GCSE Chemistry revision

The pH scale and neutralisation

Reactions of acids

AQA 4.4.2.4 Foundation & Higher

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The pH scale measures how acidic or alkaline a solution is, ranging from 0 (most acidic) to 14 (most alkaline).
A neutral substance, such as pure water, has a pH of exactly 7, sitting in the middle of the scale.
Acidic solutions have a pH below 7, while alkaline solutions have a pH above 7.
Real-World pH Examples Stomach acid has a pH of around 2, making it a strongly acidic solution that helps kill bacteria.
Acid rain has a pH of around 4, making it a weakly acidic solution.
Common household alkaline substances include washing-up liquid (pH ~9) and bleach (pH ~12).
Measuring pH with Indicators Indicators are chemical dyes that change colour depending on the pH of a solution.
Different indicators change colour at different pH levels, making some more suitable for certain ranges than others.
Universal indicator is a mixture of dyes that produces a range of colours across the full pH scale, from deep red (acidic) to bluey-purple (alkaline).
Measuring pH with a pH Probe A pH probe connected to a pH meter electronically measures the pH of a solution and gives a precise numerical reading. pH probes are more accurate and reliable than indicators because they remove the need for human judgement when interpreting colours.

1. Introduction AQA

The pH scale measures how acidic or alkaline a solution is, ranging from 0 (most acidic) to 14 (most alkaline).
A neutral substance, such as pure water, has a pH of exactly 7, sitting in the middle of the scale.
Acidic solutions have a pH below 7, while alkaline solutions have a pH above 7.
Real-World pH Examples Stomach acid has a pH of around 2, making it a strongly acidic solution that helps kill bacteria.
Acid rain has a pH of around 4, making it a weakly acidic solution.
Common household alkaline substances include washing-up liquid (pH ~9) and bleach (pH ~12).
Measuring pH with Indicators Indicators are chemical dyes that change colour depending on the pH of a solution.
Different indicators change colour at different pH levels, making some more suitable for certain ranges than others.
Universal indicator is a mixture of dyes that produces a range of colours across the full pH scale, from deep red (acidic) to bluey-purple (alkaline).
Measuring pH with a pH Probe A pH probe connected to a pH meter electronically measures the pH of a solution and gives a precise numerical reading. pH probes are more accurate and reliable than indicators because they remove the need for human judgement when interpreting colours.

An acid is any substance that forms an aqueous solution with a pH of less than 7 by releasing hydrogen ions (H⁺) in water.
A base is any substance with a pH greater than 7, and bases will neutralise acids.
An alkali is a specific subgroup of bases that is soluble in water, forming hydroxide ions (OH⁻) in solution with a pH greater than 7.

2. Defining Acids and Bases AQA

An acid is any substance that forms an aqueous solution with a pH of less than 7 by releasing hydrogen ions (H⁺) in water.
A base is any substance with a pH greater than 7, and bases will neutralise acids.
An alkali is a specific subgroup of bases that is soluble in water, forming hydroxide ions (OH⁻) in solution with a pH greater than 7.

A neutralisation reaction occurs when an acid and a base react together, nearly always producing a salt and water.
For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride (a salt) and water.
Neutralisation can be represented ionically as H⁺ + OH⁻ → H₂O, showing hydrogen ions and hydroxide ions combining to form water.
Because both the acid and base are neutralised, the pH of the product should be 7.

3. Neutralisation Reactions AQA

A neutralisation reaction occurs when an acid and a base react together, nearly always producing a salt and water.
For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride (a salt) and water.
Neutralisation can be represented ionically as H⁺ + OH⁻ → H₂O, showing hydrogen ions and hydroxide ions combining to form water.
Because both the acid and base are neutralised, the pH of the product should be 7.

The three main acids to learn are hydrochloric acid, sulfuric acid, and nitric acid.
Common bases are typically hydroxides or carbonates, such as sodium hydroxide and calcium carbonate.
These acids and bases appear frequently in GCSE Chemistry, so it is well worth committing them to memory.

4. Common Acids and Bases to Remember AQA

The three main acids to learn are hydrochloric acid, sulfuric acid, and nitric acid.
Common bases are typically hydroxides or carbonates, such as sodium hydroxide and calcium carbonate.
These acids and bases appear frequently in GCSE Chemistry, so it is well worth committing them to memory.

A neutralisation reaction occurs when an acid and a base react together, nearly always producing a salt and water.
For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride (a salt) and water: HCl + NaOH → NaCl + H₂O.
Neutralisation can also be described in terms of ions, where hydrogen ions and hydroxide ions combine to form water: H⁺ + OH⁻ → H₂O.
Because both the acid and base are neutralised, the pH of the product should be 7.

5. Neutralisation Reactions AQA

A neutralisation reaction occurs when an acid and a base react together, nearly always producing a salt and water.
For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride (a salt) and water: HCl + NaOH → NaCl + H₂O.
Neutralisation can also be described in terms of ions, where hydrogen ions and hydroxide ions combine to form water: H⁺ + OH⁻ → H₂O.
Because both the acid and base are neutralised, the pH of the product should be 7.