Class-8 icse Language of Chemistry Dalal New Simplified Dr Viraf J Dalal Solutions. Chapter-5 EXERCISE-5 -

Language of Chemistry Class-8 Dalal ICSE New Simplified Dr Viraf J Dalal Middle School Allied Publishers Solutions. Chapter-5

EXERCISE-5

Question 1.
Explain the term ‘symbol’. State a reason why – the symbol of calcium is ‘Ca’ & of copper is ‘Cu’
Answer 1:

Symbol is a short form for the name of an element. It is the first letter and another letter of the name of an element.

For calcium, ‘C’ is the first letter and ‘a’ is another letter, hence ‘Ca’ for calcium.

For copper, cuprum is the latin name of copper. ‘C’ is the first letter and ‘u’ is another letter, hence ‘Cu’

Question 2.
Define the term ‘valency’. With reference to water & ammonia as compounds respectively, state the valency of oxygen & nitrogen. Magnesium [2, 8, 2] has valency 2+. Give reasons.
Answer 2:

Valency is the number of particular atoms combined with or displaced with another atom to form a compound.

Water which is H $_2$ O, two atoms of hydrogen combine with one atom of oxygen. The valency of oxygen is 2, because it needs two atoms of hydrogen to form water.

Ammonia which is NH $_3$ , One atom of nitrogen combines with 3 atoms of hydrogen. The valency of nitrogen is 3 because it needs 3 atoms of hydrogen to form ammonia.

Magnesium has valency equal to 2⁺ because the electronic configuration of Mg is [2,8,2]. The nearest noble gas to magnesium is neon with electronic configuration of [2,8], to achieve this stable electronic configuration Mg can lose 2 valence electrons, hence its valency is 2⁺

Question 3.

Explain the term ‘variable valency’. Copper having electronic configuration 2,8,18,1 exhibits variable valency. Give a reason for the same & name the compound CuCI & CuCl₂.
Answer 3:
When an element does not have a specific valency, its valency is called variable valency. Copper has valency ‘1’ and ‘2’ that is the number of electrons lost or gained is 1 or 2 in different compounds..

Cu [I] Cl i.e. Copper [I] chloride

CuCl₂ – Cupric chloride (higher valency) i.e. Copper [II] chloride Cu[II]Cl₂

Question 4.
State the valencies of the following metallic elements –

(a) Potassium
(b) Sodium
(c) Calcium
(d) Magnesium
(e) Zinc
(f) Aluminium
(g) Chromium [write each symbol with the valency]

Answer 4:
Metals have positive valency 1, 2 or 3 ,
Valency of

(a) Potassium is 1⁺= K⁺
(b) Sodium 1⁺ = Na⁺
(c) Calcium 2⁺= Ca²⁺
(d) Magnesium 2⁺ = Mg²⁺
(e) Zinc 2⁺= Zn²⁺
(f) Aluminium 3⁺= Al³⁺
(g) Chromium 3+ = Cr³⁺

Question 5.
Certain metals exhibit variable valencies which include valencies: 1+, 2+, 3+ and 4+.
State the variable valency of the following metals –

(a) Copper
(b) Silver
(c) Mercury
(d) Iron
(e) tin
(f) Lead

[write each symbol with the variable valency]

Answer 5:

(a) Copper: Valency = 1 and 2,

is cuprous and $^{+}$ is cupric

(b) Silver: Valency is 1 and 2,

$^{+}$ is argentous and $^{+}$ is argentic

is mercurous and is mercuric.

(d) Iron: Valency is 2 and 3,

is ferrous and is ferric.

(e) Tin: Valency is 2 and 4,

is stanous and is stanic.

(f) Lead: Valency is 2 and 4,

is plumbous and and plumbic.

Question 6.

