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Exemplar Questions and Solutions

Important CBSE Questions – Long Answer Questions

1) What is rancidity? Write different methods of preventing rancidity.

Ans) Foods containing fats, oil or butter start to impart foul smell and change in taste due to oxidation is called rancidity

METHODS TO PREVENT RANCIDITY:

(i) By filling nitrogen in the food storage bags.

(ii) The food materials should be placed at very low temperatures.
(iii) Addition of antioxidants also prevents rancidity process.

(iv) Keeping food item in air tight containers

2) Name the type of reactions in the following cases:

a. Garbage producing foul smell

b. Burning of natural gas.

c. Carbon dioxide gas passed through lime water.

Ans) a)Garbage producing foul smell is a decomposition reaction wherein organic wastes are decomposed by microorganisms to produce methane gas (CH₄) and Hydrogen sulphide(H₂S) gas. It is an exothermic reaction.

b) Burning of natural gas is an oxidation reaction and is also an exothermic reaction.

c) Carbon dioxide gas passed through lime water- When carbon dioxide gas is passed through lime water, it turns milky due to the formation of calcium carbonate(white precipitate) which is insoluble in water. So it is a precipitation reaction and a combination reaction. The reaction is exothermic

3)What are the different kinds of decomposition reaction?

Ans) The different kinds of decomposition reactions are-

a) Thermal decomposition– When a decomposition reaction is carried out by heating, it is called ‘thermal decomposition’.

Example: Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction

b)Electrolytic decomposition- When a decomposition reaction is carried out by using electric current, it is called electrolytic decomposition.

Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.

This decomposition reaction takes place by the action of electricity. It is called electrolysis of water.

c)Photolytic decomposition- When a decomposition reaction is carried out by using light energy, it is called photolytic decomposition

Example: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

4)A light sensitive compound X of silver is used in photography. Once exposed to sunlight, the colourof the Compound X changes to grey.

a)Identify X

b)Write a chemical equation to express the above change

c) Identify the type of chemical reaction

Ans) The light sensitive compound X is Silver bromide

c) It is a clear example of photolytic decomposition reaction as it involves the decomposition of Silver bromide using light

5)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 sulphur dioxide.

c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and precipitate of barium sulphate.

6)List three effects of oxidation in our daily life. Are these effects useful or harmful. Justify:

Ans)The three effects of oxidation that takes place in our daily life are-

a) Corrosion- Corrosion is the atmospheric oxidation of metals. That means that oxygen combines with the metal and forms a new layer. Corrosion can be useful as well as harmful. Rusting of iron makes iron weak and can cause hazards when used in building constructions, ship building etc. But, metals like Aluminium, Chromium, Zinc etc corrode more readily than iron. But their oxides form a coating on the metal that in turn protects the metal from further attack.

b) Combustion of fuels- This is useful and is caused due to oxidation of fuels. They provide us huge amounts of heat energy.

Rancidity- It is caused by the oxidation of foods rich in fats and oils. They make the food inedible, so is considered to be harmful.

7) What are the important rules for writing word equations?

Ans) a) Always write the names of the reactants on the left hand side with a plus sign(+) in between the reactants(if there are more than one reactant).

b) Always write the names of the products on the right hand side with a plus sign(+) in between the products(if there are more than one product).

c) Put an arrow mark between the reactants and products. The arrowhead points towards the products, and shows the direction of the reaction.

For example,

When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.

The word-equation for the above reaction would be –

8)What is a chemical equation and what are the key features of a chemical equation?

Ans) A chemical equation is a symbolic representation of a chemical reaction using the chemical formulae of the reactants and the products involved in the reaction instead of using words. Chemical equations are a way of describing a chemical reaction.

Features of a chemical equation:
For example,

Here, A and B are the reactants and C and D are the products.

The reactants are written on the left side of the arrow and the products are written on the right side.
The arrow shows the direction of the reaction (the direction of change from reactants to products).
The optimum conditions that are required for the reaction to take place are generally written above or below the arrow mark.
The number of atoms of each element involved in the reaction should remain constant.
The number of molecules of each chemical is represented by a numerical coefficient, which should be a whole number.
An arrow pointing downward indicates the formation of a product in the form of a precipitate (↓) and an arrow pointing upward shows the release of a gaseous product (↑).
The physical state of the chemicals are written near the formula by using short forms such as (s) ––– solid ; (l) ––– liquid ; (g)––– gas or vapor and (aq. ) ––– aqueous solution.

