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3.3.4Chemical Reactions

Neutralisation — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 15 of 20

📚 OCR Gateway GCSE Combined Science A J250 - Chemistry Hub / Topical Revision
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Part 1 of 4Acids, bases and alkalis

Bases
  • Bases Metal oxides and metal hydroxides are bases. They react with acids to neutralise them.
  • Many bases are insoluble in water.
Alkalis
  • Alkalis An alkali is a soluble base.
    • It forms a solution with a pH above 7, for example sodium hydroxide or calcium hydroxide.
Neutralisation
  • Neutralisation acid + base → salt + water.
  • In ionic terms, H+(aq) + OH-(aq) → H2O(l).

Part 2 of 4Naming the salt

AcidSalt formedExample
Hydrochloric acidChlorideNaOH + HCl → NaCl + H2O
Sulfuric acidSulfateCuO + H2SO4 → CuSO4 + H2O
Nitric acidNitrateZnO + 2HNO3 → Zn(NO3)2 + H2O

Part 3 of 4Other acid reactions

Reactions of acids
With metal carbonates
  • With metal carbonates acid + metal carbonate → salt + water + carbon dioxide.
  • Bubbling (effervescence) is the sign that carbon dioxide is made, for example CuCO3 + 2HCl → CuCl2 + CO2 + H2O.
With reactive metals
  • With reactive metals acid + metal → salt + hydrogen.
  • For example Mg + H2SO4 → MgSO4 + H2. Fizzing and the squeaky pop test show hydrogen.
How to tell them apart
  • How to tell them apart Oxides and hydroxides give no gas.
    • Carbonates and metals both fizz, but only hydrogen burns with a squeaky pop and carbon dioxide turns limewater milky.

Part 4 of 4Making a soluble salt from an insoluble base

1
Warm the dilute acid in a beaker.
2
Add the solid (for example copper oxide) a little at a time, stirring, until no more dissolves and some remains in excess.
3
Filter the mixture to remove the unreacted solid, keeping the filtrate.
4
Heat the filtrate gently to evaporate some of the water and leave it to cool and crystallise.
5
Filter off the crystals and dry them between filter papers.

Key terms

TermMeaning
BaseA substance that neutralises an acid, such as a metal oxide or hydroxide.
AlkaliA soluble base that forms a solution with pH above 7.
SaltA compound formed when the hydrogen in an acid is replaced by a metal or ammonium ion.
NeutralisationA reaction between an acid and a base that produces a salt and water.
EffervescenceBubbling caused by a gas being released from a liquid.

Avoid these mistakes

1
Using the wrong salthydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
2
Forgetting that carbonates also make water and carbon dioxide, not only a salt.
3
Saying all bases are alkalis. Only soluble bases are alkalis.
4
Leaving the acid in excess when making salts from solids. The solid should be in excess so it can be filtered off.

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