CHEMISTRY · Stage 9 · Chapter 12: Chemical Reactions 3 and Rates
Chemistry 12.6 Making salts from acids and metals — Filtration and Crystallisation
5 min read 6 parts Lesson 6 of 13
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
Section 1 of 6What a salt is
- 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
- 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
- magnesium + hydrochloric acid → magnesium chloride + hydrogen (Mg + 2HCl → MgCl2 + H2), and zinc + sulfuric acid → zinc sulfate + hydrogen.
Section 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 |
Section 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.
Section 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 |
Section 5 of 6Choosing equipment
- Measuring cylinder
- it measures the acid volume because it measures smaller differences in volume than a beaker.
- Beaker for the reaction
- a beaker is used rather than a conical flask because the mixture is easier to stir.
- Conical flask for filtering
- it holds the filter funnel upright.
- Excess shown by
- some magnesium remaining in the beaker shows that all the acid has reacted.
Section 6 of 6Safety and yield
1Safety2 points
- 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.
2Percentage yield1 point
= 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.
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