2. Acids, Bases and Salts

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Questions :

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: We will identify the contents of each test tube by following procedure:
i. We will put the red litmus paper in the 3 test tubes stepwise. The one in which it's color changes to blue will contain the basic solution as we know that of the remaining 2 one will contain acidic and one neutral and that basic solution changes red litmus to blue. We will mark it as basic and will keep it aside.
ii. Now we will put that blue litmus paper into the remaining solutions stepwise. The one in which it's color changes to red will contain the acidic solution as of the remaining 2 one is acidic and one neutral and neutral solution doesn't change the colour of litmus. We will mark it as acidic and will keep it aside.
iii. As the one remaining test tube's solution had neither had changed the colour of red litmus to blue nor blue to red, so by properties of neutral solution we conclude that it is neutral solution and from the given we can state that it contains distilled water. We will mark it as neutral and will keep it aside.
iv. Now we had identified the contents of each test tube.

Questions :

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

Answer: We can say from the property that acids are sour that curd and sour substances contain acids. Brass is a alloy of 2 metals and copper is a metal. And if such acidic substances are kept in metal vessels they will react with each other and form the deadly salts which can give rise to extreme conditions and can even lead to death of a human and so to avoid this it is better not to keep curd and sour substances in brass and copper vessels.

2. 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: Hydrogen gas is usually liberated when an acid reacts with a metal as displacement reaction happens and due to low reactivity of hydrogen the metal displaces it from acid.
The illustration of this statement can be done by following experiment:

Procedure:

i. Take about 5 mL of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it.

Observation:

i. On the surface of zinc granules we observe the colorless ZnSO₄.
ii. A gas being evolved through the soap solution in form of bubbles.
iii. Now we will test for the presence of the hydrogen gas by taking a burning candle near a gas filled bubble and we observe that it makes a pop sound.

Conclusion:

i. Hydrogen gas is liberated by reaction. As it is only the combustible gas which makes a pop sound.
ii. Hence, we conclude that hydrogen gas is usually liberated when an acid reacts with a metal.

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 compounds formed is calcium chloride.

Answer: As the gas evolved produce effervescence and extinguishes a burning candle, it is carbon dioxide. And as one product is CaCl₂ the metal compound 'A' must be Calcium Carbonate. Then, the balanced chemical equation for the reaction is: CaCO₃(s) + 2HCl(Aq) → CaCl₂(Aq) + CO₂(g) + H₂O(l)

Questions :

1. Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Answer: The reason for this phenomenon is the dissociated H+ ion from HCl, HNO₃ which later form the hydronium ions by bonding with water molecules, and increase of hydronium ions increase the acidic character of an acid in aqueous solution. Acids like HCl, HNO₃ dissociate their H+ ion which later bond with water but in case of alcohol and glucose the dissociation process is absent, even having the hydrogen element they still doesn't dissociate H+ ion which led to absence of hydronium ions and in turn absence of acidic character.

2. Why does an aqueous solution of an acid conduct electricity?

Answer: The main reason behind conduction of electricity in liquid are ions. The quantity of ions in an aqueous solution is directly proportional to conductivity of electricity. And in an aqueous solution of an acid there are lots of ions present due to dissociation of H+ ions from acid molecules and therefore due to these H+ ions, aqueous solution of an acid conducts electricity.

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

Answer: We know that the dissociated hydrogen ions are the reason for the acidic character of an acid, but acids dissociate their ions in solvents especially like water but as here HCl is a gas, from which moisture should be removed and hence it doesn't contain moisture also as the litmus paper is also dry, therefore there will be unavailability of water to HCl for dissociation of hydrogen ions and hence it will not be able to show acidic character and finally not be able to turn blue litmus to red.

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

Answer: While diluting an acid, it is recommended that the acid should be added to water and not water to the acid because mixing of water to acid is a highly exothermic reaction and hence if water is added to concentrated acid it may release so much heat that it can even vaporise water but also spatter the acid with even an explosion which can lead to severe problems to the person present there like, acid burns and burning of clothes which can even lead to death. To avoid all this acid is added to water so that tremendous amount of heat should not be produced at a time, and hence the intensity of heat should be minimised this happens as when we add water to acid then from the very start it becomes extremely dilute and had not the capacity to explode, and thus the bad results can be avoided with such small recommendation.

