Tuesday 27 June 2017

CBSE Class 10 - Chemistry - 23 HOTS Questions on Chemical Reactions You Must Know - Q and A (#cbsenotes)

23 HOTS Questions on Chemical Reactions You Must Know

CBSE Class 10 - Chemistry -  23 HOTS Questions on Chemical Reactions You Must Know - Q and A (#cbsenotes)

Class 10 - Chemistry - Q & A

Q1: A white coloured water insoluble substance X on reacting with dilute H₂SO₄ released a colourless and odourless gas accompanied by brisk effervescence. When the gas was passed through water, the solution obtained turned blue litmus red. On bubbling the gas through lime water, it initially became milky and milkiness disappeared when the gas was passed in excess. Identify the substance X‘. Write its chemical equations of the reactions involved.

Answer: Calcium carbonate on reacting with dilute sulphuric acid releases a colourless and odourless gas carbon dioxide accompanied  by brisk effervescence.

The chemical equation for this reaction is
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

CO₂ (g) + CaCO₃ (s) + H₂O (l) ⇌ Ca(HCO₃)₂ (aq)

Q2: Ahmad took a magnesium ribbon (cleaned) and burned it on a flame. The white powder formed was taken in a test tube and water was added to it. He then tested the solution formed with red and blue litmus paper. What change was seen? Why?

Red litmus paper turned blue.
Blue litmus paper remained blue.

This is because the magnesium ribbon on burning in air forms the white magnesium oxide. MgO when dissolves in water, it forms magnesium hydroxide, which is basic in nature.

2Mg + O₂ → 2MgO

MgO + H₂O → Mg(OH)₂ (aq) + H₂(g)

Q3: Give one example of a combination reaction in which an element combines with a compound to give you a new compound.

SO₂ + O₂ → 2SO₃

8NH₃ + 3Cl₂ → 6NH₄Cl

Q4: A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer: Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.

2Cu + O₂ + Heat ⇨ 2CuO

Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide (CuO).

Q5: Marble statues often slowly get corroded when kept in open for a long time. Assign a suitable explanation.

Answer: The discoloration of marble is due to its corrosive reaction with gaseous oxides in air.  The gaseous  oxides ( CO₂, NO₂ and SO₂) present in air (chemical pollutants) mix with water in air and form acids like carbonic acid, nitric acid and sulphuric acid. These acid come down as acid rain and causes  corrosion of marble statues.

Calcium carbonate present in marble it transformed into other compounds by reaction acid as follow:-

CO₂ + H₂O + CaCO₃ → Ca(HCO₃)

CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O

Thus, the blackening of marble surface is due to the presence of carbonate, nitric and sulphuric acid.

Q6: An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green. Why? 

Answer: Iron is more reactive than copper. It displaces Cu from CuSO₄ solution and forms ferrous sulphate which is of Light Green Colour.

CuSO₄(aq) + Fe (s) → FeSO₄(aq) + Cu(s) 
Blue colour light green colour 

Q7:  A copper coin is kept in a solution of silver nitrate for some time. What will happen to the coin and the colour of the solution? 

Answer:  We know that copper is more reactive than silver, so it will displace silver from its salt solution:

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s) 
Blue colour

The solution will turn blue due to the formation of copper nitrate.

Q8: Mohan took pure water for the electrolytic decomposition of water but did not see any bubbles near the electrodes. Explain why?

Answer: Distilled water (pure water) does not contain any minerals. Electrolysis of pure water is very low because distilled water does not ionize to release ions. Hence he didn't see any bubbles near the electrodes.

To increase the conductivity of water a few drops acid is added. Sulphuric acid acts as a catalyst and speeds up the ionization process to release H⁺ ions. These H⁺ ions are responsible for the conduction of electricity.

Q9: Rancidity is a process used for spoiling of cooked food materials like vegetables, etc. When kept for long time in open. How can you prevent such process to proceed? Give an example.

Answer: It can be prevented by using various methods such as by adding antioxidants to the food materials, Storing food in air tight container and by flushing out air with nitrogen.

Q10:What happens when CO₂(g) is passed through slaked lime? Write the balanced chemical equation. Write the type of reaction that has occurred.

Answer: Slaked lime,chemically is calcium hydroxide or Ca(OH)₂.
A saturated solution of calcium hydroxide in water is known as lime water.

When carbon dioxide is passed through a freshly prepared solution of lime water, then it turns milky due to the formation of calcium carbonate, which is insoluble in water.
Following is the chemical equation for the reaction

Ca(OH)₂ + CO₂  → CaCO₃ + H₂O

Q11: Why are decomposition reactions called opposite of combination reactions? Write equations for these reactions.

Answer: Decomposition reactions are those reactions in which a compound is broken into two or more new compounds. e.g.

  CaCO₃ ------> CaO + CO₂

Combination reactions are those reactions in which two substances are combined to form a new substance. e.g.

CaO + CO₂ ------> CaCO₃ + Heat

The above examples indicate that both the reactions are opposite to each other. Thus we can say that decomposition reactions are also called opposite of combination reactions.

