Class 7 Science Chemical And Chemical Change-Oxford Solution

Class 7 Science Chemical and Chemical Change-Oxford

Page 59:

A. Fill in the blanks with the correct words.

Two or more chemical substances react to produce new substances in a (chemical) change.
Rusting being a (chemical) change, is permanent.
(Carbon dioxide) gas turns freshly prepared lime water milky.
(Iron oxide) is formed when copper sulphate reacts with iron.
(Crystals) are the purest solid form of a substance having a definite geometrical shape.

Class 7 Science Chemical and Chemical Change-Oxford-1 Mark

B. Answer the following questions orally.

What are the two essential conditions for rusting?
- The two essential conditions for rusting are the presence of water (or moisture) and oxygen (from the air).
How can rusting of iron objects be prevented?
- Rusting of iron can be prevented by coating the iron with a protective layer such as paint, oil, galvanization (zinc coating), or rust-resistant alloys.
Which gas is evolved in the reaction between baking soda and vinegar?
- The gas evolved in the reaction between baking soda and vinegar is carbon dioxide (CO₂).
Why blue-coloured copper sulphate solution acquires a green colour when iron filings are added to it?
- The green color occurs due to the formation of iron(II) sulphate and the reaction between iron and copper sulphate. This is a result of displacement reaction, where iron displaces copper from copper sulphate.
How is common salt obtained?
- Common salt is obtained through the process of evaporation of seawater or from rock salt deposits.

Class 7 Science Chemical and Chemical Change-Oxford Page 61: 1 mark

1. Objective type questions

(Compound) is formed by chemical combination of two or more elements.
(Hydrogen) is an element and (water) is a compound.
Chemical (symbols) are the abbreviations used to represent elements.
Chemical (formula) is the representation of a molecule of a compound using chemical symbols of its constituent elements.
(Valency) is the combining capacity of an element.
(Ni) is the chemical symbol of Nickel.

B. Choose the correct option.

Which out of the following cannot be broken down into simpler substances by chemical means?
- a. Element
What is the chemical symbol of iron?
- Fe

C. Match the following.

Column A	Column B
1. Calcium chloride	b. CaCl₂
2. Sulphuric acid	e. H₂SO₄
3. Nitric acid	a. HNO3
4. Potassium oxide	f. K₂O
5. Calcium oxide	d. CaO
6. Calcium carbonate	c. CaCO₃

II. Very short answer type questions

Sulphur: S
Phosphorous: P
Hydrochloric acid: HCl
Aluminium: Al
Zinc oxide: ZnO

Class 7 Science Chemical and Chemical Change-Oxford – 2 marks

III. Short answer type questions

1.Differentiate between an element, a compound, and a mixture. Give one relevant example for each.

Property	Element	Compound	Mixture

Definition

A pure substance made of only one type of atom.

A substance formed by combining two or more elements in fixed proportions.

A combination of two or more substances that can be separated by physical means.

Composition

Contains only one kind of atom.

Contains two or more elements chemically combined.

Contains different substances (elements or compounds) physically mixed.

Example

Oxygen (O2)

Water (H2O)

Air (a mixture of gases like oxygen, nitrogen, etc.)

Separation

Cannot be separated into simpler substances by physical methods.

Can be separated into elements by chemical methods.

Can be separated by physical methods (e.g., filtration, distillation).

2.What is atomicity? Give one example each for monoatomic, diatomic, and tetra-atomic elements.

Atomicity refers to the number of atoms present in a molecule of an element.
Monoatomic: Argon (Ar)
Diatomic: Oxygen (O₂)
Tetra-atomic: Phosphorus (P₄)

Name all the elements present in the following compounds: SO₂, PbCl₂, Ba(OH)₂, LiF
- SO₂: Sulphur (S), Oxygen (O)
- PbCl₂: Lead (Pb), Chlorine (Cl)
- Ba(OH)₂: Barium (Ba), Oxygen (O), Hydrogen (H)
- LiF: Lithium (Li), Fluorine (F)
Why do the cut surfaces of fruits and vegetables turn brown?
- The cut surfaces of fruits and vegetables turn brown due to oxidation. This is caused by the reaction of phenolic compounds in the cells with oxygen in the air, resulting in the formation of melanin, which is brown in color.
Write a short note on crystallization.
- Crystallization is the process of forming solid crystals from a solution. It is used to purify substances, where the solute is dissolved in a solvent and then allowed to form crystals upon cooling or evaporating the solvent. This process is commonly used to purify salt, sugar, and other chemicals.

Class 7 Science Chemical and Chemical Change-Oxford-3 marks

IV. Long answer type questions

Explain the process used for removing impurities from common salt.

Ans: Purification of Common Salt:

a. Crushing: The impure salt is crushed into smaller pieces.

b. Dissolution: The crushed salt is dissolved in water, separating soluble impurities.

c. Filtration: The solution is filtered to remove insoluble impurities (e.g., sand).

d. Evaporation: The water is evaporated, leaving behind pure salt crystals.

e. Sublimation: Any volatile impurities are removed by heating, leaving pure sodium chloride.

This process results in pure salt free from most impurities.

2. Represent the following using chemical symbols/formulae.

a. Two atoms of the element neon:

Ans: Ne₂

b. A molecule of the element nitrogen:

Ans: N₂

c. A molecule of the compound carbon dioxide:

Ans: CO₂

d. Three molecules of the compound water:

Ans: 3H₂O

e. Four molecules of the compound magnesium oxide:

Ans: 4MgO

3.Balance the following equations.

a. H₂ + O₂ → H₂O:

Balanced equation: 2H₂ + O₂ → 2H₂O

b. N₂ + H₂ → NH: This is an incomplete reaction.

The correct balanced equation is: N₂ + 3H₂ → 2NH₃ (Ammonia).

c. KClO₃ → KCl + O₂:

Balanced equation: 2KClO₃ → 2KCl + 3O₂

d. HCl + Mg(OH)₂ → MgCl₂ + H₂O:

Balanced equation: 2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O

e. K + H₂O → KOH + H₂:

Balanced equation: 2K + 2H₂O → 2KOH + H₂

4. Complete the following word equations. Give their balanced chemical equations also.

a. Magnesium + Oxygen → Magnesium oxide:

2Mg + O₂ → 2MgO

b. Iron + Oxygen + Water → Rust (Iron oxide):

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃

c. Vinegar + Baking soda → Carbon dioxide + Water + Sodium acetate:

CH₃COOH + NaHCO₃ → CO₂ + H₂O + CH₃COONa

d. Carbon dioxide + Lime water → Calcium carbonate:

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

e. Copper sulphate + Iron → Iron sulphate + Copper:

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