CBSE Class 10 Science Chapter 1 Chemical Reactions And Equations Short Answer Questions

CBSE Class 10 Science Class 10 Science Chapter 1 Chemical Reactions And Equations


Representation of Chemical Reaction

CBSE Class 10 Science Class 10 Science Chapter 1 Chemical Reactions And Equation

CBSE Class 10 Science Class 10 Science Chapter 1 Chemical Reactions And Equations Definitions


1. Chemical reaction:
Whenever a chemical change occurs, we can say that a chemical reaction occurs.

2. The following observations help us determine whether a chemical reaction occurred.

  1. Change in state
  2. Change in colour
  3. Evolution of a gas
  4. Change in temperature

3. Chemical equation: A chemical change can be represented symbolically in terms of a chemical equation.

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4. A chemical equation represents a chemical reaction occurring in a chemical change.

5. To make a chemical reaction more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by notations (g), (l), (aq) and (s) respectively.

6. Mass can neither be created nor destroyed in a chemical reaction, that is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.

7. Balancing equation: A chemical reaction is balanced so that the number of atoms to each element involved in a chemical reaction is the same on the reactant and product sides.

8. Combination reaction: The reaction in which two or more substances combine to form a new single substance.

For example:

⇒ \(\underset{\text { Calcium oxide(Quicklime) }}{\mathrm{CaO}(s)}+\underset{\text { Water }}{\mathrm{H}_2 \mathrm{O}(l)} \longrightarrow \underset{\text { Calcium hydroxide(Slaked lime) }}{\mathrm{Ca}(\mathrm{OH})_2(a q)}\)

9. Decomposition reaction:

The reaction in which a single substance decomposes to give two or more substances. It can be thermal (by heat) electrolytic (by electric current) decomposition or photochemical (by sunlight). For example,

2 Pb (NO)2(s) (Lead nitrate) →  (heat) → 2 PbO(s)(Lead oxide)  + 4 NO2 (g) +O2(g)

10. Displacement reaction:

The chemical reaction in which an element displaces another element from its compound.

For example:

⇒ \(\underset{\text { Iron }}{\mathrm{Fe}(s)}+\underset{\text { Copper sulphate }}{\mathrm{CuSO}_4(a q)} \longrightarrow \underset{\text { Iron sulphate }}{\mathrm{FeSO}_4(a q)}+\underset{\text { Copper }}{\mathrm{Cu}(s)}\)

11. Double displacement reaction:

The reaction is in which two different atoms or groups of atoms (ions) are mutually exchanged.

For example:

⇒ \(\underset{\text { Sodium sulphate }}{\mathrm{Na}_2 \mathrm{SO}_4(a q)}+\underset{\text { Barium chloride }}{\mathrm{BaCl}_2(a q)} \rightarrow \underset{\text { Barium sulphate }}{\mathrm{BaSO}_4(s)}+\underset{\text { Sodium chloride }}{2 \mathrm{NaCl}(a q)}\)

12. Precipitation reaction: The reaction that produces precipitation (insoluble salt) is called precipitation reaction.

For example:

⇒ \(\underset{\text { Silver nitrate }}{\mathrm{AgNO}_3(a q)}+\underset{\text { Sodium chloride }}{\mathrm{NaCl}(a q)} \longrightarrow \underset{\begin{array}{c}
\text { Silver chloride } \\
\text { (white pott }
\end{array}}{\mathrm{AgCl}(s)}+\underset{\text { Sodium nitrate }}{\mathrm{NaNO}_3(a q)}\)

13. Neutralisation reaction: When acids react with bases or alkalis their acidity is destroyed and a salt is produced; such reactions are known as neutralisation reactions.

For example:

⇒  \(\underset{\begin{array}{c}
\text { Hydrochloric } \\
\text { acid }
\end{array}}{\mathrm{HCl}}+\underset{\text { Sodium hydroxide }}{\mathrm{NaOH}} \longrightarrow \underset{\text { Sodium chloride }}{\mathrm{NaCl}}+\underset{\text { Water }}{\mathrm{H}_2 \mathrm{O}}\)

14. Isomerisation reaction:

Those reactions in which the atoms present in the molecules of the reactant are rearranged are known as isomerisation reactions.

