Selina ICSE Class 8 Chemistry Solutions Chapter 5 Language of Chemistry

Exercise

Q1. Define:
(a) Radical
(b) Valency
(c) Molecular formula
Answer:
(a) Radical: An atom of an element or a collection of atoms from other elements that behave as a single unit and have a positive or negative charge on them is referred to as a radical.
(b) Valency: The quantity of electrons that an atom’s valence shell contributes to or accepts during chemical fusion.
(c) Molecular formula: This is a molecule’s symbolic representation. It displays the number of atoms in each element. To create the molecule, these atoms must be combined in entire numbers.

Q2. Give the symbols and valances of following radicals:
(a) Hydroxide
(b) Chloride
(c) Carbonate
(d) Ammonium
(e) Nitrate
Answer:

Q3. Write the molecular formula for the oxide and sulphide of following elements.
(a) Sodium
(b) Calcium
(c) Hydrogen
Answer:
(a) Molecular formula of Sodium oxide Na₂O
Molecular formula of Sodium sulphide Na₂S
(b) Molecular formula of Calcium oxide CaO
Molecular formula of Calcium sulphide CaS

(c) Molecular formula of Hydrogen oxide H₂O
Molecular formula of Hydrogen sulphide H₂S

Q4. 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
Answer:
a) Baking soda — NaHCO₃
The Elements present in Baking soda are:-
1. Sodium
2. Hydrogen
3. Oxygen
4. Carbon
2. Common salt — NaCl
The Element present in Common salt are:-
1. Sodium
2. Chlorine
3. Sulphuric acid — H₂SO₄
The Element present in Sulphuric acid are:-
1. Hydrogen
2. Sulphur
3. Oxygen
4. Nitric acid — HNO₃
The Elements present in Nitric acid are:-
1. Hydrogen
2. Nitrogen
3. Oxygen

Q5. 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
Answer:
(a) Aluminum Chloride AlCl₃. Al has a valency of 3.
Here, Other radical is chloride – Cl^–. The Valency of chloride is 1.
(b) Aluminum oxide Al₂O₃,. Al has a valency of 3.
Here, Other radical is Oxide – O^2- . The valency of Oxide is 2.
(c) Aluminium nitride AlN. Al has a valency of 3.
Here, Other radical is Nitride N^3- . The valency of Nitride is 3.
(d) Aluminium sulphate Al₂(SO₄)3. Al has a valency of 3.
Here, Other radical is sulphate SO₄^2- The valency of sulphate is 2.

Q6. What is variable valency? Give two examples of elements showing variable valency.
Answer:

Q7. (a) What is a chemical equation?
(b) Why it is necessary to balance a chemical equation?
(c) What are the limitations of a chemical equation?
Answer:
(a) Chemical equations use the symbols and formulas of the substances involved in the process to symbolically express a chemical reaction.
(b) A chemical equation needs to be balanced in order for the number of atoms in the reactants and products to be equal.
(c) Below are the limitations of chemical equation:-
1. The physical states of the reactants and the result, such as whether they are solids, liquids, or gases, are not mentioned.
2. It is silent on the reactant and product concentrations.
3. It is silent on the length of time required to complete the reaction.
4. It doesn’t provide information on how quickly a reaction is happening.
5. It does not inform about the heat changes during the reaction i. e. whether the heat is given out or absorbed.
6. It does not inform about the conditions such as temperature, pressure, catalyst etc. which affect the reaction.
7. It does not inform about the nature of the reaction i.e. whether it is reversible or irreversible.

Q8. What are the ways by which a chemical equation can be made more informative?
Answer:
In the following methods, a chemical equation can provide additional information:
1. The letters (s), (l), (g), and (aq) can be used to denote the physical states of the reactants and products, respectively.
2. The addition or subtraction of a heat term on the product side might be used to indicate the evolution or absorption of heat during the reaction.
3. Above the arrow (or =) designating the separation between the reactants and products, temperature, pressure, and catalyst can be indicated.
4. The words (dil) for dilute and (cone) for concentrated are added before the formulas of the reactants and products, respectively, to indicate the concentration of each.
5. Information on reversible and irreversible reactions can be found by looking at the sign or.

Q9. State the law of conservation of mass.
Answer:
The law of conservation of mass asserts that in a chemical reaction, mass cannot be generated or destroyed.

There is neither creation nor destruction of matter during any change (physical or chemical). It may, however, transform from one form to another.Experimental Proof of the Mass Conservation Law Landolt’s tube, silver nitrate solution, sodium chloride solution, and other materials are needed. A specifically made H-shaped tube is used for the procedure. As depicted in the picture, sodium chloride solution is placed in one limb of the tube and silver nitrate solution in the other. Currently, both limbs have been sealed and weighed
Ag NO₃            +               NaCl                 →         Ag Cl↓          +        NaNO₃
(Silver nitrate)          (Sodium Chloride)            (Silver chloride)      (Sodium nitrate)
The tubes are now turned away to allow the liquids to mix and chemically react. Thus, this experiment clearly verifies the law of conservation of mass.

Q10. Differentiate between:
(a) Reactants and products
(b) A balanced and an unbalanced chemical equation
Answer:

Q11. Balance the following equations:
(a) N₂ + H₂ → NH₃
(b) H₂ + O₂ → H₂O
(c) Na₂O + H₂O → NaOH
(d) CO + O₂ → CO₂
(e) Zn + HCl → ZnCl₂ + H₂
Answer:
(a) N₂ + 3H₂ → 2NH₃
(b) 2H₂ + O₂ → 2H₂O
(c) Na₂O + H₂O → 2NaOH
(d) 2CO + O₂ → 2CO₂
(e) Zn + 2HCl → ZnCl₂ + H₂

Q12. Write balanced chemical equations for the following word equations:
(a) Iron + Chlorine → Iron (III) chloride
(b) Magnesium + dil sulphuric acid → Magnesium sulphate + water
(c) Magnesium + oxygen → Magnesium oxide
(d) Calcium oxide + water → Calcium hydroxide Products
(e) Sodium + chlorine → Sodium chloride
Answer:
(a) 2Fe + 3Cl₂ → 2FeCl₃
(b) 2Mg + 2H₂SO₄ → 2MgSO₄ + 2H₂
(c) 2Mg + O₂ → 2MgO
(d) CaO + H₂O → Ca (OH)₂
(e) 2Na + Cl₂ → 2NaCl
(e) Cl₂ + 2Na = 2NaCl

Q13. What information do you get from the following chemical equation?
Zn(s) + 2HCl (dil) → ZnCl₂ (aq) + H₂(g)
Answer:
This gives zinc chloride and hydrogen. The word equation is:
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
Formula for the products are ZnCl₂ and H₂