1. Why should a magnesium ribbon be cleaned before burning in the air—->

Solution:A magnesium ribbon is cleaned before burning to remove the thin white layer of magnesium oxide that forms when it reacts with oxygen in the air. This layer prevents magnesium from burning easily. By cleaning the ribbon, we remove this layer and allow the magnesium to burn quickly with a bright white flame.

2. Write a balanced equation for the following chemical reactions.
i) Hydrogen + Chloride —-> Hydrogen chloride
ii) Barium chloride + Aluminium sulphate —-> Barium sulphate + Aluminium chloride
iii) Sodium + Water —-> Sodium hydroxide + Hydrogen

solution-:

i) Hydrogen + Chlorine —-> Hydrogen chloride
H2 + Cl2 —-> 2HCl

ii) Barium chloride + Aluminium sulfate —-> Barium sulfate + Aluminium chloride
3BaCl2 + Al2(SO4)3 —-> 3BaSO4 + 2AlCl3

iii) Sodium + Water —-> Sodium hydroxide + Hydrogen
2Na + 2H2O —-> 2NaOH + H2

3. Write a balanced chemical equation with state symbols for the following reactions
i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and solution of Sodium chloride.
ii) Sodium hydroxide solution in water reacts with the hydrochloric acid solution to produce Sodium chloride solution and water.
Solution:

i) Solutions of Barium chloride and Sodium sulfate in water react to form insoluble Barium sulfate and a solution of Sodium chloride.
BaCl2 (aq) + Na2SO4 (aq) —-> BaSO4 (s) + 2NaCl (aq)

ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to form Sodium chloride solution and water.
NaOH (aq) + HCl (aq) —-> NaCl (aq) + H2O (l)

In-text questions set 2 Page number – 10

1. A solution of a substance, ‘X,’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Solution:

(i) The substance ‘X’ used for whitewashing is quicklime, also known as Calcium Oxide. Its formula is CaO.

(ii) When quicklime reacts with water, the reaction is:
CaO + H2O —-> Ca(OH)2 (This forms slaked lime, which is used in whitewashing).

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name this gas.

Solution:
In Activity 1.7, the amount of gas collected in one test tube is double the amount in the other because water breaks down into hydrogen (H2) and oxygen (O2) gases. During electrolysis, two molecules of hydrogen are produced for every one molecule of oxygen, so the amount of hydrogen gas collected is twice as much as the oxygen gas. The gases involved are hydrogen and oxygen.

In-text questions set 3 Page number – 13

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it—->

Solution:When an iron nail is dipped in copper sulfate solution, iron replaces copper in the solution because iron is more reactive than copper. As a result, the color of the copper sulfate solution changes. The reaction is:
Fe + CuSO4 —-> FeSO4 + Cu

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Solution:An example of a double displacement reaction, other than the one in Activity 1.10, is the reaction between silver nitrate (AgNO3) and sodium chloride (NaCl). In this reaction, the positive and negative ions switch places, resulting in the formation of a white silver chloride (AgCl) precipitate. The chemical reaction is:

AgNO3 + NaCl —-> AgCl + NaNO3

3. Identify the substances that are oxidised and that are reduced in the following equation.
i) 4Na(s) + O2(g) —-> 2Na2O(s)
ii) CuO(s) + H2(g) —-> Cu(s) + H2O(l)

Solution:
In simple words:

(i) In the first equation, Sodium (Na) is getting oxidized because it loses electrons when reacting with Oxygen (O2).

(ii) In the second equation, Copper (Cu) is getting reduced because it gains electrons from Hydrogen (H2).

Exercise Questions Page number – 14-16

1. Which of the statements about the reaction below are incorrect—->

2PbO(s) + C(s) —-> 2Pb(s) + CO2(g)

(a) Lead is getting reduced
(b) Carbon Dioxide is getting oxidised
(c) Carbon is getting oxidised
(d) Lead oxide is getting reduced
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all the above
Solution:
(i) (a) and (b)
Explanation: (a) Because Oxygen is being removed and (b) Because the removed oxygen from Lead is added to the elemental Carbon.

2. Fe2O3 + 2Al —-> Al2O3 + 2Fe

The above reaction is an example of a

1.Combination reaction
2.Double displacement reaction
3.Decomposition reaction
4.Displacement reaction

Solution:The correct answer is 4. Displacement reaction.

Explanation: In this reaction, Aluminum (Al) is more reactive than Iron (Fe), so aluminum displaces iron from its oxide (Fe2O3) and forms aluminum oxide (Al2O3). This is called a displacement reaction, where a more reactive metal replaces a less reactive metal from its compound. Since only one metal is being displaced, it is called a single displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron fillings—-> Tick the correct answer.

