This module covers how chemists recognise an acid or an alkali and what makes a solution acidic. It also covers what happens when an acid meets a base, how a salt is prepared and how an endpoint is read. One idea runs through all of it: acidity is about hydrogen ions in water, and every observation should be explained with them.
Check the current Cambridge Chemistry 0620 syllabus for the exact content in your exam year. Our Chemistry learning guide shows where this module sits among the others.
What should you already know?
You should be comfortable writing and balancing simple equations, and with moles and concentration in mol/dm³. If those feel shaky, revise formulas and equations and concentration calculations first. The equation balance reasoning trainer and the mole and equation-ratio tutor are useful for that.
An orienting example
A student has 25.0 cm³ of hydrochloric acid, concentration 0.100 mol/dm³. The table below is invented for teaching.
| Change | What happens to the hydrogen ions | Result |
|---|---|---|
| Add water until the volume is 250 cm³ | Same amount (0.00250 mol), larger volume | Concentration falls to 0.0100 mol/dm³, pH rises from 1 to 2 |
| Add 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide | Hydrogen ions are used up by hydroxide ions | Neutral solution of sodium chloride, pH 7 |
Both changes make the solution “less acidic”, yet they are different events.
Dilution changes concentration only. Neutralisation removes hydrogen ions. Keeping those two apart is the heart of this module.
In what order should you study the lessons?
- Use an indicator result to classify a solution: begin with what you can observe, and learn what a colour does and does not prove.
- Relate acidity to the relevant particles: move from colours to hydrogen ions, and separate strength from concentration.
- Distinguish neutralisation from dilution: apply the particle idea to the two changes in the example above.
- Choose a suitable salt-preparation principle: use solubility to decide between an excess solid, a titration and precipitation.
- Explain an endpoint from supplied observations: read titration results, choose valid values and explain the colour change.
Then work through the acids, bases and salts practice set.
What are the common traps?
- Treating a colourless result with phenolphthalein as proof of an acid. It only shows the solution is not strongly alkaline.
- Saying that a weak acid is the same as a dilute acid.
- Believing that adding water to an acid can push its pH above 7.
- Choosing a filtration method for a salt that dissolves in water.
- Averaging every titre, including the rough one, instead of the concordant results.
How should you use the practice set?
Attempt each question on paper before opening the answer. After a wrong answer, write the error in one line (for example “confused strength with concentration”), then return to the lesson named in the set. The mistake log and retest queue helps you keep track of repeated errors.
If you want a teacher to watch how you reason through these questions, see our online one-to-one Chemistry tuition.