CHEMICAL REACTIONS AND EQUATIONS (CBSE 10TH CHEMISTRY QUESTIONS AND ANSWERS)

 

1.Why should a magnesium ribbon be cleaned before burning in air?

Answer:

• A magnesium ribbon should be cleaned before burning in air to remove the layer of magnesium oxide (MgO) that forms on its surface due to exposure to air.

• Magnesium reacts with oxygen in the air over time, forming a thin, dull coating of magnesium oxide.

• This oxide layer prevents or slows down the magnesium metal underneath from reacting efficiently when burned.

• Cleaning the ribbon (usually by scraping it with sandpaper) removes the oxide layer, exposing pure, shiny magnesium.

• The clean surface ensures that magnesium burns quickly and brightly, showing the characteristic intense white flame.

• Cleaning ensures a faster and more complete combustion.

2. Write the balanced equation for the following chemical reactions?

a.Hydrogen + Chloride----gives----Hydrogen chloride

b.Barium Chloride + Ammonium Sulphate----gives---Barium Sulphate + Ammonium Chloride

c.Sodium+Water----gives-----Sodium Hydroxide + Hydrogen

Answer

a.Hydrogen + Chlorine → Hydrogen chloride

Unbalanced:
H₂ + Cl₂ → HCl

Balanced:
H₂ + Cl₂ → 2HCl

b.Barium Chloride + Ammonium Sulphate → Barium Sulphate + Ammonium Chloride

Unbalanced:
BaCl₂ + (NH₄)₂SO₄ → BaSO₄ + NH₄Cl

Balanced:
BaCl₂ + (NH₄)₂SO₄ → BaSO₄ + 2NH₄Cl

c.Sodium + Water → Sodium Hydroxide + Hydrogen

Unbalanced:
Na + H₂O → NaOH + H₂

Balanced:
2Na + 2H₂O → 2NaOH + H₂

3. Write a balanced chemical equation with state symbols for the following reactions

i. Solution of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride

ii.Sodium hydroxide solution(in water) react with hydrochloric acid solution (in water) to produce sodium chloride solution and water

Answer:

i. Barium chloride solution reacts with sodium sulphate solution

Reaction:
Barium chloride (aq) + Sodium sulphate (aq) → Barium sulphate (s) + Sodium chloride (aq)

Balanced Equation with State Symbols:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

BaSO₄ precipitates as an insoluble solid.

ii. Sodium hydroxide solution reacts with hydrochloric acid solution

Reaction:
Sodium hydroxide (aq) + Hydrochloric acid (aq) → Sodium chloride (aq) + Water (l)

Balanced Equation with State Symbols:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

This is a neutralization reaction.

4. A solution of X is used for white washing 

i. Name the substance X and write it's formula

ii. Write the reaction of the substance X named in (i) above with water

Answer:

i. Name the substance X and write its formula

• The substance X used for whitewashing is quicklime.

• Name: Calcium oxide

• Formula: CaO

ii. Reaction of calcium oxide (X) with water

When calcium oxide reacts with water, it forms calcium hydroxide (slaked lime) and releases heat. This is an exothermic reaction.

Balanced Chemical Equation:
CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Calcium hydroxide forms a milky suspension in water and is used for whitewashing walls.

5.Why does copper sulphate solution change when an iron nail is dipped in it?

Answer:

Why the change occurs:

• Iron (Fe) is more reactive than copper (Cu).

• Therefore, iron displaces copper from copper sulphate (CuSO₄) solution.

• Chemical Reaction:

  Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

What changes you observe:

The blue colour of the CuSO₄ solution fades because:

• Copper(II) sulphate (CuSO₄) is blue.
• Iron(II) sulphate (FeSO₄) is pale green.
• A reddish-brown layer of copper metal forms on the iron nail.

Type of reaction: This is a displacement reaction and also a redox reaction (iron gets oxidized, copper gets reduced).

6. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, or electricity?

Answer:

Here are three decomposition reactions, each involving a different form of energy:

A.Decomposition by Heat (Thermal Decomposition)

Example:
Calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide.

Equation:
CaCO₃(s) → CaO(s) + CO₂(g)
(Heat is supplied)

B.Decomposition by Light (Photodecomposition)

Example:
Silver chloride decomposes in sunlight to form silver and chlorine gas.

Equation:
2AgCl(s) → 2Ag(s) + Cl₂(g)
(Light energy is supplied)

This reaction is used in photographic films.

C. Decomposition by Electricity (Electrolysis)

Example:
Electrolysis of water breaks it down into hydrogen and oxygen gases.

Equation:
2H₂O(l) → 2H₂(g) + O₂(g)
(Electric current is supplied)

7. What is the difference between displacement and double displacement reactions ? Write equations for these reactions.

Answer:

1. Displacement Reaction

Definition:
A displacement reaction is one in which a more reactive element displaces a less reactive element from its compound.

General form:
A + BC → AC + B

Example:
Zinc displaces copper from copper sulphate solution.

Equation:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Single element (Zn) replaces another element (Cu) from its compound.

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2. Double Displacement Reaction

Definition:
A double displacement reaction involves an exchange of ions between two compounds to form two new compounds.

General form:
AB + CD → AD + CB

Example:
Barium chloride reacts with sodium sulphate to form barium sulphate and sodium chloride.

Equation:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Cations and anions are exchanged between the reactants.

Key Difference Summary: 

Feature                                            Displacement Reaction            Double Displacement Reaction

Number of exchanges               One element displaces another            Two compounds exchange ions

Type of reactants                       Element + Compound               Compound + Compound

Example reaction                  Zn + CuSO₄ → ZnSO₄ + Cu      BaCl₂ + Na₂SO₄ →BaSO₄ + 2NaCl