For example:

NH4CNO(Ammoniumcyanate) →(heat) → H4N-CO-NH4(urea)

15. Photochemical reaction: The reaction which takes place in the presence of light is called photochemical reaction.

For example:

2AgBr(s) (silver bromide) → (sunlight) → 2 Ag(Silver) +Br2 (Bromine)

16. Exothermic reaction: The reaction in which heat is given out along with the products.

⇒ \(\underset{\text { Carbon }}{\mathrm{C}(\mathrm{s})}+\underset{\text { Oxygen }}{\mathrm{O}_2(\mathrm{~g})} \longrightarrow \underset{\text { Carbon dioxide }}{\mathrm{CO}_2(\mathrm{~g})}+\text { Heat }\)

17. Endothermic reaction: The reaction in which energy is absorbed by the reactants.

N2(g)(Nitrogen) +O2(g)(oxygen) → (heat) →  2NO(g) (Nitric oxide)

18. Oxidation: It is the gain of oxygen or the loss of hydrogen.

For example:

2 Cu (Copper) +O2(Oxygen) → (heat)→ 2CuO(Copper oxide)

19. Reduction: It is the loss of oxygen or gain of hydrogen. For example

CuO(Copper oxide) +H2 → (heat)→ Cu (Copper)  +HO(water)

20. Redox reaction: The reaction in which one reactant gets oxidised while the other gets reduced.

21. Oxidising agent: The substance which helps in oxidation is called oxidising agent.

22. Reducing agent: The substance which helps in reduction is called reducing agent.

23. Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

For example:

  1. The reddish-brown coating on iron
  2. The black coating on the silver
  3. The green coating on the copper

24. Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change.

25. Antioxidants: The substances which prevent oxidation are added to foods containing fats and oils

CBSE Class 10 Science Class 10 Science Chapter 1 Chemical Reactions And Equations Short Question And Answers


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

Answer:

On exposure to air magnesium ribbon gets covered with a layer of magnesium carbonate. So magnesium ribbon should be cleaned with as and paper so that only clean magnesium ribbon is available for burning in air.

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

  1. Hydrogen + Chlorine → Hydrogen chloride
  2. Barium chloride + Aluminium sulphate chloride → Barium sulphate + Aluminium
  3. Sodium + Water  →  Sodium Hydroxide +Hydrogen

Answer:

1. H2 + Cl2 → 2 HCl

2. 3BaCl2 + Al2 (SO4)3 → 3BaSO4 + 2AlCl

3. 2Na + 2H2O→  2NaOH4+ H2

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

  1. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
  2. Sodium hydroxide solution (in water) reacts with a hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer:

1. BaCl2(aq) (Barium chloride) + Na2SO4(aq) (Sodiumsulphate)→ BaSO4(s)(Barium sulphate)+2 NaCl (aq)(Sodium chloride

2. NaOH(aq) (Sodium hydroxide)+ HCl(sq) (Hydrochloric acid) → NaCl(aq) (Sodium chloride)  + H2O(Water)

Question 4. A solution ofa substance ‘X? is used for whitewashing.

  1. Name the substance ‘X and write its formula.
  2. Write the reaction ofthe substance ‘X’ named in (i) above with water.

Answer:

1. X is quick lime and its formula is CaO.

2. CaO(s) (Quick lime) +H2O(Water) → Ca(OH)2(aq)(Slakedlime or calcium hydroxide)

Question 5.  Why is the amount of gas collected in one of the test tubes double the amount collected in the other? Name this gas.
Answer:

Water containing a few drops of dilute sulphuric acid is electrolysed using graphite anode and cathode. H2 gas is collected at the cathode and oxygen at the anode.

The electrochemical reaction is:

2HO(l) → 2H2(g)+O2(g)

Thus the amount of hydrogen gas collected is double that of oxygen. If the two gases are collected in two different test tubes, the gas collected in one test tube (i.e., H2) is double that collected in the other test tube (i.e., oxygen).

Question 6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer:

This is a displacement reaction.

Fe(s)(Iron nail)+ CuSO4(aq)  → FeSO4(aq) + Cu(s) (Copper)

Since copper sulphate is blue in colour and iron sulphate is greenish, the colour of copper sulphate changes when an iron nail is dipped in it.

Question 7. Give an example of a double displacement reaction other than the one given in
Answer:

AgNO3(ag)(Silver nitrate ) + NaCl(aq)(Sodium chloride ) →  AgCl(s)(Silver chloride) + NaNO3(aq)(Sodium nitrate)

Thus silver exchanges chloride from sodium chloride, and sodium chloride exchanges nitrate ions from silver nitrate. This is a double displacement reaction.

