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Unit 5: Reactivity

Salts — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 10 of 12 1 quiz

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Salts — HIA INTERNATIONAL notes: Explain what a salt is and prepare a salt from a metal and an acid or a metal oxide and an acid.Salt = a compound formed when the hydrogen in an acid is replaced by a metal.Table salt — sodium chloride is the salt used to flavour and preserve food…

  • Other salts
  • Which metals
  • Large crystals
  • Hydrogen test
  • Crystallisation
  • Structure — salts are made of ions and are usually crystalline solids that often dissolve in water

Section 1 of 8What a salt is

What a salt is
Salt
Salt a compound formed when the hydrogen in an acid is replaced by a metal.
Table salt
  • Table salt sodium chloride is the salt used to flavour and preserve food.
Other salts
  • Other salts copper sulfate, silver nitrate and calcium carbonate.
Structure
  • Structure salts are made of ions and are usually crystalline solids that often dissolve in water.

Section 2 of 8Acids and their salts

AcidFormulaSaltsExample
Hydrochloric acidHClChloridesSodium chloride, NaCl
Sulfuric acidH2SO4SulfatesCopper sulfate, CuSO4
Nitric acidHNO3NitratesPotassium nitrate, KNO3
Carbonic acid—CarbonatesCalcium carbonate
Citric acid—CitratesSodium citrate
  • Naming
    • the first part comes from the metal and the second from the acid.

Section 3 of 8Uses of salts

SaltUse
Sodium chloridePreserves and flavours food
Magnesium carbonateKeeps gymnasts’ hands dry
Calcium sulfateMakes blackboard chalk and plaster
Aluminium sulfateHelps dyes stick to fibres
Ammonium nitrateFertiliser
Copper sulfateCoats seeds to stop fungi

Section 4 of 8Salt from a metal and an acid

Equation
  • Equation acid + metal → salt + hydrogen, e.g. zinc + hydrochloric acid → zinc chloride + hydrogen.
Which metals
  • Which metals only metals above hydrogen react with dilute acids.
Danger
  • Danger very reactive metals such as sodium are never used.
1
Pour about 50 cm3 of dilute sulfuric acid into a beaker.
2
Add zinc until the fizzing stops, with some left over.
3
Filter off the unreacted zinc.
4
Heat the filtrate gently until crystals form at the edge.
5
Leave for a few days to crystallise slowly.
  • Large crystals
    • evaporating slowly gives larger crystals than heating to dryness.

Section 5 of 8Salt from a metal oxide and an acid

Why
  • Why unreactive metals such as copper and silver cannot displace hydrogen from acid.
Equation
  • Equation metal oxide + acid → salt + water, e.g. copper oxide + sulfuric acid → copper sulfate + water (CuO + H2SO4 → CuSO4 + H2O).
1
Pour about 100 cm3 of dilute sulfuric acid into a beaker.
2
Add black copper oxide powder and heat very gently, stirring; do not boil the acid.
3
When the mixture turns blue, turn off the heat and cool.
4
Filter off excess copper oxide.
5
Evaporate gently and leave to crystallise.

Section 6 of 8Test and safety

Hydrogen test
  • Hydrogen test a lighted splint gives a squeaky pop.
Spitting
  • Spitting take care when heating the evaporating basin because the solution may spit.
Tongs
  • Tongs use tongs to move a hot evaporating basin.

Section 7 of 8Key terms

TermMeaning
SaltA compound formed when the hydrogen in an acid is replaced by a metal.
CrystallisationForming crystals from a solution by evaporating the water.
FiltrateThe liquid that passes through filter paper.
ExcessMore than is needed to react.
ChlorideA salt of hydrochloric acid.
SulfateA salt of sulfuric acid.
NitrateA salt of nitric acid.

Section 8 of 8Exam technique

1
Salt means only table salt — a salt is any compound formed from an acid and a metal.
2
Hydrochloric acid makes sulfates — hydrochloric acid makes chlorides.
3
Copper reacts with dilute acid to make a salt — copper must be used as its oxide.
4
Heating to dryness gives the best crystals — slow evaporation gives larger crystals.

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