State which of the following ions or radicals given below of non-metallic elements exhibit-valency: 1^–, 2^– & 3^–
(a) Chloride

(b) Bromide

(c) Iodide

(d) Nitrate

(e) Hydroxide

(f) Bicarbonate

(g) Bisulphite

(h) Bisulphate

(i) Aluminate

(j) Permanganate

(k) Oxide

(l) Sulphide

(m) Sulphite

(n) Sulphate

(o) Carbonate

(p) Dichromate

(q) Zincate

(r) Plumbite

(s) Phosphate

(t) Nitride
[write each ion or radical with the correct valency]
Answer 6:

Question 7.
Differentiate between the terms – ‘Ion’ & ‘radical’ with suitable examples.
Answer 7:

An atom or group of atoms which carry a positive or negative charge due to loss or gain of electrons are called ions. Positive ions are cation Na⁺^– and negative ions are anion Cl^1-

A group of atoms of elements that behave like a single unit and have charge /valency are called radicals.

Positive radical [NH₄¹⁺ ] Ammonium
Negative radical [NO₃^1-] Nitrate

An Ion “is an atom or a group of atoms carrying a positive or negative charge due to loss or gain of electrons.”

Question 8.

Write the chemical formula of the following compounds in a step-by-step manner – (a) Potassium chloride

(b) Sodium bromide

(c) Potassium nitrate

(d) Calcium hydroxide

(e) Calcium bicarbonate

(f) Sodium bisulphate

(g) Potassium sulphate

(h) Zinc hydroxide

(i) Potassium permanganate

(j) Potassium dichromate

(k) Aluminium hydroxide

(l) Magnesium nitride

(m) Sodium zincate

(n) Copper [II] oxide

(o) Copper [I] sulphide

(p) Iron [III] chloride

(q) Iron [II] hydroxide

(r) Iron [III] sulphide

(s) Iron [III] oxide.
Answer 8:

(a) Potassium Chloride:

	Symbol	Valency
Potassium	K	1+
Chlorine	Cl	1-

Positive ions are written first

On interchanging the valencies

Formula = $K C l$

(b) Sodium Bromide

	Symbol	Valency
Sodium	Na	1+
Bromide	Br	1-

Positive ions are written first

On interchanging the valencies

Formula =

(c) Potassium Nitrate

	Symbol	Valency
Potassium	K	1+
Nitrate	NO3	1-

Positive ions are written first

On interchanging the valencies

Formula = K(NO3)

(d) Calcium hydroxide

	Symbol	Valency
Calcium	Ca	2+
Hydroxide	OH	1-

Positive ions are written first

On interchanging the valencies

Formula = $C a [O H]_{2}$

(e) Calcium bicarbonate

	Symbol	Valency
Calcium	Ca	2+
Bicarbonate	[[> widget id=”10″ <]]	1-

Positive ions are written first

On interchanging the valencies

Formula = $NO_3$

(f) Sodium Bisulphate

	Symbol	Valency
Sodium	Na	1+
Bisulphate	HSO4	1-

Positive ions are written first

On interchanging the valencies

Formula = Na(HSO4)

(g) Potassium Sulphate

	Symbol	Valency
Potassium	K	1+
Sulphate	SO4	2-

Positive ions are written first

On interchanging the valencies

Formula = K2(SO4)