9)What is meant by a balanced chemical equation? What are the features of a balanced chemical equation?

A chemical equation in which the number of atoms of each element are same on the side of reactants and products, is called a balanced chemical equation

Features of a balanced chemical equation:

a)Balanced chemical equations give us an idea about the substances taking part in a chemical reaction (those being used up) and the products formed (those being made).

b)The coefficients of a balanced equation tell us in what ratio the substances react or are produced.

c)It also gives us the number of atoms, molecules or ions of the various reactants and products.

d)We can determine the actual masses of the reactants required in a chemical reaction from a balanced chemical equation. We can also estimate the expected amounts of the products weights of the reactants and products involved in the chemical reaction.

10)Give any three limitations of a chemical equation?

A chemical equation does not give us any information about the velocity (fast or slow) of the reaction.
The nature of the reaction
Some reactions occur with explosion. This is not indicated by the chemical equation.
It does not give any information about the completion of the reaction(whether the reaction is complete or incomplete)

When a small piece of sodium metal is added to beaker containing water, it catches fire and the solution in the beaker becomes warm. What does this indicate?

Ans) Sodium metal reacts violently with water to form a colourless basic solution of sodium hydroxide and hydrogen gas along with the release of large amounts of energy in the form of heat. This heat is enough to cause hydrogen gas to catch fire. The formation of gaseous product(hydrogen) and the release of heat energy(Exothermic reaction) Which makes the solution in the beaker warm clearly indicate that a chemical reaction has taken place.

11)What are the rules that we need to follow while writing balancing a chemical equation?

Ans) The following rules are to be strictly followed while balancing a chemical equation:

While balancing a chemical equation, we should never alter the formulae of the reactants and products.
The formulae of the reactants and products should be in their molecular form and not in the atomic form
For eg.,
KClO3 → KCl + 3O
This equation is an atomic equation as oxygen is represented in the form of atoms and not molecules. The correct or the molecular form is
2KCLO₃ → 2KCl + 3O₂
Appropriate numbers must be written before the formulae of the reactants and products.
A chemical equation should be balanced by using the smallest possible numbers as coefficients of reactants and products.

When silver is kept open in air that contains even traces of hydrogen sulphide, it gets tarnished or turns black. Silver reacts readily with all sulphur-containing compounds like the white of egg (which has sulphur-containing proteins) to form black silver sulphide . The equation of blackening/tarnishing of silver ornaments is represented by

Ag(s) + H₂S(g) + O₂ (g) → Ag₂S(s) + H₂O(l). Balance the given equation.

Ans) The skeletal equation of blackening/tarnishing of silver ornaments is

Ag(s) + H₂S(g) + O₂ (g) → Ag₂S(s) + H₂O(l).

First, balance Ag atoms

2Ag(s) + H₂S(g) + O₂(g) → Ag₂S(s) + H₂O(l)

Balance oxygen atoms by placing the coefficient 2 before H₂O

2Ag(s) + H₂S(g) + O₂(g) → Ag₂S(s) + 2H₂O(l)

Balance hydrogen atoms by placing the coefficient 2 before H₂S

2Ag(s) +2H₂S(g) + O₂(g) → Ag₂S(s) + 2H₂O(l)

Balance sulphur atoms by placing the coefficient 2 before Ag₂S

2Ag(s) +2H₂S(g) + O₂(g) → 2Ag₂S(s) + 2H₂O(l)

Again balance the silver atoms by replacing the coefficient 2 before Ag with the coefficient 4

4Ag(s) +2H₂S(g) + O₂(g) → 2Ag₂S(s) + 2H₂O(l)

This is now a completely balanced equation.

12)In photosynthesis, carbon dioxide gas and water are taken up by plants to form glucose and oxygen gas is liberated. Write a chemical equation to represent this statement and balance it(Formula of glucose is C₆H₁₂O₆).

