Chapter 5 – Acids, Bases and Salts
1. Identify the odd one out and justify.
(a) Chloride, nitrate, hydride, ammonium
Ans: Ammonium
Reason: It is a basic radical and rest all are acidic radicals.
(b) Hydrogen chloride, sodium hydroxide, calcium oxide, ammonia
Ans: Hydrogen chloride
Reason: It is acidic and the rest are all basics.
(c) Acetic acid, carbonic acid, hydrochloric acid, nitric acid
Ans: Carbonic acid.
Reason: It is a dibasic acid and the rest are all monobasic acids.
(d) Ammonium chloride, sodium chloride, potassium nitrate, sodium sulphate
Ans: Ammonium chloride
Reason: It is made up of a strong acid and weak base and rest all are formed from strong acid and strong base.
(e) Sodium nitrate, sodium carbonate, sodium sulphate, sodium chloride
Ans: Sodium carbonate
Reason: It is made up of a weak acid and strong base, and rest all are formed from strong acid and strong base.
(f) Calcium oxide, magnesium oxide, zinc oxide, sodium oxide.
Ans: Zinc oxide
Reason: it is amphoteric oxide, and rest all are basic oxides.
(g) Crystalline blue vitriol, crystalline common salt, crystalline ferrous sulphate, crystalline sodium carbonate.
Ans: Crystalline common salt
Reason: It does not contain water of crystallisation. It is an ionic compound and ionic compounds are crystalline in nature and rest all have their crystalline structure because of their water of crystallization.
(h) Sodium chloride, potassium hydroxide, acetic acid, sodium acetate.
Ans: Acetic acid
Reason: It is an acid, the rest are all salts.
2. Write down the changes that will be seen in each instance and explain the reason behind it.
(a) 50ml water is added to 50ml solution of copper sulphate.
Ans: Copper sulphate solution is blue. It is a concentrated solution. When 50 ml of water is added to this concentrated solution, it becomes a diluted solution. The intensity of the blue colour is now different in this homogenous mixture.
(b) Two drops of the indicator phenolphthalein were added to 10ml solution of sodium hydroxide.
Ans: Sodium hydroxide is a base and phenolphthalein is a synthetic indicator. Sodium hydroxide solution will turn pink if phenolphthalein is added to it. It is a test for bases.
(c) Two or three filings of copper were added to 10ml dilute nitric acid and stirred.
Ans: When copper metal reacts with dilute nitric acid, the metal does not displace hydrogen from the acid like reaction with other metals. Instead the reaction produces nitric oxide, (NO).
3 Cu\(_{(s)}\) + 8 HNO₃\(_{(aq)}\) → 3 Cu(NO₃)₂\(_{(aq)}\) + 2 NO\(_{(g)}\) + 4 H₂O\(_{(l)}\)
Copper Nitric acid Copper Nitric Water
(dil) nitrate oxide
(d) A litmus paper was dropped into 2ml dilute HCl. Then 2ml concentrated NaOH was added to it and stirred.
Ans:
Blue litmus Paper :
HCl is hydrochloric acid, so the blue litmus turns red. When an equal amount of NaOH is added the colour again changes to blue and remains the same.
Red litmus paper :
Red litmus paper shows no colour change in hydrochloric acid. When some amount of NaOH is added the colour changes to blue initially but when the amount of NaOH is sufficient the blue colour disappears. Equal amounts of HCl and NaOH results in the formation of NaCl, a salt, and the solvent water. This reaction is called the neutralization reaction.
NaOH + HCl → NaCl + H₂O
Base Acid Salt Water
(e) Magnesium oxide was added to dilute HCl and magnesium oxide was added to dilute NaOH.
Ans:
(i) Magnesium oxide + dil HCl:
This is a neutralization reaction. Magnesium oxide is an insoluble base, it reacts with dilute HCl to produce a soluble salt MgCl₂ and water H₂O.
MgO + 2 HCl → MgCl₂ + H2O
(ii) Magnesium oxide + NaOH:
No chemical reaction takes place between magnesium oxide and sodium hydroxide.