1. Hydrogen gas and Iron chloride are produced.
2. Chlorine gas and Iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Solution:
The correct answer is 1. Hydrogen gas and Iron chloride are produced.

Explanation: When dilute hydrochloric acid is added to iron filings, the iron reacts with the acid, and hydrogen gas is released, while iron chloride (FeCl2) is formed. The reaction is:

2HCl + Fe —-> FeCl2 + H2

4. What is a balanced chemical equation—-> Why should a chemical equation be balanced—->

Solution:A balanced chemical equation is one where the number of atoms of each element is the same on both sides of the equation (reactants and products). Balancing a chemical equation is important because it follows the Law of Conservation of Mass, which says that mass cannot be lost or gained in a chemical reaction. There’s no set method for balancing equations; it’s mostly done through trial and error.

5. Translate the following statements into chemical equations and balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in the 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.

Solution:
Here are the chemical equations translated and balanced in simpler words:

(a) Hydrogen gas combines with nitrogen to form ammonia.
Unbalanced: H2 + N2 —-> NH3
Balanced: 3H2 + N2 —-> 2NH3

(b) Hydrogen sulphide gas burns in the air to give water and sulphur dioxide.
Unbalanced: H2S + O2 —-> H2O + SO2
Balanced: 2H2S + 3O2 —-> 2H2O + 2SO2

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Unbalanced: BaCl2 + Al2(SO4)3 —-> AlCl3 + BaSO4
Balanced: 3BaCl2 + Al2(SO4)3 —-> 2AlCl3 + 3BaSO4

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Unbalanced: K + H2O —-> KOH + H2
Balanced: 2K + 2H2O —-> 2KOH + H2

6. 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 + HCl

Solution:

(a) 2HNO3 + Ca(OH)2 —-> Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 —-> Na2SO4 + 2H2O
(c) NaCl + AgNO3 —-> AgCl + NaNO3
(d) BaCl2 + H2SO4 —-> BaSO4 + 2HCl

7. Write the balanced chemical equation for the following reactions.

Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water

Zinc + Silver nitrate —-> Zinc nitrate + Silver

Aluminium + Copper chloride —-> Aluminium chloride + Copper

Barium chloride + Potassium sulphate —-> Barium sulphate + Potassium chloride

Solution:
Here are the balanced chemical equations in simple words:

1. Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water:
Ca(OH)2 + CO2 —-> CaCO3 + H2O

2. Zinc reacts with silver nitrate to form zinc nitrate and silver:
Zn + 2AgNO3 —-> Zn(NO3)2 + 2Ag

3. Aluminum reacts with copper chloride to form aluminum chloride and copper:
2Al + 3CuCl2 —-> 2AlCl3 + 3Cu

4. Barium chloride reacts with potassium sulfate to form barium sulfate and potassium chloride:
BaCl2 + K2SO4 —-> BaSO4 + 2KCl

8. Write a balanced chemical equation for the following and identify the type of reaction of each case.

KBr + BaI2 —-> KI + BaBr2

ZnCO3 —-> ZnO + CO2

H2 + Cl —-> HCl

Mg + HCl —-> MgCl2 + H2

Solution:
2KBr + BaI2 —-> 2KI + BaBr2 (Double Displacement Reaction)

ZnCO3 —-> ZnO + CO2 (Decomposition Reaction)

H2 + Cl —-> 2HCl (Combination Reaction)

Mg + 2HCl —-> MgCl2 + H2 (Displacement Reaction)

9. What is meant by exothermic and endothermic reactions—-> Give examples.

Solution: Endothermic reactions are the chemical reaction that absorb heat from their surroundings. This means they need energy to happen. Examples: Photosynthesis, melting of ice, and evaporation.

Exothermic reactions are the chemical reaction that release heat into their surroundings. This means they give off energy. Examples: Explosions, concrete hardening, and nuclear reactions (fission and fusion).

10. Why is respiration considered to be an exothermic reaction—->

Solution:Our body needs energy to survive, and we get this energy from the food we eat. During digestion, food is broken down into simpler substances like glucose. This glucose reacts with oxygen in our body cells, producing carbon dioxide, water, and energy.

Since this process releases heat (which helps maintain our body temperature), respiration is called an exothermic reaction.

The chemical reaction for respiration is:

C6H12O6 + 6O2 —-> 6CO2 + 6H2O + Energy

11. Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.

Solution:
In a decomposition reaction, a single compound breaks down into two or more simpler substances,
Whereas, in a combination reaction, two or more substances combine to form a single compound.

