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:
Core
- Describe the characteristic properties of acids in terms of their reactions with:
(a) metals
(b) bases
(c) carbonates - Describe acids in terms of their effect on the indicators:
(a) litmus
(b) methyl orange - State that bases are oxides or hydroxides of metals and that alkalis are soluble bases
- Describe the characteristic properties of bases in terms of their reactions with acids
- Describe alkalis in terms of their effect on the indicators:
(a) litmus
(b) methyl orange - Describe how to compare neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator
- Describe the neutralisation reaction between an acid and an alkali to produce a salt and water (the ionic equation for this reaction is not required)
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.