Edexcel IGCSE Chemistry · Spec 2.28-2.32
Acids, Alkalis, Indicators & the pH Scale
Acids and alkalis, indicators, and the pH scale.
Chemistry revision video
Acids, Alkalis, Indicators & the pH Scale
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Explained
Acids, alkalis and neutralisation
An acid produces hydrogen ions in water. An alkali is a soluble base, and it produces hydroxide ions in water. Those two ions are what the pH scale is really measuring, and neutralisation is what happens when they meet.
The pH scale
The scale usually runs from 0 to 14. Below 7 is acidic, and the lower the number the more acidic the solution. Above 7 is alkaline, and the higher the number the more alkaline. Exactly 7 is neutral, which is pure water.
Indicators are the practical way to find a pH, and the colours for litmus, phenolphthalein, methyl orange and universal indicator are covered on the indicators page.
Neutralisation, in ions
Neutralisation is the reaction between hydrogen ions from the acid and hydroxide ions from the alkali. They combine to form water.
That is the whole reaction, expressed as an ionic equation: H plus and OH minus give H2O. The other ions present, such as sodium and chloride, do not take part. They stay in solution and are left behind as the salt when the water is evaporated.
Writing the ionic equation is often worth a mark on its own, and it explains why every acid and alkali neutralisation produces water regardless of which acid and which alkali were used.
Acid reactions worth knowing
- Acid plus metal gives a salt plus hydrogen.
- Acid plus base gives a salt plus water.
- Acid plus metal carbonate gives a salt plus water plus carbon dioxide.
Only the carbonate reaction produces a gas that turns limewater milky, which is how a carbonate is identified. The metal reaction produces hydrogen, tested with a lighted splint and a squeaky pop.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry record marks lost in neutralisation equations for an incorrect state symbol, and note that candidates would benefit from more practice at applying solubility rules to determine state symbols.
So the equation is not finished when the formulae are right. Decide whether each substance is aqueous, solid, liquid or gas. A soluble salt formed in solution is aqueous, and the water produced is liquid.
Naming the salt
The metal comes from the base or the metal itself, and the rest of the name comes from the acid. Hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates.
So sodium hydroxide with sulfuric acid gives sodium sulfate. Working the name out from the two reactants is quicker and safer than trying to remember individual salts.
Spec 2.28-2.32
What you need to know
- Use the pH scale to classify solutions
- Describe indicators and their colours
- Explain neutralisation
Active recall
Quick check
Answer each question before opening the answer.
What pH values are acidic, neutral and alkaline?
Below 7 is acidic, 7 is neutral, and above 7 is alkaline.
Which ion do acids and alkalis produce in water?
Acids produce H+ ions; alkalis produce OH− ions.
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