Q12: Compound ‘X’ decomposes to form compound ‘Y’ and CO₂ gas. Compound ‘Y’ is used in the manufacturing of cement.

(i) Name the compounds X and Y.

(ii) Write the chemical equation for this decomposition reaction.

Answer: Calcium carbonate (limestone) decomposes on heating to give calcium oxide and carbon dioxide.

CaCO₃ (s) ⎯→ CaO(s) + CO₂ (g)
calcium calcium   Carbon
Carbonate oxide  dioxide

Q13: During the reaction of some metals with dilute hydrochloric acid, following observations were made.
(a) Silver metal does not show any change
(b) The temperature of the reaction mixture rises when aluminium(Al) is added.
(c) The reaction of sodium metal is found to be highly explosive
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.
Explain these observations giving suitable reasons.


(a) Silver metal does not show any change.
Explanation: No change takes place because silver metal categorized as noble metal and does not react with hydrochloric acid in normal situations.

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
Explanation: The reaction between hydrochloric acid and aluminium is exothermic, thus the temperature of the reaction mixture rises when aluminium is added.

(c) The reaction of sodium metal is found to be highly explosive.
Explanation: Since, sodium is a highly reactive metal, thus it reacts with hydrochloric acid vigorously and produces large amount of heat and it is highly explosive. Even sodium reacts with water vigrouly.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.
Explanation: Bubbles of hydrogen gas are formed when lead reacts with dilute hydrochloric acid, which is seen in the course of reaction.

Q14 (NCERT Exemplar): A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and write the chemical reactions involved.

Answer: Element ‘X’ is calcium. When calcium oxide reacts with water it forms calcium hydroxide. Since calcium hydroxide is a base, thus it turns red litmus blue. Reaction involved in the reaction of calcium oxide with water can be written as follows:

CaO + H₂O ⇨ Ca(OH)₂ + Heat

Calcium belongs to group 2nd in periodic table. It is used in cement industry and also found in bones.

Q15: A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.
(b) Write a balanced chemical equation, when X is dissolved in water.


2Mg + O₂ ⎯→ 2MgO

3Mg + N₂ ⎯→ Mg₃N₂

(a) X is MgO and Y is Mg₃N₂

(b) MgO + H₂O ⇨ Mg(OH)₂

Q16: A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and give its chemical formula.

(a) A layer of black substance, called silver oxide, deposited over silver because of exposure of oxygen present in air. Finally it forms silver sulphide with the reaction of oxygen and hydrogen sulphide gas present in atmosphere, because of which silver metal turns black. This phenomenon is called tarnish or tarnishing of silver. Reaction involved in this is given here:

4Ag + O₂ + 2H₂S ⇨ 2Ag₂S + 2H₂O

The silver sulphide so formed can be cleaned using toothpaste. Toothpastes have many abrasive substance. Aluminium hydroxide - Al(OH)₃ is one of them. When toothpaste is rubbed over tarnished silver metal, the silver sulphide reacts with aluminium and aluminium sulphide is formed leaving silver metal shiny. The cleansing speed ups with use of hot water and sodium hydrogen carbonate. The reaction involve in cleansing is as follows:

3Ag₂S + 2Al ⇨ 6Ag + Al₂S₃

(b) The black substance formed over the silver metal is silver sulphide. The chemical formula of silver sulphide is Ag₂S.

Q17:  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) Identity the brown gas X evolved.
(c) Identity the type of reaction.
(d) What could be the nature(acidic,basic or neutral) of aqueous solution of the gas X?


(a) 2Cu(NO₃)₂ ⇨ 2CuO + 4NO₂ + O₂

(b) The brown gas 'X' is nitrogen dioxide.

(c) This is a Decomposition reaction

(d) Oxides of non-metals are acidic and hence, aqueous solution of this gas would be acidic. This means that its pH would be less then 7.

Q18: Give the characteristic tests for the following gases
(a) CO₂
(b) SO₂
(c) O₂
(d) H₂

Answer: Give the characteristic tests for the following gases:
CO₂: Carbon dioxide turns lime water milky as it forms calcium carbonate with lime water. This is the characteristics test for carbon dioxide.

SO₂: The smell of sulphur dioxide gas is like burning of sulphur, which is the characteristics test for sulphur dioxide. Sulhpur dioxide turns moist litmus paper from blue to red.

O₂: When a burnt candle or match stick is brought near the oxygen gas, it starts burning brightly. This is the characteristic test for oxygen gas.

H₂: When a burnt candle or match stick, it burns with pop sound, which is the characteristic test for hydrogen gas.

Q19: What happens when a piece of
(a) zinc metal is added to copper sulphate solution?
(b) aluminium metal is added to dilute hydrochloric acid?
(c) silver metal is added to copper sulphate solution?
Also, write the balanced chemical equation if the reaction occurs

(a) Zinc sulphate and copper is formed when zinc reacts with copper sulphate.