(h) Zinc Hydroxide

	Symbol	Valency
Zinc	Zn	2+
Hydroxide	OH	1-

Positive ions are written first

On interchanging the valencies

Formula = $Z n [O H]_{2}$

(i) Potassium permanganate

	Symbol	Valency
Potassium	K	1+
Permanganate	MnO4	1-

Formula = $K [M n O_{4}]$

(j) Potassium dichromate

	Symbol	Valency
Potassium	K	1+
Dichromate	Cr2O7	2-

Positive ions are written first

On interchanging the valencies

Formula= $K_{2} [C r_{2} O_{7}]$

(k) Aluminium hydroxide

	Symbol	Valency
Aluminium	Al	3+
Hydroxide	[OH]	1-

Positive ions are written first

On interchanging the valencies

Formula = $A l [O H]_{3}$

(l) Magnesium nitride

	Symbol	Valency
Magnesium	Mg	2+
Nitride	N	3-

Positive ions are written first

On interchanging the valencies

Formula = $M g_{3} N_{2}$

(m) Sodium zincate

	Symbol	Valency
Sodium	Na	1+
Zincate	ZnO2	2-

Positive ions are written first

On interchanging the valencies

Formula = $N a_{2} [Z n O_{2}]$

(n) Copper [II] oxide

	Symbol	Valency
Copper	Cu	2+
Oxide	O	2-

Positive ions are written first

On interchanging the valencies

Formula = $C u O$

(o) Copper [I] sulphide

	Symbol	Valency
Copper	Cu	1+
Sulphide	S	2-

Positive ions are written first

On interchanging the valencies

Formula = $C u_{2} S$

(p) Iron [III] chloride

	Symbol	Valency
Iron	Fe	3+
Chloride	Cl	1-

Positive ions are written first

On interchanging the valencies

Formula = $F e C l_{3}$

(q) Iron [II] hydroxide

	Symbol	Valency
Iron	Fe	2+
Hydroxide	OH	1-

Positive ions are written first

On interchanging the valencies

Formula = $F e [O H]_{2}$

(r) Iron [III] sulphide

	Symbol	Valency
Iron	Fe	3+
Sulphide	S	2-

Positive ions are written first

On interchanging the valencies

Formula = $F e_{2} S_{3}$

(s) Iron [III] oxide

	Symbol	Valency
Iron	Fe	3+
Oxide	O	2-

Positive ions are written first

On interchanging the valencies

Formula = $F e_{2} O_{3}$

Question 9.

What’ is a chemical equation. How is it represented. Differentiate between a ‘word equation’ and a ‘molecular equation’ with a suitable example.
Answer 9:

A Chemical equation is a representation of a statement of reaction with symbol and formula

example,

Statement :-

Zinc reacts with dilute sulphuric acid to produce zinc sulphate and hydrogen gas.

Word equation:

Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen

Chemical Equation :

Question 10.
State the information provided by a chemical equation. Chemical equations suffer from a number of limitations. State the main limitations of a chemical equation.
Answer 10:
a chemical equation provide Information are below

(a) Temperature and pressure require for reaction
(b) It tells us the ratio in which substances reacts or are produced. If limitations are covered.
(c) a chemical equation also tells the physical state of substances i.e. solid, liquid, gas.
(d) Whether the reaction is endothermic or exothermic.
(e) catalyst used in reaction.
(f) If reaction is reversible or not.

(g) a chemical equation tells us the formulas and symbols of the reactants and products.

main limitations of a chemical equation.

Although a chemical reaction provide many information but some information is not present in a reaction

(a) Physical states of the reactants or products. But now we write along with substances (1) for liquids, (s) for solid and (g) for gas.
(b) Time taken for rection
(c) Concentration of reactants and products we use dil. for dilute and cone, for concentrated.
(d) Nature of the chemical reaction.
(e) Speed – reaction is fast or slow.
(f) Exothermic or endothermic we write + heat or – heat towards products for exothermic and endothermic.
(g) The completion of the reaction.

Question 11.
State what is a balanced equation with a relevant example. Give a reason why an equation is balanced with reference to the law of conservation of matter.
Answer 11:
When the number of atoms of each element of the reactant/reactants is equal to the number of atoms of the elements of the product/products is same, then the equation is said to be balanced
Example :
2Mg + O₂ → 2MgO
Reactants have 2 atoms of Mg and 2 atoms of oxygen.
Products have 2 atoms of magnesium and 2 atoms of oxygen.

Hence
∴ Number of atoms of each element of reactants = Number of atoms of each element of product.

Reason for balancing equation :

Law of conservation of matter says that “Matter is neither created nor destroyed during a chemical reaction.” This is possible only if number of atoms of each element on both sides of → are equal i.e. in reactants and also in products. Hence a reaction is balanced.

