Chapter 06: Acids and Bases

Acids

Acids are compounds that ionises to form hydrogen ions (H⁺) when dissolved in water.

The presence of H⁺ ions gives an acid its acidic properties.

Characteristics of Acids

• Acids taste sour.
• Acids turn blue litmus paper red.
• Acids have pH < 7.
• Acids dissolved in water can conduct electricity due to the presence of mobile ions.

Question 1 

Three experiments, P, Q, and R were set up to test the conductivity of two solutions as shown below.  

Explanation

Hydrogen chloride (HC𝑙) can only conduct electricity when dissolved in water. This is because HC𝑙 only dissociates in water to produce H⁺ and C𝑙^– ions that act as mobile charge carriers to conduct electricity. This eliminates experiment Q, and the options B, C and D. Only experiments P and R will light up because the hydrogen chloride ionises in water and can conduct electricity, causing the bulb to light up. Hence the correct answer is A.

3 important concepts of acids:

• Strength of acid
• Basicity of acid
• Concentration of acid

1. Strength of Acid (Strong / Weak)

Testing for strength

1. Universal Indicator

• Add a few drops of Universal Indicator into solutions of equal concentrations of each acid.
• A strong acid will give a red solution while a weak acid will give a yellow or orange solution with the Universal Indicator.

2. Acid-Metal or Acid-Carbonate Reaction

• Add magnesium ribbon or a metal carbonate into solutions of equal concentrations of each acid in separate test tubes.
• The acid which can completely react with the magnesium or carbonate in a shorter time, and which produces more vigorous effervescence, is the stronger acid.

Question 2 

The table contains data about two acids.

Acid	pH Values
Nitric acid, HNO3	1.0
Ethanoic acid, CH3COOH	3.7

 

Why are the pH values different?  

1. Ethanoic acid releases more H⁺ ions in water.
2. Ethanoic acid undergoes partial dissociation.
3. Nitric acid has smaller molecules.
4. Nitric acid releases fewer H⁺ ions in water.

Explanation

Nitric acid is a strong acid, while ethanoic acid is a weak acid. This means that nitric acid will undergo complete dissociation to form more H⁺ ions than ethanoic acid, which undergoes partial dissociation. This eliminates the options A and D, as well as C, because dissociation has nothing to do with size of molecules. Hence, the correct answer is B.

2. Basicity of Acid (Monobasic, Dibasic and Tribasic)

• The basicity of an acid is the maximum number of moles of H⁺ ions per mole of an acid available for reaction with other substances.
• The basicity of an acid is not dependent on its strength.
  • Hydrochloric acid and ethanoic acid are both monobasic, but hydrochloric acid is a strong acid and ethanoic acid is a weak acid.

Question 3 

Why is sulfuric acid described as dibasic?

1. Each molecule contains two hydroxide ions.
2. Each molecule can produce two hydrogen ions.
3. Sulfur can form two acids, sulfuric acid, and sulfurous acid.
4. Sulfuric acid has the usual properties of an acid but is also a dehydrating agent.

Explanation:

The term basicity refers to maximum number of moles of H⁺ per mole of acid. One mole of sulfuric acid will give two moles of H⁺, making it dibasic. This eliminates the options A and D and C. Hence, the correct answer is B.

3. Concentration of Acid (Dilute / Concentrated)

• Concentration of an acid is the amount of acid present per unit volume of the acid solution.
• Unlike the strength of an acid, the concentration can be altered by adding or removing water.

 

Chemical Properties of Acids

Acids can undergo three reactions.

1. Acid + Base/Alkali → Salt + Water (Neutralisation)

• Three types of bases:
  • Ammonia
  • Metal Oxides
  • Metal hydroxides
• Alkalis are soluble bases that dissociate in water to produce hydroxide ions.

2. Acid + Reactive Metal → Salt + Hydrogen Gas

• Unreactive metals like copper, silver and gold will not react with acids.
• Lead appears to not react with HC𝑙 and H₂SO₄. This is because a layer of insoluble lead(II) chloride or lead(II) sulfate is formed between the lead metal and the acid, preventing further reaction.
• Effervescence of a colorless, odorless gas (H₂) can be seen during the reaction.

3. Acid + Metal Carbonates → Salt + Water + Carbon Dioxide Gas

• Effervescence of a colorless, odorless gas (CO2) can be seen during the reaction.

Question 4 

Which compound does not react with dilute hydrochloric acid?

• Iron(II) sulfate
• Iron(II) oxide
• Iron(II) hydroxide
• Iron(II) carbonate

Explanation:

Iron(II) oxide and iron(II) hydroxide are bases, and can react with hydrochloric acid to give iron(II) chloride and water. Iron(II) carbonate can also react with hydrochloric acid to give iron(II) chloride, carbon dioxide and water. This eliminates option B,C, and D. Option A is a salt, and does not react with acids, hence the correct answer is option A.

Bases/Alkali

A base is a metal hydroxide, metal oxide or ammonia.

An alkali is a soluble base that ionises to form hydroxide ions (OH^–) when dissolved in water.

The presence of OH^– ions gives an alkali its alkaline properties.