5. How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?

Answer: The concentration of hydronium ions (H₃O⁺) is inversely affected when a solution of an acid is diluted, it is No of hydronium ions per unit volume. As hydronium ions is the effect due to dissociated ions from acid, they remains constant when the mass of acid remain constant and when more water molecules are added it leads to decrease of hydronium ions per unit volume, and thus decrease in concentration of H₃O+.

6. How is the concentration of hydroxide ions (OH^–) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer: The concentration of hydroxide ions (OH^–) is affected positively when excess base is dissolved in a solution of sodium hydroxide. As it is No of hydroxide ions per unit volume. When excess base is added it leads to more hydroxide ions and thus as the volume of solvent or water is constant, there is net increase in concentration of OH^–.

Questions :

1. 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: We can find the solution with more hydrogen ion concentration from the following law that is: The hydrogen ion concentration of a solution is inversely proportional to its pH. Hence, as pH_A < pH_B, solution A will have more hydrogen ion concentration. Also, we know that hydrogen ion concentration is more in acids and less in bases and thus, solution A is acidic and solution B is basic.

2. What effect does the concentration of H+(aq) ions have on the nature of the solution?

Answer: The concentration of H⁺(aq) ions have an acidic effect on the nature of the solution, it means that more is the concentration of H⁺(aq) ions in the solution the more the solution becomes acidic and vice versa.

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

Answer: Yes, basic solutions also have H⁺(aq) ions but in very less quantity. And the reason they are basic are the OH^-(aq) ions present in plenty in them which makes them basic and also neutralize the acidic effect shown by H⁺(aq) ions. As the hydroxide ion concentration of a solution is directly proportional to its pH.

4. 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: When the pH condition of the soil of his fields will become less than 7 means when his field's soil will turn acidic he will treat it with CaO or Ca(OH)2 or CaCO3. As these all are bases and will neutralize the acidic soil and will make the soil neutral, which will be extremely beneficial for crop yield. As, plants require pH nearby 7 for better growth.

Questions :

1. What is the common name of the compound CaOCl2?

Answer: The common name of the compound CaOCl2 is bleaching powder.

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

Answer: The substance which on treatment with chlorine yields bleaching powder is calcium hydroxide[Ca(OH)₂].

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

Answer: The sodium compound which is used for softening hard water is sodium carbonate[Na₂CO₃].

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

Answer: If a solution of sodium hydrocarbonate is heated, then decomposition reaction will happen and it will give us sodium carbonate, water and carbon dioxide gas ass products. The equation of the reaction involved is: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

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

Answer: Equation: 2CaSO4.1/2H2O + 3H2O → 2CaSO4.2H2O

Exercises :

1. A solution turns red litmus blue, its pH is likely to be

(a) 1
(b) 4
(c) 5
(d) 10

Answer: (d) 10, Reason: Because only bases turn red litmus blue and pH of base is above 7, as in options only (d) is greater than 7 it is correct.

2. 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: (b) HCl, Reason: As HCl is the only acid in these options and we know how it reacts with the calcium carbonate present in the egg-shell, all the other are neutral salts which will not react with CaCO3, so option (b) is correct. The reaction is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

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
(c) 12 mL
(d) 16 mL

Answer: (d) 16 mL, Reason: By the law of constant proportions, we can calculate the required amount, 10:8=20:x, 10x=160 → x=16.

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

(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer: (c) Antacid, Reason: Antacids are bases which neutralize the effect of excess acid produced in our stomach during indigestion causing pain and irritation, and thus providing relief to us. E.g : Mg(OH₂)

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.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.

Answer: (a). Word Equation: Zinc + Sulphuric acid(dilute) → Zinc Sulphate + Hydrogen Gas
Balanced equation: H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)
(b). Word Equation: Magnesium + Hydrochloric acid(dilute) + → Magnesium Chloride + Hydrogen Gas
Balanced equation: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
(c). Word Equation: Aluminium + Sulphuric Acid(dilute) → Aluminium Sulphate + Hydrogen Gas
Balanced equation: 3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)
(d). Word Equation: Iron + Hydrochloric Acid(dilute) + → Ferrous Chloride + Hydrogen Gas
Balanced equation: 2HCl(aq) + Fe(s) → FeCl₂(aq) + H₂(g)

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

Answer: Activity:-

Procedure:

i. Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc.
ii. Fix two nails on a cork, and place the cork in a 100 mL beaker.
iii. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch.
iv. Now pour some dilute HCl in the beaker and switch on the current, observe that the bulb glows or not.
v. Repeat the same with dilute sulphuric acid, alcohol and glucose and note the observations.