Ans) The skeletal equation for the process of photosynthesis is-

CO₂(g) + H₂O(l) → C₆H₁₂O₆(aq) + O₂(g)

Let us first balance the carbon atoms. For that let us place 6 before carbon di oxide as glucose(C₆H₁₂O₆) contains 6 carbon while CO₂ contains only one carbon atom

6 CO₂(g) + H₂O(l) → C₆H₁₂O₆(aq) + O₂(g)

Now that carbon is balanced, let us try to balance the hydrogen atoms. As we know glucose contains 12 hydrogen atoms while water(H₂O) has only 2. So by placing the coefficient 6 before water molecules, we can balance the hydrogen atoms.

6 CO₂(g) + 6 H₂O(l) → C₆H₁₂O₆(aq) + O₂(g)

Let us now balance the oxygen atoms, we can find that there are 18 oxygen atoms on the LHS(reactants, ie., 6x 2 + 6= 18) while RHS has only 8 oxygen atoms, so by putting the coefficient 6 in front of O₂(RHS) we get,

6 CO₂(g) + 6 H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

Now, we can say that the above equation is balanced as we have equal number of C, H, and O atoms on LHS and RHS.

13)Write the difference between exothermic and endothermic:

Exothermic reaction	Endothermic reaction
an exothermic reaction expels heat	an endothermic reaction pulls heat into an object or area
Exothermic Reactions make their surroundings hotter	Endothermic Reactions Cool their Surroundings
Products of exothermic reactions have less energy than the reactants	Products of endothermic reactions have more energy than the reactants
Exothermic reactions are spontaneous reactions	Endothermic reactions are comparatively slower
Rusting of iron, combustion, formation of ice cubes, respiration, etc.	photosynthesis, evaporation, cooking of an egg, baking bread, etc

14)Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating.

Ans) When lead(II) nitrate is heated, it decomposes into lead (II) oxide, nitrogen dioxide and oxygen. The balanced chemical equation for this reaction is

Activity– Take about 3 g of solid lead nitrate in a boiling tube. Note the colour of the compound(Colourless). Heat it in the flame of the Bunsen burner. Observe the change taking place.

Observation– A pungent smelling brown gas (NO₂) starts evolving from it and a brown residue (PbO) is left in the test tube.

Inference– Here, lead nitrate breaks down or decomposes to form three new substances (lead (II) oxide, nitrogen dioxide and oxygen). Hence, it is an example of decomposition reactions. Decomposition reactions often involve an energy source such as heat, light, or electricity that breaks apart the bonds of compounds. In case of lead nitrate, heat is used. So, thermal decomposition has taken place.

15) Explain the trial and error method of balancing a chemical equation with an example?

Ans) In the trial and error method of balancing, there are no definite rules that need to be followed. This method is applicable for simple chemical reaction. Some simple tips that are to be kept in mind while balancing are-

Determining the reactants and products in a reaction.
Writing down the symbols and formulae of the reactant and products in the form of a skeletal equation.
Any elementary gas (O₂, H₂, N₂ ,F₂ and Cl₂) appearing on either side of the skeletal equation, is written in the atomic state.
Counting the number of atoms of each on both sides.
Selecting the elements that occur for the least number of times in the equation.
Changing the coefficient of the molecules of reactants or products as required.
Atoms of elementary gases are balanced at the last.
When the balancing is complete, the equation is converted to the molecular form by multiplying the whole equation by 2.

16)Balance the chemical equation, Hydrogen and oxygen React to form water using it and trial method

Ans)

Determining the reactants and products in a reaction. Here Hydrogen and oxygen are reactants. Water is the product.
Skeletal equation.