Characteristics of Alkalis

• Alkalis taste bitter and have a soapy feel.
• Alkalis turn red litmus paper blue.
• Alkalis have pH > 7
• Alkalis dissolved in water can conduct electricity due to the presence of mobile ions.

1. Strength of Alkali (Strong / Weak)

 

2. Concentration of Alkali (Dilute / Concentrated)

• Concentration of an alkali depends on the amount of alkali present per unit volume of the alkaline solution.
• Unlike the strength of an alkali, the concentration can be altered by adding or removing water.

Chemical Properties of Bases/Alkali

Alkalis / Bases can undergo two reactions.

• Acid + Base/Alkali → Salt + Water (Neutralisation)
• Alkali / Base + Ammonium Salt → Salt + Water + Ammonia Gas

Question 5 

Which mixture would not give off ammonia on heating?

• Ammonium sulfate and calcium hydroxide
• Ammonium chloride and potassium hydroxide
• Potassium nitrate and sodium hydroxide
• Sodium hydroxide and ammonium nitrate

Explanation:

Ammonia is given out when an ammonium salt reacts with a base/alkali. Options A, B and D are bases/alkalis reacting with ammonium salts, and will give out ammonia gas, salt, and water. The correct answer is option C because potassium nitrate and sodium hydroxide will not react to give out ammonia gas.

The pH Scale

pH measures concentration of H⁺ ions.

Type of substance	pH	Concentration of ions
Acids	< 7	Concentration of H+ ions > Concentration of OH– ions
Salts	7	Concentration of H+ ions = Concentration of OH– ions
Alkalis	> 7	Concentration of H+ ions < Concentration of OH– ions

The pH Curve

During titration, a laboratory indicator like methyl orange is used to determine the endpoint of titration. When the amount of alkali added exactly neutralises the acid present, an indicator colour change happens. This is known as the equivalence point, and the endpoint of titration.

Question 6 

What are R and S?

	R	S
A	Insoluble metal oxide	Hydrochloric acid
B	Insoluble non-metal oxide	Sodium hydroxide
C	Soluble metal oxide	Hydrochloric acid
D	Non-metal oxide	Sodium hydroxide

Explanation:

The initial pH is around 2, as seen in the pH curve. This suggests that aqueous solution S is a strong acid. This could be hydrochloric acid, hence eliminating options B and D. Solid R must then be basic, hence a metal oxide. Even as solid R was added in excess (as seen by the increasing mass on the graph), the pH remains at 7. This suggests that solid R is insoluble in water and does not affect the pH after reaching neutralisation. This makes option A the correct answer.

Different Classes of Oxides

Question 7 

Oxides of elements may be classified as acidic, basic or amphoteric. Which set of oxides is correctly classified?

	Acidic	Basic	Amphoteric
A	Carbon dioxide	Copper(II) oxide	Zinc oxide
B	Carbon dioxide	Zinc oxide	Copper(II) oxide
C	Copper(II) oxide	Carbon dioxide	Zinc oxide
D	Zinc oxide	Carbon dioxide	Copper(II) oxide
E	Zinc oxide	Copper(II) oxide	Carbon dioxide

Explanation:

There are only 3 possibilities for amphoteric oxide, and zinc oxide is one of them. This eliminates options B, D and E. Acidic oxides are non-metal oxides, hence eliminating option C. The correct answer is A because carbon dioxide is a non-metal oxide, making it an acidic oxide. Copper(II) hydroxide is a metal oxide, making it a basic oxide.

Controlling pH in Soil

Soils can become too acidic from extensive use of chemical fertilisers and acid rain. If the pH of soil becomes too acidic or basic, it will lead to poor crop yield, or plant death. An optimal soil pH would be around 7 (neutral). Plants also thrive in soil with a rich nitrogen content.

Acidic soil can be treated with bases to neutralize the acid, causing the soil to reach its optimum pH of 7.

• Calcium oxide, CaO (quicklime)
• Calcium hydroxide, Ca(OH)₂ (slaked lime)
• Calcium carbonate, CaCO₃ (limestone)

It is not recommended to add fertilisers (ammonium salts) and base together at the same time because the base and ammonium salt will react, liberating ammonia gas and hence reducing the nitrogen content in the soil, which plants need for optimum growth.

Question 8 

A sample of soil has a nitrogenous fertilizer in the form of an ammonium salt added to it. The ammonium salt dissolves in the water in the soil.

When tested a week later, the water in the soil contained 15.3% of dissolved nitrogen and had a pH of 4.6.

Calcium hydroxide was added to the soil and then the water in the soil was tested the next day, both for nitrogen content and pH.

What would be the most likely result of the second test?

	Nitrogen content	pH
A	11.4%	3.8
B	12.7%	6.9
C	15.3%	4.6
D	19.8%	4.2

Explanation:

Calcium hydroxide is a sparingly soluble base and can neutralise the excess acid in the soil, bringing its pH to about 6.9 or 7. This eliminates options A, C and D. Calcium hydroxide is also able to react with the ammonium salt in the nitrogenous fertilizer, liberating ammonia gas. This reduces the nitrogen content in the soil to below the initial 15.3%. Hence, the answer is B.  

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