Class 7 Science Chapter 4 Chemical and Chemical Changes Oxford Book Solutions

Page 59:

A. Fill in the blanks with the correct words.

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

B. Answer the following questions orally.

  1. 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).
  2. 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.
  3. 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).
  4. 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.
  5. How is common salt obtained?
    • Common salt is obtained through the process of evaporation of seawater or from rock salt deposits.

Page 61:

1. Objective type questions

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

B. Choose the correct option.

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

C. Match the following.

Column AColumn B
1. Calcium chlorideb. CaCl₂
2. Sulphuric acide. H₂SO₄
3. Nitric acida. HNO3
4. Potassium oxidef. K₂O
5. Calcium oxided. CaO
6. Calcium carbonatec. CaCO₃

II. Very short answer type questions

  1. Sulphur: S
  2. Phosphorous: P
  3. Hydrochloric acid: HCl
  4. Aluminium: Al
  5. Zinc oxide: ZnO

III. Short answer type questions

  1. Differentiate between an element, a compound, and a mixture. Give one relevant example for each.
    • Element: A substance made up of only one type of atom (e.g., Oxygen (O)).
    • Compound: A substance made up of two or more different elements chemically combined (e.g., Water (H₂O)).
    • Mixture: A combination of two or more substances that are not chemically bonded (e.g., Air).
  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)
  3. 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)
  4. 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.
  5. 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.

IV. Long answer type questions

CuSO₄ + Fe → FeSO₄ + Cu

Explain the process used for removing impurities from common salt.

The process used for removing impurities from common salt is evaporation. Sea water is evaporated to get the salt. After that, crystallization is used to purify the salt. Impurities like sand or dirt can be removed by dissolving the salt in water and then filtering the solution before crystallization.

Represent the following using chemical symbols/formulae.

a. Two atoms of the element neon: Ne₂

b. A molecule of the element nitrogen: N₂

c. A molecule of the compound carbon dioxide: CO₂

d. Three molecules of the compound water: 3H₂O

e. Four molecules of the compound magnesium oxide: 4MgO

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

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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