Lesson: Acids, Bases, and Salts

Question:1

A solution turns red litmus blue; its pH is likely to be:

(a) 1

(b) 4

(d) 10

Solution:

(d)

Question:2

A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains:

(a) NaCl

(b) HCl

(d) KCl

Solution:

(b)

Question:3

10 mL of a 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 HCl solution (the same solution as before) required to neutralise it will be:

(a) 4 mL

(b) 8 mL

(d) 16 mL

Solution:

(d)

Question:4

Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(d) Antiseptic

Solution:

(c)

Question: 5

Write word equations and then balanced equations for the reaction taking place when $-$

(a) Dilute sulphuric acid reacts with zinc granules.

(b) Dilute hydrochloric acid reacts with magnesium ribbon.

(d) Dilute hydrochloric acid reacts with iron filings.

Solution:

a) $S u l p h u r i c a c i d + Z i n c \to Z i n c s u l p h a t e + H y d r o g e n$

(b) $H y d r o c h l o r i c a c i d + M a g n e s i u m \to M a g n e s i u m c h l o r i d e + H y d r o g e n$

(d) $H y d r o c h l o r i c a c i d + I r o n \to F e r r i c c h l o r i d e + H y d r o g e n$

Question: 6

Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Solution:

· Take two nails fitted on a cork and kept in a 100-mL beaker.

· Connect the nails to two terminals of a 6-volt battery through a bulb and a switch.

· Pour dilute HCl in the beaker such that the nails dip into it.

· Switch on the current.

· Repeat the same experiment with glucose solution and alcohol solution.

Observations:

It is observed that the bulb glows in the HCl solution and does not glow in the glucose and alcohol solution.

Result:

HCl dissociates into $H^{+} a n d C l^{-}$ ions. These ions conduct electricity in the solution resulting in glowing of the bulb.

Alcohol and glucose solutions do not dissociate into ions and therefore they do not conduct electricity.

That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.

Question: 7

Why does distilled water not conduct electricity, whereas rain water does?

Solution:

Distilled water is neither acidic nor basic in nature. So it does not dissociate into ions. Therefore, it does not conduct electricity.

Rainwater becomes acidic due to its reaction with acidic gases. Like any other acid, rain water conducts electricity due to the formation of ions.

Question: 8

Why do acids not show acidic behaviour in the absence of water?

Solution:

An acid ionises on dissolving in water to produce hydrogen ions. It is the presence of these ions that makes it behave like an acid.

An acid will not show acidic behaviour in the absence of water.

Question: 9

Five solutions A, B, C, D and E when tested with universal indicator showed pH as:

4, 1, 11, 7 and 9, respectively. Which solution is:

(a) Neutral?

(b) Strongly alkaline?

(d) Weakly acidic?

(e) Weakly alkaline?

Arrange the pH in increasing order of hydrogen-ion concentration.

Solution:

(a) $N e u t r a l \to S o l u t i o n D w i t h p H 7$

(b) $S t r o n g l y a l k a l i n e \to S o l u t i o n C w i t h p H 11$

(d) $W e a k l y a c i d i c \to S o l u t i o n A w i t h p H 4$

(e) $W e a k l y a l k a l i n e \to S o l u t i o n E w i t h p H 9$

The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1.

Question: 10

Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid ( $H C l$ ) is added to test tube A, while acetic acid ( $C H_{3} C O O H$ ) 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?

Solution:

$H C l$ is a stronger acid than $C H_{3} C O O H$ . It produces hydrogen gas at a faster speed due to which the fizzing occurs. Therefore, the fizzing will occur vigorously in test tube A, in which hydrochloric acid ( $H C l$ ) is added.

Question: 11

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

Solution:

The pH of milk is 6. As it turns into curd, the pH will reduce because curd is more acidic in nature than milk.

Question: 12

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 this milk take a long time to set as curd?

Solution:

(a) In an alkaline condition, milk does not set as curd easily. The milkman shifts the pH of the fresh milk from 6 to slightly alkaline to keep it fresh for a longer time.

(b) When baking soda is added, milk becomes more basic than usual. Hence, acids produced to set the milk as curd are neutralized by the base added to milk. Therefore, it takes a longer time for the milk to set as curd.

Question: 13

Plaster of Paris should be stored in a moisture-proof container. Explain why?

Solution:

Plaster of Paris (POP) should be stored in a moisture-proof container because Plaster of Paris absorbs water (moisture) to form a hard solid called gypsum.

$\begin{array}{l} {CaSO}_{4} . \frac{1}{2} H_{2} O+1 \frac{1}{2} H_{2} O \to {CaSO}_{4} {.2H}_{2} O \\ (POP) Gypsum (Hard Solid) \end{array}$

Question: 14

What is a neutralisation reaction? Give two examples.

Solution:

A reaction in which an acid reacts with a base to give a salt and water is called neutralisation reaction. Given below are two examples of neutralisation reaction:

1. $N a O H + H C l \to N a C l + H_{2} O$

2. $M g {(O H)}_{2} + 2 H C l \to M g C l_{2} + 2 H_{2} O$

Question: 15

Give two important uses of washing soda and baking soda.

Solution:

Washing soda is used:

(a) In glass, soap, and paper industries.

(b) To remove permanent hardness of water.
Baking soda is used:

(a) In the food industries, mainly in the bakeries, to make the bread or cake fluffy.

(b) In soda-acid fire extinguishers.