Question 12.
Write balanced molecular equations for the following word equations :

Calcium + Oxygen → Calcium oxide
Calcium + Water → Calcium hydroxide + Hydrogen
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Lead sulphate + Ammonium hydroxide → Ammonium Sulphate + Lead hydroxide
Copper hydroxide + Nitric acid → Copper nitrate + Water
Lead nitrate + Sodium Chloride → Sodium Nitrate + Lead Chloride

OBJECTIVE TYPE QUESTIONS

Language of Chemistry Class-8 Dalal ICSE New Simplified Dr Viraf J Dalal Middle School Allied Publishers Solutions. Chapter-5

Question-1.

Complete the statements given below by filling in the blank with the correct words.

1. The formula of silver [I] chloride is ___ [AgCl/AgCl₂].

2. The basic unit of an element is a/an ___ [molecule/atom/ion]

3. Atoms contains ___ [netron/nucleus/, with positively charged __ [electrons/protons].

4. Element ___ [calcium/lead/carbon] has the symbol derived from its Laltin name ‘plumbum’.

5. From the elements – He, Br, Pt & O; the element which forms a polyatomic molecule is ___ & which is liquid at room temperature is ___

6. The valency of iron in FeO is ___ [2⁺/1⁺] of chlorine [chloride] in CaCl₂ is ___ [1^–/2^–] and of dichromate in K₂ Cr₂ O₇ is [2⁺/2^–].
Answer-1:
1. The formula of silver [I] chloride is AgCl.

2. The basic unit of an element is a/an atom.

3. Atoms contains nucleus with positively charged protons.

4. Element lead has the symbol derived from its Latin name ‘plumbum’.

5. From the elements – He, Br, Pt & O; the element which forms a polyatomic molecule is O & which is liquid at room temperature is Br.

6. The valency of iron in FeO is 2⁺ of chlorine [chloride] in CaCl₂ is 1^– and of dichromate in K₂ Cr₂ O₇ is 2^–.

Question.-2.

Match the statements – 1 to 10 below with their correct answers from – A to J.

Answer-2:

Question-.3.

Match the compounds in List I – 1 to 20 with their correct formulas in List II – A to T.

Answer-3:

Question-.4.

Underline the incorrectly balanced compound in each equation & rewrite the correct equation.

1. 2Na + 3H₂0 → 2NaOH + H₂
Answer:
Correct equation is :
2Na + 2H₂0 → 2NaOH + H₂

2. 4P + 4O₂ → 2P₂O₅
Answer:
Correct equation is :
4P + 5O₂ → 2P₂O₅

3. FE₂O₃ + 2H₂ → 2Fe + 3H₂0
Answer:
Correct equation is :
FE₂O₃ + 3H₂ → 2Fe + 3H₂0

4. 2A1 + 2H₂SO₄→ A1₂(SO₄)3 + 3H₂
Answer:
Correct equation is :
2A1 + 3H₂SO₄ → A1₂(SO₄)3 + 3H₂

Answer:
Correct equation is :

6. ZnO + 3NaOH → NA₂ZnO₂ + H₂0
Answer:
Correct equation is :
ZnO + 2NaOH → NA₂ZnO₂ + H₂0

7. FeCl₃ + 3NH₄OH → 2NH₄C1 + Fe(OH)₃
Answer:
Correct equation is :

8. FeS + 2HCl → 2FeCl₂ + H₂S
Answer:
Correct equation is :
FeS + 2HCl → FeCl₂ + H₂S

9. 3NH₃ + H₂So₄ → (NH₄)₂So₄
Answer:
Correct equation is :
2NH₃ + H₂SO₄ → (NH₄)₂SO₄

10. PbO₂ + 4HCl → PbCl₂ + H₂0 + Cl₂
Answer:
Correct equation is :
PbO₂+ 4HCl → PbCl₂ + 2H₂0 + Cl₂