Decomposition reaction:
CaCO3 ? CaO + CO2

Combination reaction:
CaO + CO2 ? CaCO3

Decomposition reactions require energy input (heat, light, or electricity) to break bonds,
Whereas, combination reactions often release energy in the form of heat or light.

Decomposition reaction:
2H2O ? 2H2 + O2

Combination reaction:
2H2 + O2 ? 2H2O
This is why decomposition reaction is called opposite of combination reaction.

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

Solution:

1. By Heat:
CaCO3 ? CaO + CO2

2. By Light:
2AgCl ? 2Ag + Cl2

3. By Electricity:
2H2O ? 2H2 + O2

13. What is the difference between displacement and double displacement reactions—-> Write relevant equations for the above.

Solution:
In a displacement reaction, a more reactive element replaces a less reactive element from a compound.
Example: Zn + CuSO4 ? ZnSO4 + Cu
Whereas, in a double displacement reaction, two compounds exchange ions to form two new compounds.
Example: Na2SO4 + BaCl2 ? BaSO4 + 2NaCl

Displacement reactions involve one element and one compound,
Whereas, double displacement reactions involve two compounds reacting with each other.

14. In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.
Solution:
Cu(s) + 2AgNO3(aq) —-> Cu(NO3)2(aq) + 2Ag(s)

15. What do you mean by a precipitation reaction—-> Explain by giving examples.

Solution:When two solutions of soluble salts are mixed, a double displacement reaction occurs, where the ions swap places.

If one of the new compounds formed is insoluble in water, it settles at the bottom as a solid substance called a precipitate. This type of reaction is called a precipitation reaction.

Examples:
CdSO4(aq) + K2S(aq) —-> CdS(s) + K2SO4(aq)
(Cadmium sulfate reacts with potassium sulfide, forming cadmium sulfide as a precipitate.)

2NaOH(aq) + MgCl2(aq) —-> 2NaCl(aq) + Mg(OH)2(s)
(Sodium hydroxide reacts with magnesium chloride, forming magnesium hydroxide as a precipitate.)

16. Explain the following in terms of the gain of oxygen with two examples each.

(a) Oxidation
(b) Reduction

Solution: Oxidation and Reduction in Terms of Oxygen Gain/Loss

(a) Oxidation:
– When oxygen is added to an element in a chemical reaction, the element gets oxidized.
– Examples:
1. 4Na(s) + O2(g) —-> 2Na2O(s)
(Sodium reacts with oxygen to form sodium oxide.)
2. H2S + O2 —-> H2O + SO2
(Hydrogen sulfide gains oxygen and forms water and sulfur dioxide.)

(b) Reduction:
– When oxygen is removed from a compound, the element gets reduced.
– Examples:
1. CuO(s) + H2(g) —-> Cu(s) + H2O(l)
(Copper oxide loses oxygen and forms copper.)
2. 2HgO —-> 2Hg + O2
(Mercuric oxide loses oxygen and forms mercury.)

17. A shiny brown coloured element ‘X’ on heating in the air becomes black in colour. Name the element ‘X’ and the black-coloured compound formed.

Solution:The shiny brown-colored element is copper (Cu).
When copper is heated in air, it reacts with oxygen to form copper oxide (CuO), which is black in color.
Reaction:
2Cu(s) + O2(g) —-> 2CuO(s)

18) Why do we apply paint on iron articles—->

Solution:We paint iron articles to prevent rusting.
If iron is left unpainted, it reacts with oxygen and moisture in the air to form iron(III) oxide (rust).
When painted, the paint layer acts as a protective barrier, preventing the iron from coming into contact with air and moisture, thus stopping rusting.

19) Oil and Fat containing food items are flushed with Nitrogen. Why—->

Solution:Food items with oil and fat can spoil due to rancidity, which happens when they react with oxygen, causing a bad smell and taste.
To prevent this, food packets are flushed with nitrogen. Nitrogen is unreactive, so it creates a protective environment and stops the food from spoiling.

20) Explain the following terms with one example each.

(a) Corrosion
(b) Rancidity

Solution: Explanation of Terms with Examples

(a) Corrosion:
– Corrosion happens when a metal reacts with oxygen from the air and forms a more stable compound like metal oxide. This slowly damages the metal.
– Example: Rusting of iron, where iron reacts with oxygen and moisture to form iron oxide (rust).
– Every year, millions of dollars are spent to prevent rusting in bridges, buildings, and monuments.

(b) Rancidity:
– Rancidity occurs when oils and fats in food react with oxygen, causing a bad smell and taste.
– Example: Spoiled chips or fried food taste bad due to oxidation of the fats.
– Prevention: Storing food in a refrigerator slows down rancidity because low temperatures reduce oxidation.