Acids with Bases

Definition of Acids and Bases

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. These H⁺ ions are responsible for the acidic properties of the solution.

Bases are substances that can neutralise acids. They do this by reacting with the H⁺ ions, often by providing hydroxide ions (OH⁻) or other negatively charged ions that combine with H⁺ to form water.

Some bases dissolve in water and are called alkalis; others do not dissolve and are called insoluble bases. The solubility depends on the chemical nature of the base.

Common examples of acids include:

• Hydrochloric acid (HCl) – found in stomach acid and used in industry
• Sulfuric acid (H₂SO₄) – used in car batteries and fertilisers
• Nitric acid (HNO₃) – used in explosives and fertilisers

Common examples of bases include:

• Sodium hydroxide (NaOH) – a strong base used in drain cleaners
• Calcium hydroxide (Ca(OH)₂) – used in agriculture to neutralise acidic soils
• Ammonia solution (NH₃ in water) – a weak base used in household cleaners

Neutralisation Reactions

Neutralisation is the reaction between an acid and a base to form a salt and water. This happens because the H⁺ ions from the acid react with the OH⁻ ions from the base to produce water (H₂O).

The general word equation for neutralisation is:

acid + base → salt + water

For example, when hydrochloric acid reacts with sodium hydroxide:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Here, sodium hydroxide provides hydroxide ions (OH⁻) which combine with H⁺ ions from the acid to form water, leaving sodium chloride (NaCl) as the salt.

Neutralisation reactions produce neutral solutions when the acid and base are mixed in the right proportions, meaning the solution has a pH of 7 (neutral). This is because all the H⁺ ions have reacted with OH⁻ ions to form water.

For instance, if equal amounts of hydrochloric acid and sodium hydroxide are mixed, the resulting solution contains only water and dissolved sodium chloride, which is neutral.

Reaction with Alkalis

Alkalis are bases that dissolve in water to release hydroxide ions (OH⁻). All alkalis are bases, but not all bases are alkalis because some bases do not dissolve in water.

When an acid reacts with an alkali, the reaction is a neutralisation that produces a salt and water.

For example, sulfuric acid reacting with potassium hydroxide:

H₂SO₄ (aq) + 2KOH (aq) → K₂SO₄ (aq) + 2H₂O (l)

Here, potassium hydroxide (an alkali) neutralises sulfuric acid to form potassium sulfate (a salt) and water.

Indicators such as methyl orange or phenolphthalein can be used to show when neutralisation has occurred by changing colour at the neutral point.

Indicator	Colour in Acid	Colour at Neutral	Colour in Alkali
Methyl orange	Red	Orange	Yellow
Phenolphthalein	Colourless	Colourless	Pink

Example: If 25 cm³ of hydrochloric acid reacts with 25 cm³ of sodium hydroxide solution, the solution becomes neutral because the H⁺ ions from the acid have reacted with the OH⁻ ions from the alkali to form water.

Reaction with Insoluble Bases

Some bases do not dissolve in water and are called insoluble bases. These include metal oxides and metal hydroxides such as copper(II) oxide (CuO) and iron(III) hydroxide (Fe(OH)₃).

Acids react with these insoluble bases to form a salt and water, similar to reactions with alkalis, but the base does not dissolve. Instead, the acid reacts with the solid base directly.

For example, copper(II) oxide reacting with hydrochloric acid:

CuO (s) + 2HCl (aq) → CuCl₂ (aq) + H₂O (l)

Here, copper(II) oxide (an insoluble base) reacts with hydrochloric acid to form copper(II) chloride (a soluble salt) and water.

The main difference between reactions with alkalis and insoluble bases is that alkalis dissolve in water and provide OH⁻ ions directly, while insoluble bases react as solids with acids.

Example: When magnesium hydroxide (Mg(OH)₂), an insoluble base, reacts with sulfuric acid, the products are magnesium sulfate and water:

Mg(OH)₂ (s) + H₂SO₄ (aq) → MgSO₄ (aq) + 2H₂O (l)

Inline example: For instance, if hydrochloric acid reacts with calcium hydroxide, the products are calcium chloride and water:

Ca(OH)₂ (s) + 2HCl (aq) → CaCl₂ (aq) + 2H₂O (l)