12.6Chapter 12: Chemical Reactions 3 and Rates

Making salts from acids and metals — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 13 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision

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Lesson summary

12.6 Making salts from acids and metals — Filtration and Crystallisation — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define a salt and describe how to make a pure, dry sample of a soluble salt from an acid and a metal.Salt = a compound made when a metal ion r…

Choosing reactants
Example reactions
Testing for crystallisation
Measuring cylinder
Beaker for the reaction
Conical flask for filtering

Part 1 of 6What a salt is

Salt

Salt a compound made when a metal ion replaces the hydrogen ion in an acid.

Examples include magnesium chloride, nickel nitrate and copper sulfate crystals.

Choosing reactants

Choosing reactants the metal must be moderately reactive.
Magnesium, zinc and iron are suitable, copper does not react with dilute acids, and sodium and potassium are too dangerous.

Example reactions

Example reactions magnesium + hydrochloric acid → magnesium chloride + hydrogen (Mg + 2HCl → MgCl₂ + H₂),
- and zinc + sulfuric acid → zinc sulfate + hydrogen.

Part 2 of 6Risk assessment

Method
- identify the hazards and risks linked to the reactants, products and equipment, and decide how to reduce the chance and consequences of injury.

Hazard	Risk	Reduce the chance of injury by
Magnesium	Flammable	Keep it away from flames
Dilute hydrochloric acid	Corrosive, damages eyes and skin	Wear eye protection and do not touch
Hydrogen gas	Explosive mixture with air	Use small quantities to make only a small amount of gas
Hot equipment and solutions	Burns	Wait for them to cool before touching
Breaking apparatus	Cuts	Wear eye protection

Part 3 of 6Method step by step

Measure a volume of dilute acid, such as 25 cm³, in a measuring cylinder and pour it into a beaker, warming it gently.

Add the metal in small amounts, stirring, until bubbling stops and some metal remains.

Filter the mixture into a conical flask to remove the unreacted metal, so the salt solution passes through as the filtrate.

Pour the filtrate into an evaporating basin and heat over a water bath until crystals start to form around the edge, without heating to dryness.

Let the basin cool and leave it in a warm, dry place so the water evaporates slowly and crystals form.

Filter off the crystals, wash them with a little cold distilled water and dry them between filter papers.

Testing for crystallisation
- dip a glass rod into the solution, and crystals forming on the rod as it cools show the solution is ready.

Part 4 of 6Why each step matters

Step	Reason
Excess metal	All acid is used, so the salt is not contaminated with acid
Filtration	Removes the insoluble metal
Water bath	Gentle, even heating avoids spitting and decomposing the salt
Evaporating basin	Its shape gives a large surface for water to evaporate from
Slow cooling	Forms larger, well-shaped crystals
Washing with cold water	Removes soluble impurities without dissolving the crystals

Part 5 of 6Choosing equipment

1Measuring cylinder

Measuring cylinder it measures the acid volume because it measures smaller differences in volume than a beaker.

2Beaker for the reaction

Beaker for the reaction a beaker is used rather than a conical flask because the mixture is easier to stir.

3Conical flask for filtering

Conical flask for filtering it holds the filter funnel upright.

4Excess shown by

Excess shown by some magnesium remaining in the beaker shows that all the acid has reacted.

Part 6 of 6Safety and yield

Safety

Safety wear eye protection, as dilute acids can irritate the eyes, hot solutions can spit and the hydrogen produced is flammable.
Warm the acid with a hot water bath or electric hotplate, not a flame.

Percentage yield

Percentage yield actual mass of crystals ÷ theoretical mass × 100.

Yield is below 100% because some salt stays in solution, some is lost in transfer and some sticks to the filter paper.

Key terms

Term	Meaning
Salt	a compound made when a metal ion replaces the hydrogen ion in an acid.
Filtrate	the liquid that passes through a filter.
Residue	the solid left on the filter paper.
Crystallisation	the formation of crystals from a saturated solution.
Evaporating basin	a dish for heating solutions to remove solvent.
Risk assessment	identifying hazards and how to reduce the risks.