Acids react with metals, bases and carbonates, and every one of those reactions makes a salt. Learn the four reaction patterns and the three indicator colours, and most acid questions answer themselves.
Acids turn blue litmus red, turn methyl orange red and leave thymolphthalein colourless. They react with metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide. Bases are oxides or hydroxides of metals; soluble bases are alkalis. Alkalis turn red litmus blue, methyl orange yellow and thymolphthalein blue, and release ammonia from ammonium salts. Acids contain H+ ions and alkalis contain OH− ions.
Fig. 1: the four reaction patterns to learn
Section 01
Indicators and ions.
Add an acid, then an alkali, to each indicator in the simulator and note the colours.
Indicator
In acid
In alkali
litmus
red
blue
methyl orange
red
yellow
thymolphthalein
colourless
blue
Acids produce hydrogen ions, H+(aq), in water.
Alkalis produce hydroxide ions, OH−(aq), in water. Examples: sodium hydroxide, potassium hydroxide, calcium hydroxide, aqueous ammonia.
Base or alkali? Copper(II) oxide is a base but not an alkali, because it is insoluble in water. All alkalis are bases, but not all bases are alkalis.
Stage 1 · Learn
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What colour is methyl orange in an acid?
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What are the products when magnesium reacts with dilute hydrochloric acid?
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Which ion is present in all acidic solutions?
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What is an alkali?
Section 02
Reactions of acids.
Every reaction of an acid on this page produces a salt. The metal or ammonium part of the salt comes from the base; the rest comes from the acid.
Metals: zinc + hydrochloric acid → zinc chloride + hydrogen. Fizzing; the gas gives a squeaky pop. Very unreactive metals, such as copper, do not react with dilute acids.
Carbonates: sodium carbonate + nitric acid → sodium nitrate + water + carbon dioxide. The gas turns limewater milky.
Neutralisation: acid + alkali → salt + water. The ionic equation is always H+(aq) + OH−(aq) → H2O(l).
Ammonium salts: warm an ammonium salt with an alkali and ammonia is given off; it turns damp red litmus blue. This is the test for the ammonium ion.
Examiner trap
Metal + acid gives hydrogen; carbonate + acid gives carbon dioxide. Mixing these up is the commonest slip. Also, a base is not always an alkali: say "alkali" only for a base that dissolves in water.
Stage 2 · Exam
Exam-style questions
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Dilute sulfuric acid is added to copper(II) carbonate. Which products form?
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Which is the ionic equation for neutralisation?
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A white solid is warmed with aqueous sodium hydroxide. A gas is given off that turns damp red litmus paper blue. The solid contains:
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Which substance is a base but not an alkali?
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Properties of acids and bases, mastered.
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