H₂ +O₂ → H₂O

Start counting the number of atoms each on both sides. We have 2 hydrogen atoms on RHS and LHS whereas we have 2 oxygen atoms on the LHS and one on the RHS.
Start balancing O atoms. LHS contains 2 and RHS has 1. So, Oxygen atoms need to be balanced.
Changing the coefficient of the molecules of reactants or products as required. Balance the oxygen atoms by placing coefficient 2 in front of H₂O.
Now there are 2 oxygen atoms on the reactant side and 2 oxygen atoms on the products side.
H₂ +O₂ → 2 H₂O
But the number of hydrogen atoms are not balanced now. To balance hydrogen atoms place the coefficient 2 on the left-hand side of hydrogen in reactants side.
2H₂ +O₂ → 2 H₂O
Thus, the atoms of each element on the left and right hand side of the equations are balanced according to the law of conservation of mass

17)Balance the following chemical equations and identify the type of chemical reaction:

a)Al(s) + Cl₂(g) → AlCl₃(s)

Ans) 2Al(s) + 3Cl₂(g) → 2AlCl₃(s). It is a combination reaction

Ans)

This is an example of thermal decomposition reaction

c)CaO(s) + SiO₂(s) → CaSiO₃(s)

Ans) CaO(s) + SiO₂(s) → CaSiO₃(s). This is a balanced equation and this reaction is an example of combination reaction

Ans)

This is an example of photolytic decomposition reaction.

Ans)

This is a balanced reaction. It is an example of double displacement reaction and precipitation reaction.

18) On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed

a) Write a balanced chemical equation of the reaction.

b) Identify the brown gas X evolved.

Ans) The brown gas ‘X’ is nitrogen dioxide (NO₂)

c) Identify the type of reaction.

Ans) The above reaction is a thermal decomposition reaction as decomposition of copper nitrate takes place due to heating.

d) What could be the pH range of aqueous solution of the gas X?

Ans) Nitrogen dioxide dissolves in water to form an acidic solution as oxides of non-metals are acidic. Hence, an aqueous solution of this gas would be acidic with a pH of less than 7.

19) Give the characteristic tests for the following gases:

a) CO₂

Ans) The specific characteristic test for CO2 is to bubble the test gas through limewater(calcium hydroxide) solution. When carbon dioxide is bubbled through limewater, a solid precipitate of

calcium carbonate(limestone) is formed and this makes the lime water milky.

Calcium hydroxide + carbon dioxide → calcium carbonate + water.

Ca(OH)_2(aq) + CO_2(g) → CaCO_3(s) + H₂O_(l)

Carbon dioxide turns lime water milky as it forms calcium carbonate with lime water. This is the characteristics test for carbon dioxide.

b) SO₂

Ans) Sulphur dioxide gas turns moist litmus paper from blue to red. Sulphur dioxide gas will put out a lit splint. But, the specific test for SO₂ is that it turns acidified potassium dichromate(VI) solution from orange to green.

c) O₂

Ans) When a glowing wooden splint is introduced into in a test tube containing a sample of the gas. The glowing splint re-ignites to a flame if the sample of gas is oxygen. This is the characteristic test for oxygen gas.

d) H₂

Ans) When we introduce the light of a wooden splint, but do not blow out the flame into the mouth of the test tube containing a sample of the gas. If the gas is hydrogen, you will hear a slight explosive “pop” sound. When a burnt candle or match stick, is introduced into in a test tube of hydrogen, it burns with pop sound, which is the characteristic test for hydrogen gas.

2H₂(g) + O₂(g) → 2H₂O(l) + energy

20) What happens when a piece of

a) Zinc metal is added to copper sulphate solution?

Ans) Zinc being more reactive than copper displaces copper from its solution and a solution of zinc sulphate and copper is formed. This reaction is an example of displacement reaction as Zn displaces Cu from CuSO₄ to form ZnSO₄

Zn(s) + CuSO₄(aq) → Cu(s) + ZnSO₄(aq)

b) Aluminium metal is added to dilute hydrochloric acid?

Ans) When aluminium metal is added to dilute HCl, it displaces hydrogen from the acid and forms aluminium chloride and hydrogen gas as Aluminium is more reactive than hydrogen.

2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g)

c) Silver metal is added to copper sulphate solution? Also, write the balanced chemical equation if the reaction occurs

Ans) When silver metal is added to copper sulphate solution, no reaction takes place because silver is less reactive than copper cannot displace copper from its salt solution.

Ag (s) + CuSO₄(aq) → No reaction

21) What happens when zinc granules are treated with dilute solution of H₂SO₄, HCl, HNO₃, NaCl and NaOH, also write the chemical equations if reaction occurs.