(f) Zinc oxide was added to dilute HCl and zinc oxide was added to dilute NaOH.
Ans:
(i) Zinc oxide reacts with dilute hydrochloric acid to form zinc chloride and water. It is a neutralization reaction.
ZnO\(_{(s)}\) + 2 HCl\(_{(aq)}\) ZnCl₂ \(_{(aq)}\) + H₂O\(_{(l)}\)
(ii) Zinc oxide reacts with sodium hydroxide to form sodium zincate and water.
ZnO(s) + 2 NaOH\(_{(aq)}\) → Na₂ZnO₂\(_{(aq)}\) + H₂O\(_{(l)}\)
(g) Dilute HCl was added to limestone.
Ans: When hydrochloric acid is added to limestone, carbon dioxide is liberated. Limestone is calcium carbonate. Carbon dioxide is prepared in the laboratory using these chemicals.
CaCO₃ + 2 HCl → CaCl₂ + CO₂ + H₂O
(h) Pieces of blue vitriol were heated in a test tube. On cooling, water was added to it.
Ans: On heating, the crystalline structure of blue vitriol breaks down to form a colourless powder and water is released. This water is part of the crystal structure of blue vitriol. It is called water of cystallization.
On adding water to the white powder, a solution was formed which has the same colour as the copper sulphate salt solution.
(i) Dilute H₂SO4 was taken in an electrolytic cell and electric current was passed through it.
Ans: When a dilute solution of sulfuric acid is electrolysed, gases are produced at both the anode and the cathode electrode. The gas produced at the cathode burns with a ‘pop’ sound, when a sample is lit with a lighted splint. This shows that the gas is hydrogen. The gas produced at the anode relights a glowing splint dipped into a sample of the gas. This shows that the gas is oxygen. The gases are produced when ions move towards the electrodes.
At the cathode:
2H⁺ + 2e⁻ → H₂
At the anode:
4OH⁻ − 4e⁻ → 2H₂O + O₂
3. Classify the following oxides into three types and name the types.
CaO
MgO
CO₂
SO₃
Na₂O
ZnO
Al₂O₃
Fe₂O₃
Ans: There are three types of oxides : Basic oxides, Acidic oxides and Amphoteric oxides. Generally metal oxides are basic in nature. Exception Al₂O₃ and ZnO which are amphoteric. Generally non-metal oxides are acidic in nature.
| Basic oxides | Acidic oxides | Amphoteric oxides |
|---|---|---|
CaO, MgO, Na₂O, ZnO, Fe₂O₃ | CO₂. SO₃ | ZnO, Al₂O₃ |
4. Explain by drawing a figure of the electronic configuration.
a. Formation of sodium chloride from sodium and chlorine.
Ans:
b. Formation of magnesium chloride from magnesium and chlorine.
Ans:
5. Show the dissociation of the following compounds on dissolving in water, with the help of a chemical equation and write whether the proportion of dissociation is small or large.
Hydrochloric acid
Ans:
HCl\(_{(g)}\) \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) H⁺ \(_{(aq)}\) + Cl⁻\(_{(aq)}\)
Hydrochloric acid is a strong acid, as on dissolving in water, it dissociates almost completely and the resulting aqueous solution contains mainly H⁺ ions and the concerned acidic radical. The proportion of dissociation is large.
Sodium chloride
Ans:
NaCl\(_{(g)}\) \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) Na⁺\(_{(aq)}\) + Cl⁻\(_{(aq)}\)
When an ionic compound begins to dissolve in water, the water molecules push themselves in between the positive and negative ions of the compound and separate them from each other. The proportion of dissociation is large.
Potassium hydroxide
Ans:
KOH\(_{(s)}\) \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) K⁺\(_{(aq)}\) + OH⁻\(_{(aq)}\)
Potassium hydroxide is a strong base, as on dissolving in water, it dissociates almost completely and the resulting aqueous solution contains mainly OH⁻ ions and the concerned basic radicals. The proportion of dissociation is large.
