Study Tips
How to Understand Acids, Bases and pH
By Dr Ayesha Khan · · 4 min read
Quick answer
Acids release hydrogen ions in solution and have a low pH; bases and alkalis do the opposite and have a high pH. The pH scale runs from 0 to 14, with 7 neutral. Neutralisation is an acid reacting with a base to form a salt and water. Learning the key reactions and the scale is the foundation.
The pH scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. Below 7 is acidic, 7 is neutral, above 7 is alkaline. The further from 7, the stronger the acid or alkali.
Knowing the scale and being able to place substances on it is the foundation of the topic and a common exam requirement.
What acids and bases are
An acid releases hydrogen ions when dissolved in water, which is what makes it acidic. A base does the opposite; a soluble base is called an alkali and releases hydroxide ions.
Understanding acids and bases in terms of ions, rather than just as labels, explains their behaviour and the reactions between them.
Neutralisation
When an acid reacts with a base, they neutralise each other, producing a salt and water. This is one of the most important reactions in the topic and appears in many forms.
The general pattern — acid plus base gives salt plus water — is worth learning thoroughly, as it underlies a large share of the questions.
Key reactions to know
Acids react with metals to give a salt and hydrogen, with carbonates to give a salt, water and carbon dioxide, and with bases to give a salt and water. These standard reactions recur throughout.
Learning these general equations, and being able to apply them to specific acids and metals, covers most of the reaction questions.
Strong versus concentrated
A common confusion is between strong and concentrated. Strength is about how fully an acid ionises; concentration is about how much acid is dissolved in a given volume. They are different things.
Keeping this distinction clear prevents a frequent error, because a strong acid can be dilute and a weak acid can be concentrated.
Indicators and testing
Indicators change colour depending on pH, letting you identify acids and alkalis. Universal indicator gives a range of colours across the scale; others simply distinguish acid from alkali.
Knowing which indicator does what, and the colours involved, is standard content and often tested in practical questions.
Frequently asked questions
What does the pH scale measure?+
How acidic or alkaline a solution is, from 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is alkaline, and the further from 7, the stronger the acid or alkali.
What is neutralisation?+
An acid reacting with a base to produce a salt and water. It is one of the most important reactions in the topic, and the pattern 'acid plus base gives salt plus water' underlies many questions.
What's the difference between strong and concentrated?+
Strength is about how fully an acid ionises in water; concentration is about how much acid is dissolved in a given volume. A strong acid can be dilute, and a weak acid can be concentrated.
What reactions do acids undergo?+
With metals to give a salt and hydrogen, with carbonates to give a salt, water and carbon dioxide, and with bases to give a salt and water. Learning these general patterns covers most reaction questions.
How do indicators work?+
They change colour depending on pH, letting you identify acids and alkalis. Universal indicator shows a range of colours across the scale, while others simply distinguish acid from alkali.
Try your first session free
Meet your tutor, set your goals, and see the difference one-to-one attention makes. No card required, no commitment.