Question 8. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

  1. 4Na(s) + O2(g) →  2Na2O(s)
  2. CuO(s) + H2(g) →  Cu(s) + H2O(Z)

Answer:

  1. Na(s) is oxidised to Na2O
  2. (s) and oxygen is reduced to O2- (NaO2(s)).
  3. CuO(s) is reduced to Cu(s) and H2(g) is oxidised to H2O(Z).

Question 9. What is a balanced chemical equation? Why should chemical equations be balanced1
Answer:

In a balanced equation the number ofatoms ofeach element on both sides ofthe equation are equal. The chemical equation must be balanced with the aspect of mass because matter can neither be created nor destroyed.

The equation must be balanced concerning charge because the electrons lost by atoms to form cations are equal to electrons gained by other atoms to form anions. Free electrons do not exist in solution.

Question 10. Write the balanced chemical equations for the following reactions:

  1. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
  2. Zinc + Silver nitrate → Zinc nitrate + Silver
  3. Aluminium + Copper chloride → Aluminium chloride + Copper
  4. Barium chloride + Potassium sulphate → Barium, sulphate + Potassium chloride

Answer:

Ca(OH)2+ CO2 → CaCO3 + H2O

Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

2Al + 3CuCl → 2AlCl3 + 3Cu

BaCl2+ K2SO4  → 4 BaSO4 + 2KCl

Question 11. Why is respiration considered an exothermic reaction? Explain.
Answer:

During digestion, food is broken down into simpler substances. Food like rice, potatoes and bread are made up of carbohydrates. These carbohydrates are further broken down into glucose. Glucose during respiration (inhalation of oxygen) is oxidised with the

Liberation of energy as shown below:

C6H12 O6 (Glucose)+ 6O2(aq)(Oxygen) + 6CO2(aq)  → 6CO(Carbon dioxide)(aq) + 6H2O(l) (Water)+ energy

Thus, respiration is an exothermic process.

Question 12. In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write down the reaction involved.
Answer:

The involved reaction is:

⇒ \(\mathrm{Cu}(s)+2 \mathrm{AgNO}_3(a q) \longrightarrow \mathrm{Cu}\left(\mathrm{NO}_3\right)_2(a q)+2 \mathrm{Ag}(s)\)

Question 13. What do you mean by a precipitation reaction? Explain by giving an example.
Answer:

When two reactants in a solution react and one or more of the products are insoluble or form a precipitate, the reaction is called a precipitation reaction.

For example, when a solution of iron chloride and ammonium hydroxide is mixed, a brown precipitate of iron hydroxide is formed.

FeCl3(aq)(Ferric chloride) + 3NH4OH(aq)(Ammoniumhydroxide) → Fe(OH)3(s) (Ferriuc hydroxide)  +3NH4Cl(aq) (Ammonium chloride)

Question 14. Why do we apply paint on iron articles?
Answer:

It is observed that iron articles are shiny when new but get coated with a reddish-brown surface when left for some time as the iron articles get rusted. This process is called corrosion. To prevent rusting of iron, paint is applied on iron articles.

Question 15. What happens chemically when quicklime is added to water filled in a bucket?
Answer:

Quicklime reacts with water to form slaked lime and produces a lot of heat and hissing sound

⇒ \(\underset{\begin{array}{c}
\text { Quicklime } \\
\text { (Calcium oxide) }
\end{array}}{\mathrm{CaO}(s)}+\underset{\text { Water }}{\mathrm{H}_2 \mathrm{O}(l)} \longrightarrow \underset{\begin{array}{c}
\text { Slaked lime } \\
\text { (Calcium hydroxide) }
\end{array}}{\mathrm{Ca}(\mathrm{OH})_2(s)}+\text { Heat + Hissing sound }\)

Question 16. Study the reactions given below. State which of the following chemical reactions will take place or not, giving suitable reason for each:

  1. Zn(s) + CuSO4  (aq) → ZnSO4(aq) + Cu(s)
  2. Fe(s) + ZnSO4(aq)→ FeSO4(aq) + Zn(s)
  3. Zn(s) + FeSO4(aq) →  ZnSO4(aq) + Fe(s)

Answer:

  1. Zinc is more reactive than copper; so it will displace copper from CuS04. This reaction will take place.
  2. Iron is less reactive than zinc, so it cannot displace zinc from zinc sulphate. The reaction will not take place.
  3. Zinc is more reactive than iron, it will displace iron from FeS04. The reaction will take place.

