Lesson: 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?

## Solution:

The content of the tube will be an acid if the colour of red litmus does not change when dipped into it.

It is a base if the colour of red litmus changes to blue.

If there is a slight change in the colour of red litmus (purple), then it is distilled water.

# Question:2

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

## Solution:

Curd and other sour substances contain acids. Therefore, when they are kept in brass or copper vessels, the metal reacts with the acid. This results in release hydrogen gas and other harmful products. This spoils the food.

# 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?

## Solution:

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

Take few pieces of zinc granules in a test tube.

Add 5 ml of dilute ${H}_{2}S{O}_{4}$.

Shake it and pass the gas produced through a soap solution.

Bubbles are formed in the soap solution are formed. These soap bubbles contain hydrogen gas.

${H}_{2}S{O}_{4}+Zn\to ZnS{O}_{4}+{H}_{2}↑$

Now test if the evolved gas is hydrogen or not.

When a candle is brought near the soap bubbles the gas burns with a pop sound.

This proves that the evolved gas is hydrogen.

# Question:4

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 compounds formed is calcium chloride.

## Solution:

The metal compound formed due to the reaction is calcium chloride. This means that the metal compound has calcium metal. The gas that extinguishes a burning candle is carbon dioxide. This means the metal compound is calcium carbonate.

$CaC{O}_{3}\left(s\right)+2\text{\hspace{0.17em}}HCl\left(aq\right)\to CaC{l}_{2}\left(aq\right)+C{O}_{2}\left(g\right)+{H}_{2}O\left(l\right)$

# Question:5

Why do $HCl\text{\hspace{0.17em}}$, $HN{O}_{3}$, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

## Solution:

When $HCl\text{\hspace{0.17em}}$ or $HN{O}_{3}$ are mixed with water, they dissolve in water to form hydrogen ion $\left({H}^{+}\right)$ or hydronium ion $\left({H}_{3}{O}^{+}\right)$. Due to the presence of these ions, these compounds show their acidic character.

When alcohol and glucose are mixed with water, they do form hydrogen or hydronium ions. Hence, they do not show acidic character.

# Question: 6

Why does an aqueous solution of acid conduct electricity?

## Solution:

Acids ionises in aqueous solutions.These ions are responsible for conducting electricity.

# Question: 7

Why does dry $HCl$ gas not change the colour of the dry litmus paper?

## Solution:

Dry $HCl$ does not show it acidic property unless it is dissolved in water. When dissolved in water, they form ${H}^{+}$ or ${H}_{3}{O}^{+}$ ions. Due to these ions, these compounds show their acidic character. This ionization does not happen in case of dry $HCl$ gas and hence dry $HCl$ gas does not change the colour of the dry litmus paper.

# Question: 8

While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

## Solution:

The reaction of acid and water is exothermic in nature. If water is added to an acid, the heat generated can cause burns. To slow down the generation of heat, it is always recommended that acid should be added to water.

# Question: 9

How is the concentration of hydronium ions ( ${H}_{3}{O}^{+}$ ) affected when a solution of an acid is diluted?

## Solution:

When water is added to a solution of acid, the solution becomes diluted.

The concentration of hydronium ions ( ${H}_{3}{O}^{+}$ ) decreases when a solution of an acid is diluted. This means that the strength of the acid decreases.

# Question: 10

How is the concentration of hydroxide ions ( $O{H}^{–}$ ) affected when excess base is dissolved in a solution of sodium hydroxide?

## Solution:

Sodium hydroxide is basic in nature. When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions ( $O{H}^{–}$ ) would increase.

# Question: 11

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?

## Solution:

An acid has a pH value less than 7 while a base has a pH value more than 7.

The solution with $pH=6$ is acidic and has more hydrogen ion concentration than the basic solution.

# Question: 12

What effect does the concentration of ${H}^{+}$ (aq) ions have on the nature of the solution?

## Solution:

Concentration of ${H}^{+}$ (aq) can have varied effects on the nature of the solution. If the concentration of ${H}^{+}$ ions increases, the solution becomes more acidic. If the concentration of ${H}^{+}$ ions decreases, the solution becomes more basic in nature.

# Question: 13

Do basic solutions also have ${H}^{+}$ (aq) ions? If yes, then why are these basic?

## Solution:

Yes, basic solutions also have ${H}^{+}$ (aq) ions. They are basic despite having ${H}^{+}$ (aq) solution. This is due to the presence of less concentration of ${H}^{+}$ (aq) compared to the concentration of $O{H}^{–}$ ions in the solutions.

# Question: 14

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)?

## Solution:

A plant grows the best when the soil is neither acidic not basic. The farmer would treat the soil with quick lime or slaked lime or chalk when the soil is acidic in nature. Quick lime or slaked lime or chalk are basic in nature and neutralizes the acidic nature of the soil when they are added to it.

# Question:15

What is the common name of the compound $CaOC{l}_{2}$?

## Solution:

The common name of the compound $CaOC{l}_{2}$ is bleaching powder.

# Question:16

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

## Solution:

Calcium hydroxide $\left[Ca{\left(OH\right)}_{2}\right]$

# Question: 17

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

## Solution:

Washing soda $\left(N{a}_{2}C{O}_{3}.10\text{\hspace{0.17em}}{H}_{2}O\right)$

# Question: 18

What will happen if a solution of sodium hydro carbonate is heated?

Give the equation of the reaction involved.

## Solution:

When a solution of sodium hydro carbonate is heated, sodium carbonate and water are formed. Carbon dioxide too is released in this process.

# Question:19

Write an equation to show the reaction between Plaster of Paris and water.