The pH scale, neutralisation, naming salts and writing their formulae, and the required practical for making crystals of a soluble salt.
What you will learn
- Describe a solution from its pH
- Name the salt made from an acid and a base, and choose its formula
- Work out neutralisation volumes in proportion
- Plan the soluble salt practical step by step
The pH scale
Acids make hydrogen ions, H⁺, in water.
Alkalis make hydroxide ions, OH⁻. The pH scale runs from 0 to 14:
- 0 to 2: strongly acidic (car battery acid, stomach acid)
- 3 to 6: weakly acidic (clean rainwater)
- 7: neutral (pure water)
- 8 to 10: weakly alkaline (sea water, baking soda)
- 11 to 14: strongly alkaline (oven cleaner)
Measure pH with universal indicator (compare the colour with a chart) or, more precisely, with a pH probe. A strong acid ionises completely in water and a weak acid only partly (lesson 4 explains this). Universal indicator goes red in strong acid, orange or yellow in weak acid, green at pH 7 (neutral), blue in weak alkali and purple in strong alkali. A pH meter gives a number, so it does not depend on judging a colour.
A base is any substance that neutralises an acid: metal oxides, hydroxides and carbonates. An alkali is a base that dissolves in water. In every neutralisation in water the same thing happens:
H⁺(aq) + OH⁻(aq) → H₂O(l)
The rest of this lesson and its 14 practice questions are locked.
In this lesson
- The pH scale
- Naming salts
- Required practical: making a soluble salt
- Try it: Charges from the periodic table
14 practice questions, marked as you go; about 25 minutes in all.
Key terms
Strong acid, Weak acid, Base, Alkali, Independent variable, Dependent variable, Control variables, Anomalous
In this unit: Chemical changes: reactivity, acids and electrolysis
The reactivity series and metal extraction, oxidation and reduction, acids, salts and pH, titrations, and electrolysis of melts and solutions.