Ans) The reaction of Zn granules with

(a) Dilute H₂SO₄

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Zinc being more reactive than hydrogen displaces hydrogen from dilute acids(dil Sulphuric acid). It reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas is evolved. This is a displacement reaction of a non-metal by a metal.

(b) Dilute HCl

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Zinc being more reactive than hydrogen displaces hydrogen from dilute acids(dil hydrochloric acid). It reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas is evolved. This is a displacement reaction

Zinc reacts with dilute nitric acid to give zinc nitrate and hydrogen gas.

Zn(s) + 2HNO₃(aq) → Zn(NO₃)₂(aq) + H₂(g)

(d) NaCl solution

Zn(s) + NaCl (aq) → No reaction

As Zinc is less reactive than sodium, no reaction takes place between them

(e) NaOH solution

Zinc reacts with sodium hydroxide to produce zincate sodium and hydrogen. Heat (near 550°C) is required to catalyse the reaction.

Sodium Zincate is only formed only if NaOH is provided in excess.

Zn+2NaOH → Zn(OH)2+2Na

22) On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained.

(a) Write a balanced chemical equation of the reaction involved

Ans) BaCl₂ + Na₂SO₃ → BaSO₃ + 2NaCl

(b) What other name can be given to this precipitation reaction?

Ans) The other name that can be given to this precipitation reaction is double displacement reaction.

Ans) Barium chloride reacts with sodium sulphite and forms barium sulphite(white precipitate). When dilute hydrochloric acid is added to Barium Sulphite(BaSO₃), it decomposes to produce Barium chloride, sulphur dioxide gas and water.

BaSO₃(s) + 2HCl (aq) → BaCl₂+ H₂O + SO₂(g)

White ppt.

As BaCl₂ is a soluble substance, hence the white precipitate of barium sulphite disappears.

23) You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO₃, ZnCl₂ and H₂O. In which of the above containers these solutions can be kept?

Ans) (A) When solutions are kept in copper container

(a) Dilute HCl

The metal copper is less reactive than hydrogen and they do not replace hydrogen from water or acid. Copper does not react with dilute HCl. Therefore, it can be kept.

(b) Dilute HNO₃

Nitric acid is an oxidizing acid that will dissolve most metals to form soluble metal nitrates. Thus, Copper is oxidized by dilute nitric acid, HNO₃, while the nitric acid gets reduced to nitrogen dioxide. If we are using concentrated nitric acid, HNO₃, and in excess then the ratio is 1:4 copper to nitric acid. If we are using dilute nitric acid, HNO₃, then the ratio is 3:8 copper to nitric acid.

4 HNO₃(conc) + Cu(s) → Cu(NO₃)2(s and aq) + 2 NO₂(g) + 2 H₂O(l)

3Cu+8HNO₃(dil) → 3Cu(NO₃)2+2NO + 4H₂O

Zinc is more reactive than copper (Cu) therefore, no reaction takes place. Hence, ZnCl₂ can be kept in copper vessels.

Cu(s) + ZnCl₂ (aq) → no reaction

Copper does not react with water. Therefore, water can be kept in copper vessels.

(B) When solutions are kept in aluminium containers

(a) Dilute HCl

Aluminium reacts with dilute HCl to form hydrogen gas and aqueous aluminum chloride. Therefore, dil HCl cannot be kept in aluminium containers.

2 Al(s) + 6HCl(aq) → 2 AlCl₃(aq) + 3 H₂(g)

(b) Dilute HNO₃

When dil HNO₃ is placed in aluminium containers, a thin protective oxide layer is formed around the metal surface as HNO₃ is an oxidising agent. This thin protective oxide layer does-not react with HNO₃ and henceforth prevents further reaction. Therefore, dil HNO₃ can be stored in aluminium containers.

Aluminium being more reactive than zinc can displace zinc ion from the solution. Therefore, ZnCl2 cannot be kept in aluminium containers.

2 Al + 3 ZnCl₂ → 2 AlCl₃ + 3Zn

(d) H₂O

Aluminium does not react with either cold or hot water. Therefore, water can be kept in aluminium containers.

But, aluminium can displace hydrogen from steam to form aluminium hydroxide and hydrogen gas