Home / iGCSE / Coordinated Sciences / CIE iGCSE Co-ordinated Sciences-C7.1 The characteristic properties of acids and bases- Study Notes

CIE iGCSE Co-ordinated Sciences-C7.1 The characteristic properties of acids and bases- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-C7.1 The characteristic properties of acids and bases – Study Notes

CIE iGCSE Co-ordinated Sciences-C7.1 The characteristic properties of acids and bases – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Characteristic Properties of Acids

Acids are substances that produce hydrogen ions (\( \text{H}^+ \)) in aqueous solution. Their characteristic properties can be described through their reactions with different classes of compounds:

(a) Reaction with Metals

Acids react with reactive metals (e.g., magnesium, zinc) to form a salt and hydrogen gas. The general equation is:

\( \text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{H}_2 \)

Example: \( 2\text{HCl} + \text{Mg} \rightarrow \text{MgCl}_2 + \text{H}_2 \)

Example 

Write a balanced chemical equation for the reaction between zinc and hydrochloric acid.

▶️Answer/Explanation

Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas:
\( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \)

Example 

What gas is produced when magnesium reacts with dilute sulfuric acid, and how would you test for it?

▶️Answer/Explanation

The gas produced is hydrogen:
\( \text{Mg} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2 \)
Test: Collect the gas and bring a lit splint near it — it burns with a squeaky pop.

(b) Reaction with Bases (Neutralisation)

Acids react with bases (alkalis or insoluble metal oxides/hydroxides) to form a salt and water. This is called a neutralisation reaction. The general equation is:

\( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O} \)

Example:

\( \text{H}_2\text{SO}_4 + \text{CuO} \rightarrow \text{CuSO}_4 + \text{H}_2\text{O} \)

\( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)

Example 

Write an equation for the reaction of hydrochloric acid with sodium hydroxide.

▶️Answer/Explanation

\( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
This is a neutralisation reaction producing sodium chloride and water.

Example 

Explain why adding copper(II) oxide to sulfuric acid forms a blue solution.

▶️Answer/Explanation

Copper(II) oxide is a black solid (insoluble base). It reacts with sulfuric acid:
\( \text{H}_2\text{SO}_4 + \text{CuO} \rightarrow \text{CuSO}_4 + \text{H}_2\text{O} \)
The product is copper(II) sulfate, which is soluble and forms a blue solution.

(c) Reaction with Carbonates

Acids react with carbonates (and hydrogencarbonates) to form a salt, carbon dioxide, and water. The general equation is:

\( \text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{H}_2\text{O} + \text{CO}_2 \)

Example: \( 2\text{HCl} + \text{CaCO}_3 \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \)

Example 

What happens when dilute hydrochloric acid is added to sodium carbonate?

▶️Answer/Explanation

The reaction produces sodium chloride, carbon dioxide, and water:
\( 2\text{HCl} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \)
Effervescence is observed due to carbon dioxide gas.

Example 

How can you confirm that the gas produced when calcium carbonate reacts with hydrochloric acid is carbon dioxide?

▶️Answer/Explanation

\( \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \)
The gas is carbon dioxide. Test: Bubble it through limewater. Limewater turns milky due to formation of calcium carbonate.

Effect of Acids on Indicators

Acids are substances that produce hydrogen ions (\( \text{H}^+ \)) in aqueous solution. They can be identified by their effect on common acid-base indicators.

(a) Litmus

Acids turn blue litmus paper red. Red litmus paper remains red in acids.

Example 

What colour change will be observed when blue litmus paper is dipped into hydrochloric acid?

▶️Answer/Explanation

The blue litmus paper will turn red, showing that the solution is acidic.

Example 

If red litmus paper is placed in nitric acid, what happens?

▶️Answer/Explanation

No colour change occurs because the paper is already red in acidic conditions.

(b) Methyl Orange

Acids turn methyl orange from orange (neutral) to red in acidic conditions.

Example 

What colour change is seen when methyl orange is added to dilute sulfuric acid?

▶️Answer/Explanation

Methyl orange turns red in acidic solutions like sulfuric acid.

Example 

If a student adds methyl orange to hydrochloric acid, what will the indicator show?

▶️Answer/Explanation

The indicator changes colour to red, confirming the presence of an acid.

Bases and Alkalis

Bases are defined as oxides or hydroxides of metals. They react with acids to form a salt and water (neutralisation reaction).

Examples:

\( \text{CuO} \) (copper(II) oxide) – an insoluble base
\( \text{Mg(OH)}_2 \) (magnesium hydroxide) – a base

Alkalis are bases that are soluble in water. They release hydroxide ions (\( \text{OH}^- \)) in aqueous solution.

Examples:

\( \text{NaOH} \) (sodium hydroxide)
\( \text{KOH} \) (potassium hydroxide)
\( \text{Ca(OH)}_2 \) (calcium hydroxide, sparingly soluble)

Example 

Is copper(II) oxide a base or an alkali? Explain.