Question 17. What is meant by the skeletal-type chemical equation? What does it represent? Using the equation for electrolytic decomposition ofwater, differentiate between a skeletal chemical equation and a balanced chemical equation.
Answer:

The equations in which gaseous are written in atomic form instead of molecular form and the equation is not balanced are called skeletal type chemical equations.

They represent gaseous elements formed in the atomic state and the equation is not balanced

⇒ \(\mathrm{H}_2 \mathrm{O}→ {\text { Electrolysis }} →\mathrm{H}+\mathrm{O} \text { (Sketal equation) }\)

Hydrogen and oxygen are written in atomic forms and the equation is not balanced.

H2 O → 2H2 + O2 (Balanced chemical equation)

H2 O → H2 + O2 is the Skeletal equation.

Question 18. What is redox reaction? When a magnesium ribbon burns in the air with a dazzling flame and forms white ash, is magnesium oxidised or reduced? Why?
Answer:

The reaction in which oxidation (loss ofelectrons) and reduction (gain of electrons) take place simultaneously is called a redox reaction.

⇒ \(\underset{\text { Magnesium }}{2 \mathrm{Mg}(s)}+\underset{\text { Oxygen }}{\mathrm{O}_2(g)} \longrightarrow \underset{\text { Magnesium oxide }}{2 \mathrm{MgO}(s)}\)

Magnesium is getting oxidised because it is losing electrons from Mg2+ and oxygen is gaining electrons to form O2- therefore it is getting reduced.

Question 19. Distinguish between a displacement reaction and a double displacement reaction. Identify the displacement and the double displacement reaction from the following reactions:

  1.  HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
  2. Fe(s) + CuSO4(aq) → FeSO4(aq) +Cu(s)

Answer:

A displacement reaction is a reaction in which more reactive metal can displace less reactive metal from its salt solution.

Double displacement reaction: The reaction is in which a compound exchanges its ions to form two new compounds called double displacement reaction.

  1. Double displacement reaction
  2. Displacement reaction

Question 20.

1. When a metal ‘X’ is added to the salt solution ofa metal Y following chemical reaction takes place: Metal X + Salt solution of ‘Y’→ Salt solution of ‘X’ + Metal ‘Y’

2. Mention the inference you draw regarding the reactivity of metals ‘X and Y’ and also about the type of reaction. State the reason for your conclusions.
Answer:

It is a displacement reaction.

  1. Metal X is more reactive than metal Y.
  2. Metal X is displacing metal Y from its salt solution.

A more reactive metal displaces a less reactive one from its salt solution. X is more reactive than Y.

Question 21. Write the essential condition for the following reactions to take place: 2Ag + Cl2 and write one use of this reaction.
Answer:

1. To carry out the reaction 2AgCl → 2Ag + Cl2 presence ofsunlight is essential.

This reaction is used in black-and-white photography

Question 22. Name the reducing agent in the following reaction

3MnO2 + 4Al → 3Mn + 2Al2O3

State which is more reactive, Mn or Al and why?

Answer:

‘Al’ is a reducing agent.

‘Al’ is more reactive than Mn.

∴ ‘Al’ displaces Mn from its oxide.

Question 23. Identify the type ofreaction from the following equations.

  1. CH4 + 2O2 → CO2 + 2H2O
  2. Pb(NO3)2+ 2KI→ Pbl2+ 2KNO3
  3. CaO + H2O→ Ca(OH)2
  4. CuSO4 + Zn→ZnSO4 + Cu

Answer:

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

Question 24. Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.
Answer:

Any two of these observations will suggest a chemical reaction has taken place

  1. Change in state
  2. Change in colour
  3. Evolution of gas
  4. Change in temperature

For example, lead nitrate is a white crystalline solid, on heating it gives a yellowish-brown solid (lead monoxide), brown gas is evolved and a colourless gas is evolved.

It shows chemical reaction has taken place.

CBSE Class 10 Science Chapter 1 Chemical Reactions White Crysrtalline

Question 25. State what changes in the colour of the iron nail and copper sulphate solution are observed when an iron nail is dipped into copper sulphate solution for about 20 minutes. Write the equation for the reaction that takes place. Also, name the type ofreaction involved in the above process.
Answer:

  1. The iron nail becomes brownish and the blue colour of the copper sulphate solution fades.
  2. Fe(s) + CuSO4(aq)→  FeSO4(aq) + Cu(s)
  3. Displacement reaction

Question 26. What is observed when a solution of potassium iodide is added to a solution of lead nitrate? Name the type ofreaction. Write a balanced chemical equation to represent the above chemical reaction.
Answer:

A yellow precipitate of lead iodide is formed. It is a precipitation reaction.