Ammonia
Ans:
NH₃\(_{(g)}\) + H₂O\(_{(l)}\) \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) NH₄⁺\(_{(aq)}\) + OH⁻\(_{(aq)}\)
Ammonia dissolves in water to form NH₄OH (ammonium hydroxide). NH₄OH does not dissociate completely as it is a weak base. The aqueous solution contains a small proportion of OH⁻ ions and the concerned basic radical along with a large proportion of undissociated molecules of the base i.e. NH₄OH.
Acetic acid
Ans:
(CH₃COOH) \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) CH₃COO⁻ + H⁺
Acetic acid is a weak acid, on dissolving in water it does not dissociate completely, and the resulting aqueous solution contains H⁺ ion and the concerned acidic radical in small proportion along with a large proportion of the undissociated molecules of the acid. The proportion of dissociation is small.
Magnesium chloride
Ans:
MgCl₂ \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) Mg₂⁺ + 2 Cl⁻
Magnesium chloride dissolves in water and forms magnesium ions and chloride ions. When an ionic compound begins to dissolve in water, the water molecules push themselves in between the ions of the compound and separate them from each other. The proportion of dissociation is large.
Copper sulphate
Ans:
CuSO₄\(_{(s)}\) \(\xrightarrow[\mathrm{dissociation}]{\mathrm{Water}}\) Cu₂⁺ + SO₄²⁻
When copper sulphate dissolves in water, water molecules push themselves in between the ions of the compound and separate them from each other. The proportion of dissociation is large.
6. Write down the concentration of each of the following solutions in g/L and mol/L.
a. 7.3g HCl in 100ml solution
b. 2g NaOH in 50ml solution
c. 3g CH₃COOH in 100ml solution
d. 4.9g H₂SO₄ in 200ml solution
Ans:
7. Answer the following questions.
a. Classify the acids according to their basicity and give one example of each type.
Ans:
Basicity of acids : The number of H⁺ ions obtainable by the dissociation of one molecule of an acid is called its basicity. The acids are classified as monobasic, dibasic and tribasic acids based on the number of H⁺ ions present.
Examples of monobasic acid : HCl, HNO₃, CH₃COOH
Example of dibasic acid : H₂SO₄, H₂CO₃
Example of tribasic acid : H₃BO₃, H₃PO₄
b. What is meant by neutralization? Give two examples from everyday life of the neutralization reaction.
Ans: In neutralisation reaction, an acid reacts with a base to form salt and water. In a neutralisation reaction the acid dissociates to form H⁺ ions and base dissociates to form OH⁻ ions. They combine to form H₂O molecules which mix with the solvent.
Examples in daily life :
(i) When people suffer from acidity, they take some antacids to neutralise the acid in their stomach.
(ii) If an ant stings us the pain is due to formic acid. It is neutralised by rubbing moist baking soda which is basic in nature.
c. Explain what is meant by electrolysis of water. Write the electrode reactions and explain them.
Ans:
Electrolysis of water :
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through acidified water.
Cathode reaction :
2H₂O\(_{(l)}\) + 2e⁻ → H₂(\(_{(g)}\) + 2OH⁻\(_{(aq)}\)
Anode reaction :
2H₂O\(_{(l)}\) → O₂\(_{(g)}\) + 4 H⁺\(_{(aq)}\) + 4e⁻
It is found that in the above activity that the volume of gas formed near the cathode is double that of the gas formed near the anode. Hydrogen gas is formed near the cathode and oxygen gas near the anode. From this, it is clear that electrolysis of water has taken place and its constituent elements have been released.
8. Write the chemical equations for the following activities.
(a) NaOH solution was added to HCl solution.
Ans: When NaOH reacts with HCl, it gives NaCl and water.
NaOH\(_{(aq)}\) + HCl\(_{(aq)}\) → NaCl\(_{(aq)}\) + H₂O\(_{(l)}\)
Sodium Hydrochloric Sodium Water
hydroxide acid chloride
(b) Zinc dust was added to dilute H₂SO₄.
