Acids and alkalis: pH, neutralisation and salts
TutorAura guide · Updated
Acids produce hydrogen ions, H⁺, in water. Alkalis are soluble bases that produce hydroxide ions, OH⁻, in water. The pH scale, from 0 to 14, measures how acidic or alkaline a solution is: below 7 is acidic, 7 is neutral and above 7 is alkaline. When an acid and an alkali react, they neutralise each other:
Key takeaways
- pH below 7: acidic; 7: neutral; above 7: alkaline.
- Acids release H⁺ ions; alkalis release OH⁻ ions.
- Acid + alkali → salt + water.
- Strong means fully ionised, not concentrated.
The pH scale and indicators
| pH | Type | Example |
|---|---|---|
| 0 to 2 | Strong acid | Stomach acid |
| 3 to 6 | Weak acid | Vinegar, orange juice |
| 7 | Neutral | Pure water |
| 8 to 11 | Weak alkali | Toothpaste |
| 12 to 14 | Strong alkali | Oven cleaner |
Universal indicator changes colour across the whole range (red for strong acid, green for neutral, purple for strong alkali). A pH probe gives a precise number. Litmus only shows acid (red) or alkali (blue).
Making salts
| Reaction | Products |
|---|---|
| Acid + metal | Salt + hydrogen |
| Acid + alkali (or base) | Salt + water |
| Acid + carbonate | Salt + water + carbon dioxide |
The salt's name comes from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates. Hydrochloric acid + sodium hydroxide → sodium chloride + water.
Strong vs weak, concentrated vs dilute
- Strong acids (hydrochloric, sulfuric, nitric) are fully ionised in water.
- Weak acids (ethanoic, citric, carbonic) are only partially ionised.
- Concentrated and dilute are about how much acid is in a given volume.
So you can have a dilute strong acid or a concentrated weak one. A decrease of 1 on the pH scale means the H⁺ concentration is 10 times higher: pH 3 has 100 times the H⁺ concentration of pH 5.
Titrations
A titration measures exactly how much acid neutralises a known volume of alkali, using a burette, a pipette and an indicator. From the volumes and one known concentration, you can calculate the other concentration. Work the numbers with our concentration calculator and practise with our titration questions.
Acids and bases in everyday life
Indigestion tablets contain bases that neutralise excess stomach acid. Farmers add lime (a base) to soil that is too acidic for their crops. Toothpaste is mildly alkaline, helping to neutralise acids made by bacteria in the mouth, and many household cleaners are alkaline because alkalis break down grease.
Common mistakes
- Confusing strong with concentrated.
- Forgetting carbon dioxide when an acid reacts with a carbonate.
- Saying all bases are alkalis: only soluble bases are alkalis.
Learn how ions move in electrolysis.
Practice questions
- Name the products when zinc reacts with hydrochloric acid.
- Name the products when calcium carbonate reacts with nitric acid.
- How many times greater is the H⁺ concentration at pH 2 than at pH 4?
- Which salt forms when sulfuric acid neutralises sodium hydroxide?
Answers: 1. Zinc chloride and hydrogen. 2. Calcium nitrate, water and carbon dioxide. 3. 100 times. 4. Sodium sulfate.
Making a soluble salt
A common practical makes copper sulfate crystals:
- Warm dilute sulfuric acid.
- Add insoluble copper oxide a little at a time until no more dissolves (it's in excess), so all the acid has reacted.
- Filter to remove the excess copper oxide.
- Gently heat the solution to evaporate some water, then leave it to cool so crystals form.
Frequently asked questions
What is the difference between an acid and an alkali?
Acids produce H⁺ ions in water and have a pH below 7; alkalis produce OH⁻ ions and have a pH above 7.
What is neutralisation?
The reaction between an acid and a base, making a salt and water: H⁺ + OH⁻ → H₂O.
What is the difference between a strong and a concentrated acid?
Strong means the acid is fully ionised in water; concentrated means there's a lot of acid per unit volume.
Sources
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