Pb(NO3)2 (aq) + 2KI (aq)→Pbl2 (s) + 2KNO3 (aq)

It is also called a double displacement reaction.

Question 27. Explain giving chemical equation of any two uses of chemical decomposition reaction in industry.
Answer:

1. In manufacturing cement, quicklime is used and it is obtained by thermal decomposition of limestone.

CaCO3(s) → (Heat) →CaO(s) + CO2(g)

2. In manufacturing photochromic glass, silver chloride/bromide is used which turns grey in sunlight.

Question 28. 2 g of lead nitrate powder is taken in a boiling tube. The boiling tube is heated over a flame. Now answer the following:

  1. State the colour ofthe fumes evolved and the residue left.
  2. Name the type of chemical reaction that has taken place stating its balanced chemical equation.

Answer:

1. Brown-coloured fumes of NO2 gas are evolved and a yellowish-brown residue is formed.

2. Decomposition reaction has taken as,

2Pb(NO3)2(s) → (Heat) → 2PbO(s)+4NO2(g)+ O2(g)

Question 29. What is meant by a balanced chemical equation? Why chemical equations are balanced? Balance the chemical equation given below:

Al2O3 + NaOH→ NaAlO2 + H2O

Answer:

A chemical equation in which the number of atoms of each type involved is the same on the reactants and products is called a balanced chemical equation.

To justify the law of conservation of mass in a reaction, a chemical equation is to be balanced.

Al2O3 + 2NaOH→ 2NaAlO2 + H2O

Question 30. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the action and name the type ofreaction.
Answer:

A white precipitate of barium sulphate is formed.

If both reactants are in solid state, the reaction will not take place between them.

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

It is a double displacement as well as a precipitation reaction.

Question 31. Write balanced chemical equations for the following reactions.

  1. Silver bromide on exposure to sunlight decomposes into silver and bromine.
  2. Sodium metal reacts with water to form sodium hydroxide and hydrogen gas.

Answer:

1. 2AgBr(s) →  (Sunlight) → 2Ag(s)+Br(g)

2. 2Na(s) + 2H2O(l)→ 2NaOH(aq) + H2(g)

Question 32. Give an example each for thermal decomposition and photochemical decomposition reactions. Write relevant balanced chemical equations also.
Answer:

Thermal decomposition reaction

(Copper carbonate -Green)CuCO(s)→ (Heat) → CuO(s)(Copper oxide -Black)+CO2(g)

Photochemical decomposition reaction

2 AgCl(s) → (Sunlight) → 2 Ag(s)+Cl2(g)

Question 33.

  1. Why is a combustion reaction an oxidation reaction?
  2. How will you test whether the gas evolved in a reaction is hydrogen? 

Answer:

1. A combustion reaction is an oxidation reaction because it is always carried in the presence of air or oxygen.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

2. Bring a burning matchstick close to the mouth ofthe tube from which hydrogen gas escapes. The gas will immediately catch fire and this will be accompanied by a pop sound.

Question 34. An aluminium can was used to store the ferrous sulphate solution. It is observed that in a few days, holes appeared in the can. Explain the observation and write a chemical equation to support your answer.
Answer:

Aluminium being more reactive displaces ferrous ions from ferrous sulphate solution and this results in the appearance of holes in the aluminium can.

⇒ \(2 \mathrm{Al}(s)+\underset{\text { (Ferrous sulphate) }}{3 \mathrm{FeSO}_4(a q)} \longrightarrow \underset{\text { (Aluminium sulphate) }}{\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3(a q)}+3 \mathrm{Fe}(s)\)

Question 35. Identify the type ofreaction from the following equations:

  1. CH4 + 2O2-4 →  CO2 + 2H2O
  2. Pb(NO3)2 + 2KI→  Pbl2 + 2KNO3
  3. CaO + H2O→ Ca(OH)2
  4. CuSO4 + Zn→ ZnSO4 + Cu

Answer:

1. Combustion reaction and oxidation reaction

2. Double displacement and precipitation reaction

3. Combination reaction

4. Displacement reaction

Question 36.

