Chapter 2 – Acids, Bases and Salts

Question 1.
You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Answer:

The content is each of the tube can be determined using red litmus paper.

  • If the litmus paper turns blue, the solution is basic.
  • If the litmus paper doesn’t change color, the solution is acidic.
  • If the litmus paper turns purple, the solution is distilled water.

Question 2.
Why should curd and other sour substances not be kept in containers made of brass or copper?

Answer:

Curd or any other sour substance are acidic and these substances combine with metal. This reaction turns the food to poison which is bad for health.

Question 3.
Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Answer:

When an acid reacts with metal, usually salt and hydrogen is released.

Metal + Acid → Salt + Hydrogen gas

Question 4.
A metallic 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 compounds formed is calcium chloride.

Answer:

The gas evolved in this reaction and that produces an effervescence is CO2.  

The reaction is mentioned below:

CaCO3(s) + HCl(aq) ➝ CaCl2(aq) + CO2 + H2O(l)

Question 5.
Why do HCl, HNO3,H2SO4 etc, show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose(C6H12O6) do not show acidic character?. Explain.

Answer:

In the listed acids, hydrogen atom is released in aqueous solution as hydrogen ion and hence they show acidic character.

The compounds glucose or alcohol, also have hydrogen but they don’t show nature of acidic nature. They will not separate to hydrogen ions, on dissolving in water.

Question 6.
Why does an aqueous solution of an acid conduct electricity?

Answer:

Charged particles like hydrogen ion are the reason behind the conductivity of electricity in acid.

Question 7.
Why does dry HCl gas not change the colour of dry litmus paper?
Answer:

A dry HCl has no free hydrogen ion, and hence it doesn’t show any acidic character. It just shows acidic character when we moisten the litmus paper.

Question 8.
While diluting an acid, why is it recommended to add acid to water and not water to the acid?

Answer:

Adding water to acids like HCl etc have a strong affinity for water. They show an exothermic process and emit a lot of heat. So it is always recommended to add acid to water to avoid such emissions.

Question 9.
How is concentration of hydronium ions (H3O+) affected when solution of an acid is diluted?

Answer:

On dilution, as the volume of the solution increases and hence the number of ions per unit volume decreases.

Question 10.
How is concentration of hydroxyl (OH) ions affected when excess of base is dissolved in solution of sodium hydroxide?

Answer:

Sodium hydroxide is a strong base. When base is dissolved in sodium hydroxide, the hydroxide ions are released but they will reach saturation in some time. After, the saturation there will be no effect even after adding base further.

Question 11.
You have two solutions A and B. The pH of solution A is 6 and that of solution B is 8. Which solution has more hydrogen ion concentration? Which of these is acidic and which one is basic?

Answer:

The pH of any solution is inversly proportional to concentration of hydrogen ion. Lesser the pH, more than concentration of Hydrogen ion.

Hence the solution A is acidic in nature because its pH is less than 7 and the solution B is basic in nature because its pH is more than 7.

Question 12.
What effect does concentration of H+(aq) have on acidic nature of a solution?

Answer:

The acidic nature of any solution is directly proportional to the concentration of Hydrogen ion. As the concentration of Hydrogen ion increases, acidic nature increases. If the hydrogen ion decreases, then solution turns basic.

Question 13.
Do basic solutions also have H+(aq)? If yes, then why are these basic?

Answer:

Yes basic solutions have hydrogen ion. But hydroxide ions in basic solution are more in basic solution. Hence the solution is basic in nature.

Question 14.
Under what soil conditions, do you think a farmer would spread or treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer:

If the soil is acidic in  nature, with pH more than 7, then the soil needs to be treated with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

Question 15.
What is the common name of the compound CaOCl2?

Answer:

Bleaching powder is the common name of the compound.

Question 16

 Name the substance which on treatment with chlorine yields bleaching powder?

Answer:

Calcium hydroxide is the substance which is yielded on treatment with chlorine.

Question 16

Name the sodium compound which is used for softening hard water?

Answer:

Sodium carbonate is used for softening hard water.(Na2CO3.10H2O)

Question 17

What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Answer:

On heating, sodium carbonate and carbon dioxide gas is evolved.

2NaHCO3 ➝ Na2CO3 + CO2 + H2O

Question 18.
Write the chemical equation for the reaction between Plaster of Paris and water.

