Acids, Bases and Salts textual questions

Chapter:- 2. Acids, Bases and Salts

textual questions

1.Five solutions A, B, C, D and E when tested with universal indicator showed p^H as 4, 1, 11, 7, and 9, respectively, which solution is (AS₁) (a) neutral (b) strongly alkaline (c) strongly acidic (d) weakly acidic (e) weakly alkaline Arrange the p^H in increasing order of hydrogen ion concentration.

Ans:-

p H VALUE	NATURE OF THE SUBSTANCE
p H < 7	ACID
p H < 7 and is closer to 0	STRONG ACID
p H < 7 and is closer to 7	WEAK ACID
p H > 7	BASE
p H > 7 and is closer to 14	STRONG BASE
p H > 7 and is closer to 7	WEAK BASE
p H = 7	NEUTRAL

frrom the above table

1. p^H solution A = 4. So, it is weakly acidic.
2. p^H of solution B = 1. So, it is strongly acidic.
3. p^H of solution C = 11. So, it is strongly alkaline.
4. p^H of solution D = 7. So, it is neutral.
5. p^H of solution E = 9. So, it is weakly alkaline.
The p^H in increasing order of hydrogen ion concentration is 11 < 9 < 7 < 4 < 1

2.What is a neutralization reaction? Give two examples. (AS₁)

Ans: -
Neutralization reaction: - The reaction between an acid and a base to give salt and water is called a neutralization reaction.
In general,
Acid + Base → Salt + Water
Ex: - HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O

3.What happens when an acid or base is mixed with water? (AS₁)

Ans: -
Mixing an acid or base with water results in a decrease in the concentration of ions (H₃O⁺/OH^-) unit volume.
This process is called dilution and the acid or the base is said to be diluted.

4.Why does tooth decay start when the p^H of the mouth is lower than 5.5? (AS₁)

Ans: -
1. Tooth decay starts when the p^H of the mouth is lower than 5.5.
2. Tooth enamel is made of calcium phosphate which is the hardest substance in the body.
3. But it is corroded when the p^H in the mouth is below 5.5.
4. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth.
5. Using tooth pastes, which are generally basic, neutralize the excess acid and prevent tooth decay.

5.Why does not distilled water conduct electricity? (AS₂)

Ans: -
1.Distilled water is the purest form of water.
2. It didn't have ions (H₃O⁺ /OH^-) to conduct electricity.

6.Why pure acetic acid does not conduct electricity? (AS₂)

Ans: -
1.Acids produce ions (H⁺/H₃O⁺) when they dissolve and dissociate in water. So, acids show electric conductivity only in the presence of water.
2. As pure Acetic acid does not contain water, it does not dissociate into H+ ions.
4. Hence it does not conduct electricity.

7.A milkman adds a very small amount of baking soda to fresh milk. (AS₂)
a) Why does he shift the p^H of the fresh milk from 6 to slightly alkaline?
b) Why does this milk take a long time to set as curd?

Ans: -
a). The milkman adds a little baking soda to fresh milk to make it preserved for a long time.
b). Addition of baking soda reduces the acidic nature of lactic acid present to convert milk into curd. It takes more time to produce lactic and long time to set as curd.

8.Plaster of Paris should be stored in a moisture-proof container. Explain why? (AS₂)

Ans: -
1. Plaster of Paris is a white powder and on mixing with water, it sets into a hard-solid mass due to the formation of gypsum.

CaSO4 . 1/2H2O	+	1 /2 H2O	→	CaSO4 .2H2O
(Plaster of Paris)				(Gypsum)

9.Compounds such as alcohols and glucose contain hydrogen but are not categorized as acids. Describe an activity to prove it. (AS₃)

Ans: -
1. Prepare solutions of glucose and alcohol.
2. Connect two different coloured electrical wires to graphite rods separately in a 100 ml beaker as shown in figure.
3. Connect free ends of the wire to 230 volts AC plug and complete the circuit by connecting a bulb to one of the wires.
4. Now pour some glucose solution in the beaker and switch on the current.
5. We will notice that the bulb does not glow.
6. Repeat this experiment with alcohol solution in the beaker. The bulb does not glow again.
7. Because glucose and alcohol solutions did not contain H + ions to conduct electricity.
8. So glucose and alcohols are not categorized as acids because they do not produce H⁺ ions in water.

10.What is meant by "water of crystallization" of a substance? Describe an activity to show the water of crystallisation. (AS₃)

Ans: -
Water of crystallization: The fixed number of water molecules present in one formula unit of a salt is called Water of crystallization.
Activity: -
Materials required: Copper sulphate crystals, boiling tube, test tube holder, Bunsen burner.
1. Observe the color of copper sulphate crystals (CuSO₄.5H₂0). They are blue in color.
2.Take a few crystals of copper sulphate in a test tube and heat it strongly.
2. We observed that after heating, the blue colour of the copper sulphate turns to white powder (CuSO₄).
3. We also observed the water droplets on the sides of the test tube.
4. Now add a few drops of water to the white copper sulphate obtained after heating.
5. We noticed that the blue colour of copper sulphate was restored.
6. Because Copper sulphate crystals contain water molecules in their crystals, without water molecules it will be in the form of powder.
7. When these crystals are heated, water present in crystals is evaporated and salt turns to white powder.
8.When water is added to white powder, the blue colour reappears and again the crystals are formed.

11.Draw a neat diagram showing acid solution in water conducts electricity. (AS₅)

Ans: -

12.How does the flow of acid rain into a river make the survival of aquatic life in a river difficult? (AS₇)

Ans: -
1. Living organisms can survive only in a narrow range of p^H change.
2. When p^H of rain water is less than 5.6, it is called acid rain.
3. When acid rain flows into the rivers, it lowers the p^H of the river water.
4. Then the survival of aquatic life in such rivers becomes difficult.

13.What is baking powder? How does it make the cake soft and spongy? (AS₇)

Ans: -
1.Baking powder: Baking powder is a mixture of baking soda and a mild edible acid such as tartaric.
2. When baking powder is heated or mixed in water, the following reaction takes place.
3. NaHCO₃ + H⁺ → CO₂ + H₂O ⁺ sodium salt of acid.
4. Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.

14.Give two important uses of washing soda and baking soda (AS₇)

Ans: -
Uses of Washing soda (Sodium carbonate - Na₂CO₃):
1. Washing soda is used in glass, soap and paper industries.
2. It is used in the manufacture of sodium compounds such as borax.
3. It can be used as a cleaning agent for domestic purposes.
4. It is used for removing permanent hardness of water.
Uses of Baking soda (Sodium hydrogen carbonate - NaHCO₃):
1. Sodium hydrogen carbonate is used in the preparation of Baking powder.
2. It is used as antacid to neutralize excess acid in the stomach and provides relief
3. It is also used as soda-acid in fire extinguishers
4. It acts as a mild antiseptic.