1. A solution of substance ‘X’ is used for whitewashing. What is substance ‘X? State the chemical reaction of X’ with water.

Why does the colour of copper sulphate solution change when an iron nail is dipped in it? 
Answer:

‘X’ is calcium oxide (CaO)

CaO(s) + H2O(l)→  Ca(OH)2(aq) + Heat

It is because iron displaces copper from CuS04 to form FeSO4 which is pale green

Question 37. Using a balanced chemical equation explain the difference between a displacement reaction and a double displacement reaction.
Answer:

In a displacement reaction, more reactive metal can displace less reactive metal from its salt solution

Example:  Cu(s) + 2AgNO(aq)→ Cu(NO3)2(aq) + 2Ag(s)

In a double displacement reaction, two compounds exchange their ions to form two new compounds,

Example: NaOH + MCI→ NaCl + H2O

Question 38. When you have mixed the solutions of lead (II) nitrate and potassium iodide

  1. What was the colour ofthe precipitate formed and can you name the precipitate?
  2. Write the balanced chemical equation for this reaction.
  3. Is this also a double displacement reaction?

Answer:

The colour of the precipitate is yellow. The name of the compound formed as a precipitate is lead iodide (Pbl2).

Pb(NO3)2 (aq) + 2KI(aq)→ Pbl2(s) + 2KNO3(aq)

Yes, it is also a double displacement reaction.

Question 39. Balance the following chemical equations:

  1. BaCl2 + H2SO4 → BaSO4 + HCl
  2. Ca(OH)2 + HNO3→ Ca(NO3)2 + H2O
  3. Pb(NO3)2→ PbO + NO2 +O2
  4. MnO2 + HCl→MnCl2 + H2O + Cl2

Answer:

  1. BaCl2 + H2SO4 → BaSO4 + 2HCl
  2. Ca(OH)2 + 2HNO3→ Ca(NO3)2 + 2H2O
  3. 2Pb(NO3)2→ 2PbO + 4NO2 +O2
  4. MnO2 + 4HCl→MnCl2 + 4H2O + Cl2

Question 40. Define combination reaction. Give one example ofa combination reaction which is also exothermic.
Answer:

A reaction in which two elements or compounds combine to form a single compound is called a combination reaction.

CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat

It is also an exothermic reaction because heat is evolved.

Question 41. Write the balanced chemical equations for the following reactions.

  1. Sodium carbonate in reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.
  2. Sodium hydrogen carbonate in reaction with hydrochloric acid gives sodium chloride, and water and liberates carbon dioxide.
  3. Copper sulphate on treatment with potassium iodide precipitates cuprous iodide(Cu2I2), liberates iodine gas and also forms potassium sulphate.

Answer:

  1. Na2CO3 + HCl → NaCl + NaHCOg
  2. NaHCO3 + HCl → NaCl + H2O+ CO2
  3. 2CuSO4 + 4Kl→Cu2I2 + 2K2SO4 + I2

Question 42. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type ofthe chemical reaction.
Answer:

KCl(aq) + AgNO3 →AgCl(s) ↓ + KNO3(aq)

Double displacement and precipitation reaction

Question 43. Ferrous sulphate decomposes with the evolution of gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.
Answer:

2FeSO(s) → (Heat) → 2Fe2SO3(s) + SO2(g) + SO3(g)

It is an example of a thermal decomposition reaction.

Question 44. Why do fireflies glow at night?
Answer:

Fireflies have a protein which in the presence ofan enzyme undergoes aerial oxidation.

This is a chemical reaction which involves the emission of visible light.
Therefore, fireflies glow at night.

Question 45. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?
Answer:

Grapes when attached to the plants are living and therefore their immune system prevents fermentation. The microbes can grow in the plucked grapes and under anaerobic conditions, these can be fermented. This is a chemical change.

Question 46. Which among the following are physical or chemical changes?

  1. Evaporation of petrol
  2. Burning liquefied Petroleum Gas (LPG)
  3. Heating of iron rod to red hot
  4. Curdling of milk
  5. Sublimation of solid ammonium chloride

Answer:

  1. 1, 3 and 5 are physical changes.
  2. 2 and 4 an’ chemical changes

Question 47. A substance X, which is an oxide of 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 and also write the chemical reaction involved.
Answer:

The name of the substance is Calcium oxide.

CaO + H2 (l) → Ca(OH)(aq)

Question 48. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why1.
Answer:

Zinc is more reactive than copper.