Zn + CuSO₄ ⇨ Cu + ZnSO₄

(b) Hydrochloric acid forms aluminium chloride and hydrogen gas when reacts with aluminium metal.

2Al + 6HCl ⇨ 2AlCl₃ + 3H₂

(c) When silver metal is added to copper sulphate solution, no reaction takes place because silver is less reactive than copper.

Q20: 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.


Hydrogen gas and zinc sulphate are formed when zinc granules are treated with sulphuric acid.
Zn + H₂SO₄ ⇨ ZnSO₄ + H₂

Zinc chloride and hydrogen gas are formed, when zinc granules are treated with hydrochloric acid.

Zn + 2HCl ⇨ ZnCl₂ + H₂

Similarly, hydrogen gas and zinc nitrate are formed when zinc granules are treated with nitric acid.

Zn + 2HNO₃ ⇨ Zn(NO₃)₂ + H₂

Zn + HNO₃ --> ZN(NO₃)₂ + H₂O +NO₂

Since, zinc is less reactive than sodium, thus no reaction takes place when zinc granules are treated with sodium chloride.

Sodium zincate and hydrogen gas are formed when zinc granules are treated with sodium hydroxide and water.

Zn + 2H₂O + 2NaOH ⇨ Na₂Zn(OH)₄ + H₂

2NaOH + Zn --> Na₂ZnO₂ + H₂

Q21: 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
(b) What other name can be given to this precipitation reaction?
(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?

(a)A white precipitate of barium sulphite is formed when barium chloride is added to the solution o sodium sulphite.

BaCl₂ + Na₂SO₃ ⇨ BaSO₃ + 2NaCl

(b) This precipitation reaction is also a double displacement reaction.

(c) Barium chloride, suphur dioxide and water are formed hen dilute hydrochloric acid is added to the solution of barium sulphite and sodium chloride. Since barium chloride is a soluble substance, thus white precipitate of barium sulphite disappears.

BaSO₃ + HCl ⇨ BaCl₂ + SO₂ + H₂O

Q.22 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?


With Cu container:
The solution of dilute HCl, ZnCl₂ and H₂O can be kept in the container made of copper.  Since copper is a less reactive metal and is placed below the hydrogen in activity series.  It does not react with HCl, ZnCl2 and H2O.

Cu + HCl(dil)  -----------> No reaction

Cu + ZnCl₂  -------------> No reaction

Cu + H₂O  ----------------> Nop reaction

Cu reacts with dil HNO₃ and gets oxidised to give nitric oxide. HNO₃(dil) cannot be kept in copper container.

Cu(s)  +  8HNO₃(aq)  ——>  3Cu(NO₃)₂(aq)  +  2NO(g)  +  4H₂O(l)

With Al container:
While aluminium is highly reactive metal and reacts with HCl and ZnCl₂.

2Al + 6 HCl ----------> 2AlCl₃ + 3H₂ (g)

2Al + 3ZnCl₂ ---------> 2AlCl₃  + 3Zn

HNO₃ may be kept in Al container, since it forms a protective layer of Aluminium Oxide (Al₂O₃). It there is no layer of Al₂O₃ is there or breached, dil HNO₃ reacts with Al to form aluminium nitrate.

4Al   +  3O₂  --------> 2Al₂O₃ 
3Al + 4HNO₃ ----------> 3Al(NO₃)₃ + NO + 2H₂O

Aluminum metal rapidly develops a thin layer of aluminum oxide that prevents the metal from reacting with water. When this layer is breached a reaction develops, releasing highly flammable hydrogen gas.

2Al + 6H₂O -------> 2Al(OH)₃ + 3H₂

Q23: Identify the oxidizing agent, reducing agent, substance oxidized and reduced in the following reactions

(a) Fe₂O₃ + 3CO → 2Fe + 3CO₂
(b) H₂O + F₂ → HF + HOF  
(c) CuO + H₂ → Cu + H₂O   
(d) 3Fe + 4H₂O → Fe₃O₄ + 4H₂     
(e) CuSO₄ + Zn → Cu + ZnSO₄      
(a) Fe₂O₃ + 3CO → 2Fe + 3CO₂

(a) Fe₂O₃ + 3CO → 2Fe + 3CO₂
Oxidising agent = Fe₂O₃
Reducing agent = CO
Fe₂O₃ is reduced to Fe (loss of oxygen)
CO is oxidised. (gains oxygen.)

(b) H₂O + F₂ → HF + HOF
       Oxidising agent = F₂
Fluorine is reduced.

(c) CuO + H₂ → Cu + H₂O  
Oxidising agent = CuO
Reducing agent = H₂
CuO is reduced.
H₂ is oxidised.

(d) 3Fe + 4H₂O → Fe₃O₄ + 4H₂    

Oxidising agent = H₂O
Reducing agent = Fe
Fe is oxidised.
H₂O is reduced.

(e) CuSO₄ + Zn → Cu + ZnSO₄      
Oxidising agent = CuSO₄
Reducing agent = Zn
CuSO₄ is reduced.
Zn is oxidied.


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