ACIDS BASES AND SALTS

NCERT Questions (Page 18 )

Q.1

You have been provided with three test tubes. One of them contains distilled water and the other two contains an acidic and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tubes?

Ans.

Red litmus paper becomes blue when comes in contact with Basic solution. So dip red litmus in test tubes to find which one has basic solution. If it turns blue when dipped into Test tube 1 then test tube 1 has basic solution, if it does not change its colour, dip into another test tube. In this way we can find which test tube has basic solution.

Now, dip the blue litmus in both the remaining test tubes. That test tube which changes blue litmus into red litmus has Acidic solution. That test tube which does not change colour has distilled water.

NCERT Questions page 22)

Q.1

Why should curd and sour substances not be kept in brass and copper vessels?

Ans.

Sour substances loke pickles, curd etc. are acidic in nature. They are highly reactive to brass and copper metals.

If we keep curd and sour substances in brass and copper vessels, a chemical reaction starts taking place between the sour substance and the metal which in turn form toxic materials dangerous for our body.

Q2.

Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the pressure of this gas?

Ans.

Hydrogen gas is liberated when an acid reacts with a metal.

for example

Zn + 2HCl >  ZnCl2 + H2

When Zinc reacts with hydrochloric acid, it forms zinc chloride and liberates hydrogen.

Test for Hydrogen

Hydrogen gas is insoluble in water. So, when soap solution is mixed with hydrogen. Hydrogen does not mix with the solution instead starts making bubbles. These bubbles ultimately makes a blast.

Q.3

Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compound formed is calcium chloride.

Ans.

The compound formed is calcium chloride and the fire extinguisher is CO2.

The reaction goes like this

CaCO3 + 2HCl > CaCl2 + CO2 +  H2O

That is,

When Calcium Carbonate reacts with hydrochloric acid, it forms Calcium chloride and liberate carbon dioxide and water.

(NCERT Q./Ans Page 28)

Q1. You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Answer

Since Solution A has more hydrogen ions, so A is more acidic. Solution B is basic.

Q2. 

What affect does the concentration of H+ ions have the nature of the solution?

Answer

The solution which have more H+ ions is more acidic in nature.

Q3.

Do basic solutions have H+ ions? If yes, then why are these basic?

Answer

Basic solutions do have H+ ions. But these solutions have H- ions more than H+ ions. As these solutions have excess amount of H-ions, they are basic in nature.

Q4.

Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime( calcium hydroxide) or chalk ( calcium carbonate)? 

Answer

The ideal pH scale range of soil to grow plants is 6 to 8. So if the soil is more acidic, a farmer treats the soil with bases like quick lime/ slacked lime/ chalk so as to maintain its pH scale.

CHEMICAL REACTION AND EQUATION ( Textbook question answer)

NCERT TEXTBOOK EXERCISE

Which of the statements about the reaction below are in correct

Q1. Which of the statements about the reaction below are incorrect?

2PbO + C à 2Pb + CO₂

(a) Lead is getting reduced.

(b) Carbon dioxide is getting reduced.

(c) Carbon is getting oxidised.

(d) Lead oxide is getting reduced.

(1) (a) & (b)

(2) (a) & (c)

(3) (a), (b) & (c)

(4) all

Ans. (1). (a) & (b)

Q2. Fe₂O₃ + 2Al à Al₂ + O₃ + 2Fe

Q2. Fe₂O₃ + 2Al à Al₂ + O₃ + 2Fe

The above reaction is an example of a

(a) combination reaction

(b) double displacement reaction.

(c) decomposition reaction

(d) displacement reaction

Ans.

(d) displacement reaction

What happens when dilute hydrochloric acid is to iron fillings? Tick the correct answer

Q3. What happens when dilute hydrochloric acid is to iron fillings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(c) No reaction takes place.

(d) Iron salt and water are produced.

Ans

.

(a)  Hydrogen gas and iron chloride are produced.

What is balanced chemical reaction? Why should chemical reactions be balanced?

Q4. What is balanced chemical reaction? Why should chemical reactions be balanced? 

 Ans. A balanced chemical equation is that equation in which number of elemental atoms is equal on both the sides of an equation.