Zn + 2HCl → ZnCl2 + H2

Al + HCl →   No reactions.

Question 49. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

  1. Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
  2. Name the black substance formed and give its chemical formula.

Answer:

  1. Metals such as silver when attacked by substances around it such as moisture, acids, gases, etc, are said to corrode and this phenomenon is called corrosion.
  2. The black substance is formed because silver (Ag) reacts with H2S present in the air. It forms a thin black coating of silver sulphide (Ag2S).

Question 50. Give reasons why:

  1. Sodium acts as a reducing agent while chlorine acts as an oxidising agent in the following reaction: 2Na+Cl → 2 NaCl(s)
  2. White-coloured silver chloride turns grey when kept in sunlight.

Answer:

  1. Sodium reduces chlorine atoms to chloride ions, therefore it acts as a reducing agent while chlorine oxidises sodium atoms to sodium ions, therefore it acts as an oxidising agent.
  2. This is due to the decomposition of silver chloride into silver and chlorine by light.

Question 51. Give one example of a combination reaction in which an element combines with a new compound to give a new compound
Answer:

8NH3 + 3Cl2 → 6NH4Cl + N2

O2 + 2SO2 →  2SO3

CBSE Class 10 Science Class 10 Science Chapter 1Chemical Reactions And Equations Multiple Choice Questions


Question 1. Which of the following is not a physical change?

  1. Boiling of water to give water vapour
  2. Melting of ice to give water
  3. Dissolution of salt in water
  4. Combustion liquefied Petroleum Gas (LPG)

Answer: 4. Combustion liquefied Petroleum Gas (LPG)

Question 2. The following reaction is an example ofa

4NH3(g) + 5O2(g)→4NO(g) + 6H2O(g)

1. Displacement reaction

2. Combination reaction

3. Redox reaction

4. Neutralisation reaction

  1. 1 and 4
  2. 2 and 3
  3. 1 and 3
  4. 3 and 4

Answer: 4.  3 and 4

Question 3. Which of the following statements about the given reaction are correct?

3Fe(s) + 4H2O(g) > Fe3O4) + 4H2(g)

1. Iron metal is getting oxidised

2. Water is getting reduced

3. Water acts as a reducing agent

4. Water acts as an oxidising agent

  1. 1, 2 and 3
  2. 3 and 4
  3. 1, 2 and 4
  4. 2 and 4

Answer: 3. 1, 2 and 4

Question 4. Which of the following are exothermic processes?

1. Reaction of water with quick lime

2. Dilution of an acid

3. Evaporation of water

4. Sublimation of camphor (crystals)

  1. 1 and 2
  2. 2 and 3
  3. 1 and 4
  4. 3 and 4

Answer: 1. 1 and 2

Question 5. Three beakers labelled as A, B, and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous CuS04 and NaCl were added to the beakers A, B, and C respectively.  It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in the case of beaker C, the temperature of the solution fell. Which one of the following statement(s) is (are) correct?

1. In beakers A and B, an exothermic process has occurred.

2. In beakers A and B, an endothermic process has occurred.

3. In beaker C exothermic process has occurred.

4. In beaker C endothermic process has occurred.

  1. 1 Only
  2. 2 Only
  3. 1 and 4
  4. 2 and 3

Answer: 3.  1 and 4

Question 6. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate. The light purple colour of the solution fades and finally disappears. Which ofthe following is the correct explanation for the observation?

  1. KMnO4 is an oxidising agent, it oxidises FeSO4
  2. FeSO4 acts as an oxidising agent and oxidises KMnO4
  3. The colour disappears due to dilution: no reaction is involved
  4. KMnO4 is an unstable compound and decomposes in the presence of FeSO4 to a colourless compound.

Answer: 1. KMnO4 is an oxidising agent, it oxidises FeSO4

Question 7. Which among the following is (are) double displacement reaction(s)?

1. Pb + CuCl2→ PbCl2 + Cu

2. Na2SO4 + BaCl2→  BaSO4+ 2NaCl

3. C +O2 → CO2

4. CH4 + 2O2→ CO2 + 2H2O

  1. 1 and 4
  2. 2 Only
  3. 1 and 2
  4. 3 and 4

Answer: 2.  2 Only

Question 8. Which among the following statements) is (are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to

1. The formation of silver by decomposition of silver chloride

2. Sublimation of silver chloride

3. Decomposition of chlorine gas from silver chloride

4. Oxidation of silver chloride.

  1. 1 only
  2. 1 and 3
  3. 2 and 3
  4. 4 only

Answer: 1.  1 only

Question 9. Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking oflime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about the slaking of lime and the solution formed?