Ans: When zinc reacts with dilute sulphuric acid, it forms zinc sulphate and hydrogen gas is liberated.
Zn\(_{(s)}\) + H₂SO₄\(_{(aq)}\) → ZnSO₄\(_{(aq)}\) + H₂\(_{(g)}\)
Zinc Sulphuric Zinc Hydrogen
acid sulphate
(c) Dilute nitric acid was added to calcium oxide.
Ans: When dilute nitric acid reacts with calcium oxide, it forms calcium carbonate and water.
2HNO₃\(_{(l)}\) + CaO\(_{(g)}\) → Ca(NO₃)₂\(_{(s)}\) + H₂O\(_{(l)}\)
nitric calcium calcium water
acid (dil) oxide nitrate
(d) Carbon dioxide gas was passed through the KOH solution.
Ans: When carbon dioxide reacts with potassium hydroxide, it forms potassium carbonate and water.
CO₂\(_{(s)}\) + 2KOH\(_{(aq)}\) → K₂CO₃ \(_{(aq)}\) + H₂O\(_{(l)}\)
Carbon Potassium Potassium Water
dioxide hydroxide carbonate
(e) Dilute HCl was poured on baking soda.
Ans: When sodium bicarbonate reacts with hydrochloric acid, it forms sodium chloride, carbon dioxide and water.
NaHCO₃\(_{(s)}\) + HCl\(_{(aq)}\) → NaCl \(_{(aq)}\) + CO₂\(_{(g)}\) + H₂O\(_{(l)}\)
Sodium Hydrochloric Sodium Carbon Water
bicarbonate acid chloride dioxide
9. State the differences.
a. Acids and bases
Ans:
| Acids | Bases |
|---|---|
(i) A substance which liberates H⁺ ions when dissolved in water is an acid | (i) A substance which liberates OH⁻ ions in water is called a base. |
(ii) Blue litmus turns red in an acid. | (ii) Red litmus turns blue in a base. |
(iii) The pH of an acid is less than 7. | (iii) The pH of a base is greater than 7. |
(iv) Acids are sour to taste e.g. HCl, H₂SO₄ | (iv) Bases are bitter to taste. e.g. NaOH, KOH. |
b. Cation and anion
Ans:
| Cations | Anions |
|---|---|
(i) Cations are ions with a net positive charge. | (i) Anions are ions with a net negative charge. |
(ii) Cations are generally formed by metals. When metals donate electrons, they have excess protons, hence they form cations. | (ii) Anions are generally formed by non-metals. When non-metals accept electrons, they have excess electrons, hence they form anions. |
(iii) Cations are attracted towards the cathode which are negatively charged electrodes. | (iii) Anions are attracted towards the anode which are positively charged electrodes. |
(iv) e.g: Na⁺, Ca₂⁺, Mg₂⁺, K⁺ etc. | (iv) e.g: O₂⁻, S₂⁻, Cl⁻, Br⁻, etc. |
c. Negative electrode and positive electrode.
Ans:
| Negative Electrode | Positive Electrode |
|---|---|
(i) Negative electrodes are called cathode. | (i) Positive electrodes are called Anode. |
(ii) Positively charged cations move towards the cathode or negative electrode. | (ii) Negatively charged anions move towards the anode or positive electrode. |
10. Classify aqueous solutions of the following substances according to their pH into three groups : 7, more than 7, less than 7.
Common salt, sodium acetate, hydrochloric acid, carbon dioxide, potassium bromide, calcium hydroxide, ammonium chloride, vinegar, sodium carbonate, ammonia, sulphur dioxide.
Ans:
| ph < 7 | ph = 7 | ph > 7 |
|---|---|---|
sulphur dioxide, hydrochloric acid, carbon dioxide, ammonium chloride, vinegar | common salt, potassium bromide | sodium acetate, sodium carbonate, ammonia, calcium hydoxide |