For example

2H₂ + O2 à 2H₂O

According to Law of Conservation of mass, during chemical reaction mass can neither be created nor be destroyed. That means the mass of reactants must be equal to the mass of products. So, chemical reactions should be balanced.

Q5. Translate the following statements into chemical reactions and then balance them:

a) Hydrogen gas combines with the nitrogen to form ammonia.

(b)  Hydrogen sulfide gas burns in air to give water and sulfur dioxide.

(c)  Barium chloride reacts with aluminum sulfate to give aluminum chloride and precipitate of barium sulfate.

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

Ans.

(a)  3H₂ + N₂ à 2NH₃

(a)  2H₂S + 3O₂ à 2SO₂ +2H₂O

(b)  3BaCl₂ + 2AL(SO₄)  à  2AlCl₃ + 3BaSO₄

(c)  2K + 2H2O à 2KOH + H₂

Q6. Balance the following chemical equations:

(a)  HNO₃ + Ca(OH)₂ à Ca(NO₃)₂ + H₂O

(b)  NaOH + H₂SO₄ à Na₂SO₄ + H₂O

(c)  NaCl + AgNO₃ à AgCl +NaNO₃

(d)  BaCl₂ + H₂SO₄ à BaSO₄ + HCl

Ans.

(a)  2HNO₃ +Ca(OH)₂ à Ca(NO₃)₂ + 2H₂O

(b)  2NaOH + H₂SO₄ à Na₂SO₄ + 2H₂O

(c)   NaCl +AgNO₃ à AgCl + NaNO₃

(d)  BaCl₂ + H2SO₄ à BaSO₄ + 2HCl

Q7. Write the balanced chemical equation 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 sulfate --> Barium sulfate + Potassium chloride

Ans.

(a) Ca(OH)₂ + CO₂ à  CaCO₃ + H₂O

(b) Zn + 2AgNO₃ à Zn(NO₃)₂ + 2Ag

(c) 2Al +3CuCl₂ à 2AlCl₃ + 3Cu

(d) BaCl₂ + K₂SO₄ à  BaSO₄ + 2KCl

Q8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide + Barium iodide = Potassium iodide + Barium bromide.

(b) Zinc carbonate = Zinc oxide + Carbon dioxide

(c) Hydrogen + Chlorine = Hydrogen chloride

(d) Magnesium + Hydrochloric acid= Magnesium chloride

Ans.

(a) 2KBr + BaI₂  = 2KI + BaBr₂

Reaction type - Double displacement reaction

(b) ZnCO₃ = ZnO + CO₂

Reaction Type - decomposition reaction

(c) H2 + Cl₂  = 2HCl

Reaction Type - Combination reaction

(d) Mg + 2HCl =MgCl₂ + H₂

Reaction type - displacement reaction.

Q9. What does one mean by exothermic reaction and endothermic reactions? Give examples.

Ans.

Exothermic reactions are those reactions which gives out energy in the form of heat, light etc.

For example

C + O₂ à CO₂ + Energy (393 Kj)

In the above example,

·        Carbon and Oxygen reacts.

·        The reaction produces carbon dioxide and releases 393 Kj of energy

Endothermic reaction is that chemical reaction in which energy is absorbed in the form of heat, light etc.

For example

N₂ + O₂ à 2NO

In the above example

1.    Nitrogen (N₂) and oxygen (O₂) react.

2.    During the reaction process, they absorb some amount (180.5 Kj) of energy.

3.    After the reaction, the product we get is Nitric oxide (2NO).

Thus we see while Exothermic reaction liberates energy, Endothermic reaction absorb energy.

Q10. Why is respiration considered as exothermic reaction?

Ans.

We need energy to work and stay alive. So, we eat carbohydrate rich food. Wheat and rice have carbohydrates which supplies us with energy.

During digestion, food breaks down into carbohydrates. Carbohydrate further splits up into glucose.

When we take oxygen (=Respiration), glucose gets oxidised and liberates energy in our body.

CH₆H₁₂O₆ + 6O₂ à  6CO₂ + 6H₂O + Energy

Thus, Respiration is an exothermic reaction.