1. It is an endothermic reaction

2. It is an exothermic reaction

3. The pH ofthe resulting solution will be more than seven

4. The pH ofthe resulting solution will be less than seven

  1. 1 and 2
  2. 1 and 3
  3. 1 and 4
  4. 4 only

Answer: 2.  1 and 3

Question 10. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which ofthe following correctly represents the type ofthe reaction involved?

1. Displacement reaction

2. Combination reaction

3. Precipitation reaction

4. Double displacement reaction

  1. 1 only
  2. 2 only
  3. 4 only
  4. 2 and 4

Answer: 4.  2 and 4

Question 11. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during the electrolysis of water is

  1. 1:1
  2. 2: 1
  3. 4:1
  4. 1:2

Answer: 2.  2: 1

Question 12. Which ofthe following is(are) an endothermic process(es)?

1. Dilution of sulphuric acid

2. Sublimation of dry ice

3. Condensation of water vapours

4. Evaporation of water

  1. 1 and 3
  2. 2 only
  3. 3 only
  4. 2 and 4

Answer: 4.  2 and 4

Question 13. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which ofthe following can be used in place of lead nitrate?

  1. Lead sulphate (insoluble)
  2. Lead acetate
  3. Ammonium nitrate
  4. Potassium sulphate

Answer: 2.  Lead acetate

Question 14. Which of the following gases can be used for storage of fresh samples of oil for a long time?

  1. Carbon dioxide or oxygen
  2. Nitrogen or oxygen
  3. Carbon dioxide or helium
  4. Helium or nitrogen

Answer: 4.  Helium or nitrogen

Question 15. The following reaction is used for the preparation of oxygen gas in the laboratory

3Fe(s) + 4H2 O(g)→ Fe3 O4 (s) + 4H2 (g)

Which of the following statement (s) is(arc) correct about the reaction?

  1. It is a decomposition traction and endothermic
  2. It is u i’ombinntion traction
  3. It is (i decomposition reaction accompanied by the release of heat
  4. It is a photochemical decomposition reaction and exothermic

Answer: 1.  It is a decomposition traction and endothermic

Question 16. Which one ofthe following processes involves chemical reactions?

  1. Storing ofoxygen gas under pressure in a gas cylinder
  2. Liquefaction ofair
  3. Keeping petrol in a china dish in the open
  4. Heating copper wire in presence ofair at high temperature

Answer: 4.  Heating copper wire in the presence ofair at high temperature

Question 17. In which of the following chemical equations, do the abbreviations represent the correct states ofthe reactants and products involved at reaction temperature?

  1. 2H2(l) + O2(Z)→2H2O(g)
  2. 2H2(g) +O2(l) → 2H2O(l)
  3. 2H2(g) + O2(g)→ 2H2O(l)
  4. 2H2(g) + O2(g)→ 2H2O(g)

Answer: 3.  2H2(g) + O2(g)→ 2H2O(l)

Question 18. Which of the following are combination reactions?

1. \(2 \mathrm{KCIO}_3 →{\text { Heat }}→ 2 \mathrm{KCI}+30_2\)

2. \(\mathrm{MgO}+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{Mg}(\mathrm{OH})_2\)

3. \(4 \mathrm{Al}+3 \mathrm{O}_2 \longrightarrow 2 \mathrm{Al}_2 \mathrm{O}_3\)

4. \(\mathrm{Zn}+\mathrm{FeSO}_4 \longrightarrow \mathrm{ZnSO}_4+\mathrm{Fe}\)

  1. 1 and 3
  2. 3 and 4
  3. 2 and 4
  4. 2 and 3

Answer: 4.  2 and 3

Question 19. Which ofthe statements about the reaction below are incorrect?

2PbO(s) + C(s)→ 2Pb(s) + CO2(g)

1. Lead is getting reduced.

2. Carbon dioxide is getting oxidised.

3. Carbon is getting oxidised.

4. Lead oxide is getting reduced.

  1. 1 and 2
  2. 1 and 3
  3. 1, 2 and 3
  4. All

Answer: 1 and 2 are incorrect.

Question 20. 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

Answer: 4. Displacement reaction

Question 21. 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.

Answer: 1. Hydrogen gas and iron chloride are produced.

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