oxygen
Common saltNaClSodium, chlorine
Sulphuric acidH2SO4Hydrogen, sulphur, oxygen
Nitric acidHNO3Hydrogen, nitrogen, oxygen

Question 5

The valency of aluminium is 3. Write the valency of other radicals present in the following compounds.

(a) Aluminium chloride

(b) Aluminium oxide

(c) Aluminium nitride

(d) Aluminium sulphate

Exercise

Answer:

CompoundsRadicalValency
Aluminium chloride (AlCl3)Chloride (Cl-)1
Aluminium oxide (Al2O3)Oxide (O2-)2
Aluminium nitride (AlN)Nitride (N3-)3
Aluminium sulphate (Al2(SO4)3)Sulphate (SO42-)2

Question 6

What is variable valency? Give two examples of elements showing variable valency.

Exercise

Answer:

Some elements exhibit more than one valency and they are said to have variable valency.

Examples: Iron and copper.

MetalRadicalsValency
Iron (Fe)Ferrous, Ferric2, 3
Copper (Cu)Cuprous, Cupric1, 2

Question 7

(a) What is a chemical equation?

(b) Why is it necessary to balance a chemical equation?

(c) What are the limitations of a chemical equation?

Exercise

Answer:

(a) A chemical equation is the symbolic representation of a chemical reaction using symbols and formulae of the substances involved in the reaction.

(b) It is necessary to balance a chemical equation so as to make the number of atoms of the reactants equal to the number of atoms of the products because a chemical reaction is just a rearrangement of atoms and atoms are neither created nor destroyed during the chemical reaction.

(c) The limitations of a chemical equation are:

It does not inform about:

  1. the physical states of the reactants and the products i.e. whether they are solids, liquids or gases.
  2. the concentration of reactants and products.
  3. the time taken for the completion of reaction.
  4. the rate at which a reaction proceeds.
  5. the heat changes during the reaction i.e. whether heat is given out or absorbed.
  6. the conditions such as temperature, pressure, catalyst, etc. which affect the reaction.
  7. the nature of the reaction i.e. whether it is reversible or irreversible.

Question 8

What are the ways by which a chemical equation can be made more informative?

Exercise

Answer:

A chemical equation can be made more informative by the following ways:

  1. The physical state of the reactants and products can be indicated by putting s for solid, l for liquid, g for gas, aq. for aqueous state besides the symbols for the reactants and products.
  2. Evolution or absorption of heat during the reaction can be denoted by adding or subtracting a heat term on the product side.
  3. Temperature, pressure and catalyst can be indicated above the arrow (⟶) separating the reactants and products.
  4. Concentration of reactants and products are indicated by adding the words (dil.) for dilute and (conc.) for concentrated before their formulae.
  5. By the sign ⟶ or ⇌ information about irreversible and reversible reactions can be known.
  6. Upward arrow (↑) indicates a gas is evolved, downward arrow (↓) indicates a precipitate is formed, sign delta (Δ) indicates heat is evolved or absorbed.

Question 9

State the law of conservation of mass.

Exercise

Answer:

Law of conservation of mass states that matter can neither be created nor be destroyed, it can only be transformed from one form to another.

Question 10

Differentiate between:

(a) Reactants and Products

(b) A balanced and an unbalanced chemical equation

Exercise

Answer:

(a) Differences between Reactants and Products

ReactantsProducts
The substances that are used as the starting material and which react with one another are called reactants.The substances which are formed as a result of the reaction are called products.
Reactants are written on the left hand side of the equation.Products are written on the right hand side of the equation.
In equation C + O2 ⟶ CO2, C and O2 are reactants.In equation C + O2 ⟶ CO2, CO2 is product.

(b) Differences between a balanced and an unbalanced chemical equation

Balanced chemical equationUnbalanced chemical equation
In balanced chemical equation the total number of atoms on the reactant side as well as the product side are equal.In unbalanced chemical equation the total number of atoms on the reactant side and product side are not equal.
Example:
Zn + H2SO4 ⟶ ZnSO4 + H2		Example:
H2 + O2 ⟶ H2O

Question 11

Balance the following equations:

  1. N2 + H2 ⟶ NH3

  2. H2 + O2 ⟶ H2O

  3. Na2O + H2O ⟶ NaOH

  4. CO + O2 ⟶ CO2

  5. Zn + HCl ⟶ ZnCl2 + H2

Exercise

Answer:

  1. N2 + 3H2 ⟶ 2NH3

  2. 2H2 + O2 ⟶ 2H2O

  3. Na2O + H2O ⟶ 2NaOH

  4. 2CO + O2 ⟶ 2CO2

  5. Zn + 2HCl ⟶ ZnCl2 + H2

Question 12

Write balanced chemical equations for the following word equations:

  1. Iron + Chlorine ⟶ Iron(III) chloride

  2. Magnesium + dil. sulphuric acid ⟶ Magnesium sulphate + hydrogen

  3. Magnesium + oxygen ⟶ Magnesium oxide

  4. Calcium oxide + water ⟶ Calcium hydroxide

  5. Sodium + Chlorine ⟶ Sodium chloride

Exercise

Answer:

  1. 2Fe + 3Cl2 ⟶ 2FeCl3

  2. Mg + H2SO4 ⟶ MgSO4 + H2

  3. 2Mg + O2 ⟶ 2MgO

  4. CaO + H2O ⟶ Ca(OH)2

  5. 2Na + Cl2 ⟶ 2NaCl

Question 13

What information do you get from the following chemical equation?

Zn(s) + 2HCl (dil) ⟶ ZnCl2 (aq) + H2 (g)

Exercise

Answer:

We get the following information from the given chemical equation:

  1. Zinc and Hydrochloric acid are reactants. Zinc is in solid state that reacts with dilute HCl.
  2. The reaction is irreversible.
  3. The products formed are aqueous solution of zinc chloride and hydrogen gas.
  4. The equation is balanced i.e. number of atoms of each element on reactants and products sides are equal.