▶️Answer/Explanation

Copper(II) oxide is a base because it is a metal oxide. However, it is insoluble in water, so it is not an alkali.

Example 

Why is sodium hydroxide considered an alkali?

▶️Answer/Explanation

Sodium hydroxide is a base because it contains hydroxide ions. Since it dissolves in water to release \( \text{OH}^- \) ions, it is classified as an alkali.

Characteristic Properties of Bases

Bases are oxides or hydroxides of metals. One of their main characteristic properties is their reaction with acids. Bases neutralise acids to form a salt and water. This is called a neutralisation reaction.

General Equation:

\( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O} \)

Example 

Write a balanced chemical equation for the reaction between hydrochloric acid and magnesium hydroxide.

▶️Answer/Explanation

\( 2\text{HCl} + \text{Mg(OH)}_2 \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O} \)
The acid (HCl) reacts with the base (magnesium hydroxide) to form magnesium chloride and water.

Example 

Explain why copper(II) oxide reacts with sulfuric acid but is not an alkali.

▶️Answer/Explanation

\( \text{H}_2\text{SO}_4 + \text{CuO} \rightarrow \text{CuSO}_4 + \text{H}_2\text{O} \)
Copper(II) oxide is a base because it neutralises acids. However, it does not dissolve in water, so it is not an alkali.

Effect of Alkalis on Indicators

Alkalis are soluble bases that release hydroxide ions (\( \text{OH}^- \)) in aqueous solution. They can be identified by their effect on acid-base indicators.

(a) Litmus

Alkalis turn red litmus paper blue. Blue litmus paper remains blue in alkalis.

Example 

What happens when red litmus paper is dipped into sodium hydroxide solution?

▶️Answer/Explanation

The red litmus paper turns blue because sodium hydroxide is an alkali.

Example 

If blue litmus paper is placed in potassium hydroxide solution, what colour change will be observed?

▶️Answer/Explanation

No change occurs, because the paper is already blue in alkaline conditions.

(b) Methyl Orange

In alkaline solutions, methyl orange changes from orange (neutral) to yellow.

Example 

What colour will methyl orange show in calcium hydroxide solution?

▶️Answer/Explanation

Methyl orange turns yellow in calcium hydroxide solution because it is alkaline.

Example 

A student adds methyl orange to a sodium hydroxide solution. What will be observed?

▶️Answer/Explanation

The indicator turns yellow, confirming the solution is alkaline.

Universal Indicator and pH

The universal indicator is a mixture of dyes that shows a wide range of colours depending on the pH of the solution. It is used to determine whether a solution is acidic, neutral, or alkaline, and also gives an indication of the relative acidity or alkalinity.

pH Scale (0 to 14):

  • pH 7 → Neutral
  • pH less than 7 → Acidic (lower pH = stronger acid)
  • pH greater than 7 → Alkaline (higher pH = stronger alkali)

Colours of Universal Indicator:

  • Strong acids (pH 0-2) → Red
  • Weak acids (pH 4-6) → Orange to Yellow
  • Neutral (pH 7) → Green
  • Weak alkalis (pH 8-11) → Blue
  • Strong alkalis (pH 12-14) → Purple

Example 

A solution turns universal indicator bright red. What can you deduce about its pH and relative acidity?

▶️Answer/Explanation

The solution has a pH between 0 and 2. It is strongly acidic (high relative acidity).

Example 

Two solutions are tested with universal indicator: one turns green, the other turns purple. Compare their neutrality, acidity, and alkalinity.

▶️Answer/Explanation

The green solution has pH 7, which means it is neutral. The purple solution has pH 12–14, showing it is strongly alkaline (high relative alkalinity).

Neutralisation Reaction between an Acid and an Alkali

A neutralisation reaction occurs when an acid reacts with an alkali (a soluble base). The hydrogen ions (\( \text{H}^+ \)) from the acid combine with hydroxide ions (\( \text{OH}^- \)) from the alkali to form water. A salt is also produced in the reaction.

General Equation:

\( \text{Acid} + \text{Alkali} \rightarrow \text{Salt} + \text{Water} \)

Examples:

\( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
\( \text{H}_2\text{SO}_4 + 2\text{KOH} \rightarrow \text{K}_2\text{SO}_4 + 2\text{H}_2\text{O} \)

Example 

Write the neutralisation reaction between nitric acid and sodium hydroxide.

▶️Answer/Explanation

\( \text{HNO}_3 + \text{NaOH} \rightarrow \text{NaNO}_3 + \text{H}_2\text{O} \)
The acid (nitric acid) reacts with the alkali (sodium hydroxide) to form sodium nitrate and water.

Example 

What are the products when hydrochloric acid reacts with potassium hydroxide?

▶️Answer/Explanation

\( \text{HCl} + \text{KOH} \rightarrow \text{KCl} + \text{H}_2\text{O} \)
The products are potassium chloride (a salt) and water.

Scroll to Top