Observation:

i. The bulb glow in case of HCl, H₂SO₄.
ii. The bulb do not glow in case of alcohol, glucose.

Conclusion:

i. Alcohol and glucose are not acids as acids conduct electricity through them.
ii. HCl, H₂SO₄ are acids as they conducted electricity through them.
We know that compounds such as alcohol and glucose contain hydrogen but from these activity we conclude that they are not categorised as acids as they didn't showed the property of acids.

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

Answer: The reason for electricity to conduct in a liquid is the ions present in it, but as we know distilled water is pure water and doesn't contain any impurities and ions, hence it doesn't conduct electricity. But rain water is contaminated and contains dissolved gases in it which form acids on coming in contact with water in clouds, like CO₂ forms carbonic acid, SO₂ forms sulphuric acid, nitrogen oxides form nitric acids and as acids dissociate ions in water, rain water contains ions. Thus, due to presence of ions in rain water it conducts electricity but distilled water does not.

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

Answer: The acidic behaviour of an acid is due to the dissociated hydrogen ions, and as acids dissociate their H+ ion only in the presence of water-like polar molecules and best in water, and not in other substances, without water they will not dissociate their H+ ion and thus will not show acidic behaviour in the absence of water.

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?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?

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

Answer: The pH of a substance is inversly proportional to its hydrogen-ion concentration. Also, pH=7 is neutral, pH>7 is basic and pH<7 is acidic. Then, pH of A,B,C,D and E in increasing order of hydrogen-ion concentration is:
pH_C< pH_E< pH_D< pH_A< pH_B
Thus, Solution D is neutral.
Solution C is strongly alkaline.
Solution B is strongly acidic.
Solution A is weakly acidic.
Solution E is weakly alkaline.

10. 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. In which test tube will the fizzing occur more vigorously and why?

Answer: In the test tube A the fizzing will occur more vigorously. The reason behind fizzing here is the released hydrogen gas on reaction of Mg with HCl, CH₃COOH. And as pH of HCl is more than of acetic acid, it is more strong acid. The more an acid is strong the more it reacts vigorously, also the amount of magnesium to both for reaction is equal. So, the HCl will react more with the magnesium and will produce more hydrogen gas, and the more the hydrogen gas produced the more will be fizzing and hence, in the test tube 'A' the fizzing will occur more vigorously than test tube 'B'.

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

Answer: The pH will decrease as the fresh milk will turn into curd, this is because milk is changed into curd due to the change of lactose sugar present in milk to lactic acid by lactobacilli. As, milk changes to curd gradually the concentration of an acid increases in it and acid always have less pH then due to this its pH will gradually decrease.

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?

Answer: (a) He does this so that his milk would not get converted into curd in a short time, his milk would not become sour and he can sell his customers milk for longer time.
(b) The milk contains a base in it so when the lactic acid will be formed in the milk instead of turning the milk into curd it will be neutralized by the base first and after the neutralization of base the process of setting curd will start and thus more time will be required for it as both the acid and time is utilized in neutralization of base.

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

Answer: POP is a white powder which on mixing with water turns into a hard, solid mass called gypsum. If moisture in atmosphere come in contact with it will gradually turn to gypsum which can not be used for any purpose, and had to be again heated to turn it back to Plaster of Paris and so time and energy both will be wasted. To avoid this Plaster of Paris should be stored in a moisture-proof container where it will not be in contact with water molecules and thus will remain in its original form, for longer time which will be beneficial for its instant availability for instant use.

14. What is a neutralisation reaction? Give two examples.

Answer: The reaction in which an acid and a base react to form a salt and water, is known as a neutralisation reaction.
Examples: i. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.
ii. KOH + HCl → KCl + H₂O.

15. Give two important uses of washing soda and baking soda.

Answer: Uses of Washing Soda(Na₂CO₃.10H₂O):-
i. It is used in glass, soap and paper industries.
ii. It is used for removing permanent hardness of water.

Uses of baking soda(NaHCO₃):-
i. It is an important ingredient in antacids.
ii. It is also used in soda-acid fire extinguishers.

Best of luck and keep learning and be growing.