Answer:

Question 19.
What is the common name of the compound CaOCl2?

Answer:

The common name of the compound is bleaching powder.

 

Chapter End Questions

Question 20
A solution turns red litmus blue. Its pH is likely to be
(a) 2
(b) 4
(c) 7
(d) 10

Answer: 10

Question 21

A solution reacts with crushed egg-shells to give a gas that turns lime water milky. The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Answer: HCl

Question 22

10 mL of solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL

Answer: 16mL

Question 23
Which of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer: Antacid

Question 24
Write the word equation and the balanced equations for the reactions when:
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.

Answer:

(a) Zinc + Sulphuric add ➝ Zinc sulphate + Hydrogen

Zn(s) + H2SO4(dil.) ➝ ZnSO4(aq) + H2(g)

(b) Magnesium + Hydrochloric acid ➝ Magnesium chloride + Hydrogen

Mg(s) + 2HCl (dil.) ➝ MgCl2(aq) + H2(g)

(c) Aluminium + dilute sulphuric acid ➝ Aluminium sulphate + Hydrogen

2Al(s) + 3H2SO4(dil.) ➝ Al2(SO4)3(aq) + 3H2(g)

(d)  dilute Hydrochloric Acid + Iron → Iron Chloride + Hydrogen Gas

Fe(s) + 2HCl(dil.) ➝ FeCl2(aq) + H2(g)

Question 25
Compounds such as alcohol and glucose also contain hydrogen but are not characterized as
acids. Describe an activity to prove it.

Answer:

In a glass beaker, take a solution of glucose (C6H12O6). Fix two small iron nails in a rubber cork and place the cork in the beaker as shown below. Connect the iron nails to the terminals of a 6 volt battery through a bulb. Switch on the current. The bulb will not glow. This shows that the electric current has not passed through the glucose solution. As the current is carried by the movement of ions, it shows that the solution doesn’t conduct electricity.

Question 26
Why does distilled water not conduct electricity whereas rain water does?

Answer:

Distilled water, doesn’t contain any ionic compounds as it is a very weak electrolyte. So no electricity is conducted but rain water contains dissolved acids so rain water is good conductor of electricity.

Question 27
Why does an acid not show any acidic behaviour in the absence of water?

Answer:

The acidic behaviour of a substance is because of Hydrogen ions. By adding water, the ions are dissociated and hence show acidic behaviour.

Question 28
Five solutions A, B, C, D and E when tested with universal indicator show pH as 4, 1, 11, 7 and 9, respectively. Which solution is:
(a) neutral
(b) strongly alkaline
(c) strongly acidic
(d) weakly alkaline
(e) weakly acidic
Arrange the pH in increasing order of H+ concentration.
Answer:
(a) Neutral: D with pH = 7
(b) Strongly alkaline: C with pH = 11
(c) Strongly acidic: B with pH = 1
(d) Weakly alkaline: E with pH = 9
(e) Weakly acidic: A with pH = 4
Increasing order of H+ concentration:
C<E<D<A<B

Question 29

Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Answer:

Fizzing will occur more in Tube A, because HCl is a very strong acid there is a lot of liberation of hydrogen gas from test tube A.

Question 30
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Answer:

Fresh milk is converted into curd because of lactic acid, which decreases the pH of the milk.

Question 31
A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does milk take a long time to set as curd?

Answer:

(a) This is done to prevent milk from getting sour because of production of lactic acid.

(b) This is because as the lactic acid produced will first neutrilise the acid and then the pH is reduced to turn milk to curd.

Question 32
Why should Plaster of Paris be stored in a moisture-proof container?

Answer:

Plaster of paris, gets hydrated and change to gypsym in the presence of moisture.

Question 33
What is neutralisation reaction? Give two examples.

Answer:

A reaction of acid and base gives salt and water, which is known as neutralisation.

Examples:

Acid + Base ➝ Salt + Water
HCl(aq) + NaOH(aq) ➝ NaCl(aq) + H2O(l)
HNO3(aq) + KOH(aq) ➝ KNO3(aq) + H2O(l)

Question 34
Give two important uses of washing soda and baking soda.

Answer:

Baking Soda:

  • It can be used to test garden soil for acidity. If bubbles erupt then the soil Is too acidic
  • It is used in making aerated soft drinks.

Washing Soda:

  • It is used as an electrolyte
  • It is used as a domestic cleaning agent.

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