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

Ans.

Yes, the opposite of decomposition reaction is composition reaction.

In decomposition reaction, a compound breaks down into simpler elements/substances. For example,

2Na₃ à N₂ + 3H₂

Ammonia (2Na₃) breaks down and forms Nitrogen (N₂) and Hydrogen (3H₂).

In combination reaction,

N₂ + 3H₂ à 2NH₃

Simpler substances like Nitogen (N₂) and Hydrogen (3H₂) reacts to form Ammonia (2NH₃)

Q12. Write one equation each for decomposition reaction where energy is supplied in the form of heat, light or electricity?

Ans

Heat ( Thermal Decomposition)

CaCO₃ – heat à  CaO + CO₂

( CaCO₃ = Calcium carbonate; CaO= Calcium oxide; CO₂ = Carbon dioxide )

Light (Photolysis)

2AgCl -- Sunlight à  2Ag + Cl₂

(2Ag = Silver chloride, 2Ag = Silver, Cl₂ = Chlorine )

Electricity (Electrolysis)

2H₂O --- electricity à 2H₂ + O₂

( 2H₂O = Water, 2H₂ = Hydrogen, O₂ = Oxygen )

Q13. What is the difference between displacement and double displacement reaction? Write equations for these reactions.

Ans.

Displacement reaction takes place between metals. In it more reactive metal replaces less reactive metal.

for example

Fe + CuSO4 --> Cu + FeSO4

In the above reaction Iron (metal) replaces Copper (metal).

Whereas in double displacement reaction, two different ions are displaced by each other.

for example

AgNO3 + NaBr --> AgBr + NaNO3

Above reaction shows double displacement reaction. Silver (Ag) and Sodium (Na) ions replaces each other.

Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Ans.

2AgNO3 + Cu --> Cu(NO3)2 + 2Ag

What do you mean by a precipitation reaction? Explain by giving examples.

Q15. What do you mean by a precipitation reaction? Explain by giving examples.

Ans.

When two solutions are mixed together, they chemically react and form one or more insoluble substance. Since the insoluble substance is called precipitate, the chemical reaction is called precipitate reaction.

for example

FeCl3 + 3NH4OH ---- > Fe(OH)3 +3NH4

In the above equation the solution of iron chloride and ammonium hydroxide is being mixed. It produces insoluble substance iron hydroxide.

Q16. Explain the followings in terms of gain and loss of oxygen with two examples each:

(a) Oxidation (b) Reduction

Ans. 

(a) Oxidation is a chemical process in which a substance gains oxygen.

for example -

 2Mg + O2 --- > 2MgO

2H2 + O2 --- > H2O

(b) Reduction is a chemical process in which a substance loss oxygen. 

for example - 

ZnO + C --- > Zn + CO

CuO + H2 --- > Cu + H2O

 A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed

Q17. A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

Ans.

The shiny brown coloured element is Copper (Cu).
The black cloured compound formed is Copper oxide (CuO)

2Cu + O2 --- > 2CuO

 Why do we apply paint on iron articles?

Q18. Why do we apply paint on iron articles?

Ans.

Iron materials tend to corrode in the presence of air and moisture. A brow coating forms on their surface if they are exposed to air. This brown layer spoils the iron. So, for the safety of iron materials we paint them. Paint gives safeguard to iron materials.

Oil and fat containing food items are flushed with nitrogen. Why?

Q19. Oil and fat containing food items are flushed with nitrogen. Why? 

Ans.

Oil and fat tend to react with oxygen are easily get spoiled. So, when oily food is packed with nitrogen gas so that nitrogen may cover the food and not let the oily food oxidised. 

(a) Corrosion (b) Rancidity

Q20. Explain the following terms with one example each.

(a) Corrosion (b) Rancidity

Ans.

(a) Corrosion- Corrosion is a chemical process in which metals are gradually eaten away when they come in contact with the air and moisture. 

Rusting of iron is one typical example of corrosion.


(b) Rancidity - Ranticidy is a process in which oil or fat oxidised gradually. It spoils the food. It produces a foul odour and taste in them.

Fried food easily gets spoiled due to rancidity.