1.What are the factors that indicate chemical reaction?
(Chapter 1: Chemical Reactions and Equations)

Ans :
Following factors determine that a chemical reaction has taken place :

• change in state
• change in colour
• evolution of a gas
• change in temperature

2.Why should a magnesium ribbon be cleaned before burning in air?
(Chapter 1: Chemical Reactions and Equations)

Ans : Magnesium reacts with oxygen present in the air to form magnesium oxide.The layer of the magnesium oxide has to be cleaned before burning so that you get better results.

3. Explain combination reaction.
(Chapter 1: Chemical Reactions and Equations)

Ans : When two or more substances i.e. elements or compounds (reactants) form a single product it is called a combination reaction.
For example : Calcium oxide reacts with water to produce slaked lime (calcium hydroxide).
Chemical Equation is:

                                    CaO(s)     +   H2O(l) --------->   Ca(OH)2 (aq)
                                (quick lime)                                     (slaked lime)
                                

In the above reaction calcium oxide and water combine to form a single product, calcium hydroxide.
(i) Burning of coal C(s) + O₂(g) → CO₂(g)
(ii) Formation of water from H₂(g) and O₂(g) 2H₂(g) + O₂(g) → 2H₂O(l)

4. Explain exothermic chemical reactions
(Chapter 1: Chemical Reactions and Equations)

Ans : Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
Examples :
Burning of natural gas CH₄(g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

5. Explain Decomposition Reaction
(Chapter 1: Chemical Reactions and Equations)

Ans : When a single reactant breaks down to give simpler products (more than one).It is a decomposition reaction.
Ferrous sulphate crystals (FeSO₄, 7H₂O) lose water when heated and the colour of the crystals changes.
It then decomposes to ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃).
Ferric oxide is a solid, while SO₂ and SO₃ are gases.

                                                          Heat
                                    2FeSO4(s) ----->  Fe2O3(s)   + SO2(g) + SO3(g)
                                     (Ferrous sulphate)               (Ferric oxide)
                                

6.Decomposition of Lead Nitrate produces ____________ gases.
a) O₂ and CO₂
b) O₂ and NO₂
c) NO₂ and NO₃
d) NO and NO₂
(Chapter 1: Chemical Reactions and Equations)

Ans : b) O₂ and NO₂

7. Electrolysis of water is an example of a ____________ reaction
a) Combination
b) Double displacement
c) Decomposition
d) Displacement
(Chapter 1: Chemical Reactions and Equations)

Ans : c) Decomposition

8.With respect to hydrogen, differentiate between oxidation and reduction
(Chapter 1: Chemical Reactions and Equations)

Ans :

Oxidation	Reduction
If a substance loses hydrogen during a reaction, it is oxidised	If a substance gains hydrogen during a reaction, it is reduced

9.Why is it necessary to balance the chemical equation ?
(Chapter 1: Chemical Reactions and Equations)

Ans : The chemical equation has to be balanced so that the law of conservation of mass is not violated.
In the chemical equation the total mass of the elements present in the products has to be equal to the total mass of the elements present in the reactants.

10.Respiration is an example of
a) Exothermic reaction
b) Endothermic reaction
c) Displacement reaction
d) Precipitation reaction
(Chapter 1: Chemical Reactions and Equations)

Ans : a) Exothermic reaction

11.Brown fumes emitted when lead nitrate is heated are of
a) Nitrogen dioxide
b) Lead oxide
c) Lead
d) Nitric acid
(Chapter 1: Chemical Reactions and Equations)

Ans : a) Nitrogen dioxide

12.What is the difference between Combination Reaction and Decomposition Reaction ?
(Chapter 1: Chemical Reactions and Equations)

Ans :

Combination Reaction	Decomposition Reaction
In a combination reaction, two or more reactants combine to form a single product.	In a decomposition reaction, a single compound breaks down into two or more simpler substances.
Its general form is : A + B → AB	Its general form is : AB → A + B
An example of combination reaction is : Formation of water (2H₂ + O₂ → 2H₂O), where hydrogen and oxygen combine to form water.	An exmaple of decomposition reaction is : Decomposition of water (2H₂O → 2H₂ + O₂), where water breaks down into hydrogen and oxygen gases.
Combination reactions involve the coming together of substances to create a new compound. They often release energy in the form of heat and light, making them exothermic reactions.	Decomposition reactions involve the breakdown of a compound into its constituent elements or simpler compounds. They usually require an input of energy (heat, light, electricity) to proceed, making them endothermic reactions.

13.What is corrosion?
(Chapter 1: Chemical Reactions and Equations)

Ans :
Corrosion is the process through which metals, such as iron, gradually deteriorate when exposed to substances in their environment like moisture and acids. This results in the formation of undesirable coatings on the metal's surface, as seen in the reddish-brown rust on iron, tarnishing on copper and silver, and other similar effects. The corrosion process leads to damage in various metal objects, including car bodies, bridges, ships, and iron railings, which require significant financial investment for repair and replacement.

14.What is Rancidity?
(Chapter 1: Chemical Reactions and Equations)

Ans :
Rancidity is the process in which fats and oils undergo oxidation, leading to changes in their taste and smell, making them unpleasant to consume. Antioxidants are often added to prevent this process, and storing food in airtight containers or using methods like flushing with nitrogen helps slow down oxidation.

15.Why do chips manufacturers use nitrogen gas?
(Chapter 1: Chemical Reactions and Equations)

Ans :
Chips manufacturers use nitrogen gas to prevent the chips from getting oxidized. By flushing bags of chips with nitrogen, manufacturers create an oxygen-free environment inside the bag. This helps to inhibit the oxidation process that leads to rancidity and helps maintain the freshness and quality of the chips for a longer period.

16.Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans :
(a) Hydrogen gas combines with nitrogen to form ammonia:

---

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide:

---

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate:

---

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas:

17.Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl
(Chapter 1: Chemical Reactions and Equations)

Ans :
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Balance chemical reaction is

---

(b) NaOH + H2SO4 → Na2SO4 + H2O

---

(c) NaCl + AgNO3 → AgCl + NaNO3

---

(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

18.Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
(Chapter 1: Chemical Reactions and Equations)

Ans :
Here is a balanced chemical equation for the reactions
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

---

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

---

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

---

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

19.Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
(Chapter 1: Chemical Reactions and Equations)

Ans :
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

20. What does one mean by exothermic and endothermic reactions? Give examples.
(Chapter 1: Chemical Reactions and Equations)

Ans :
In an exothermic reaction, energy is released in the form of heat or light during the course of the reaction.Exothermic reactions often feel warm to the touch.
Example of exothermic reaction is:

---

In an endothermic reaction, energy is absorbed from the surroundings, causing the temperature of the surroundings to decrease.
Example of endothermic reaction is :

21.What is the balanced chemical equation for the reaction between quicklime (calcium oxide) and water?
a) CaO + H2O → Ca(OH)2
b) Ca(OH)2 + H2O → CaO
c) CaO + H2O → CaCO3
d) CaCO3 + H2O → CaO + CO2
(Chapter 1: Chemical Reactions and Equations)

Ans : a) CaO + H2O → Ca(OH)2

22.Define a double displacement reaction.
(Chapter 1: Chemical Reactions and Equations)

Ans :
A double displacement reaction is a type of chemical reaction in which the positive and negative ions of two compounds exchange places, resulting in the formation of two new compounds.
Example of a double displacement reaction:

23.CuO + H₂ → Cu + H₂O
This chemical reaction is an example of
a) Exothermic
b) Redox
c) Decomposition
d) Neutralization

Ans : b) Redox