Making salts in different ways
In this topic you will
- explain what a salt is
- give some examples of salts and their uses
- prepare a salt by reacting a metal with an acid
- prepare a salt by heating a metal oxide with an acid
- prepare a salt by using an acid and a carbonate
- prepare a salt using neutralisation
- carry out risk assessments for practical work
- use word and symbol equations
Key words
- carbonate
- chloride
- citrates
- crystallisation
- formula (plural: formulae)
- nitrate
- salt
- sulfate
- bases
- crystallise
- erodes
- limestone
- neutralisation
What is a salt?
When you think about salt, you probably think of the salt you put in your food as flavouring. This is sodium chloride. You have met other salts during your science course: copper sulfate, silver nitrate and calcium carbonate, for example.
Many salts have important uses in everyday life. The photographs show some examples.
- Sodium chloride is used to preserve food and flavour meals.
- Magnesium carbonate is used by gymnasts to keep hands dry for grip.
- Calcium sulfate is used to make blackboard chalk.
- Aluminium sulfate helps dyes stick to fabric fibres.
Acids and salts
Every day, the chemical industry makes hundreds of thousands of tonnes of different salts. Many of the methods for making salts start with acids.
All acids contain hydrogen. The table below gives you the formulae of the three common acids you find in the laboratory. The table also shows some examples of the salts that can be formed from these acids.
Two other acids you may meet are carbonic acid and citric acid. Carbonic acid is a weak acid that is formed when carbon dioxide reacts with water. Salts made from carbonic acid are called carbonates. Citric acid is found in fruits, such as oranges and lemons. Salts formed using citric acid are called citrates.
| Name of acid | Formula | Salts formed from the acid | Example of salt | Formula of salt |
|---|---|---|---|---|
| hydrochloric acid | HCl | chlorides | sodium chloride | NaCl |
| sulfuric acid | H₂SO₄ | sulfates | copper sulfate | CuSO₄ |
| nitric acid | HNO₃ | nitrates | potassium nitrate | KNO₃ |
Questions
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Acids taste sour, turn blue litmus red, react with metals to produce hydrogen, and have a pH less than 7.
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Examples include lemon juice (citric acid), vinegar (acetic acid), fizzy drinks (carbonic acid), and cleaning agents (hydrochloric acid).
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Nitric acid (HNO₃) contains hydrogen, nitrogen, and oxygen.
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Sulfuric acid (H₂SO₄) contains 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms.
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Both contain chlorine as part of their formula: HCl and NaCl.
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HCl is an acid that contains hydrogen; NaCl is a salt that contains sodium instead of hydrogen.
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Sodium citrate is the salt listed among the ingredients.
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Sodium citrate helps regulate acidity, preserve flavour, and improve texture in processed foods like jam.
Making salts using a metal and an acid
You have reacted metals with dilute acids before. It is often a good way to make salts.
The general equation for the reaction of metals with acid is:
acid + metal → salt + hydrogen
The word and symbol equations for the reaction between zinc and hydrochloric acid are:
zinc + hydrochloric acid → zinc chloride + hydrogen
Zn + 2HCl → ZnCl₂ + H₂
Questions
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Zinc chloride is the salt produced in the reaction.
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You would use sulfuric acid to make magnesium sulfate.
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iron + hydrochloric acid → iron chloride + hydrogen
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Sodium is highly reactive and would cause an explosive or dangerous reaction with hydrochloric acid.
Think Like a Scientist
Investigation: Making the salt zinc sulfate
You will need:
250 cm³ beaker, dilute sulfuric acid, measuring cylinder, zinc metal, evaporating basin, pipeclay triangle and tripod, Bunsen burner, tongs, heatproof mat, safety glasses.
Safety:
Be careful when heating the evaporating basin as the solution may spit and burn you.
Method:
Step 1: Pour about 50 cm³ of sulfuric acid into a 250 cm³ beaker.
Step 2: Add 1–5 g of zinc metal to the acid in the beaker.
Step 3: Once the mixture stops fizzing, pour it into the evaporating dish.
Step 4: Heat the evaporating dish very gently until crystals begin forming at the edge.
Step 5: Remove from heat and leave for a few days to allow crystallisation. Use tongs to move the dish to a heatproof mat if needed.
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zinc + sulfuric acid → zinc sulfate + hydrogen
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Use low heat, avoid boiling, and stop heating when crystals begin to form to prevent decomposition or splashing.
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Leaving the solution to evaporate slowly is better for forming larger, more regular crystals.
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Use the same solution in two dishes: heat one until nearly dry and leave the other to evaporate slowly, then compare the crystals.
Making salts using a metal oxide
Some metals will not react with acids to make salts. For example, silver and copper are too unreactive to displace hydrogen from an acid, so we have to find another way of making salts from unreactive metals. We can do this by starting with a metal oxide.
The general equation for this reaction is:
metal oxide + acid → salt + water
In this example we can make copper sulfate by heating copper oxide with sulfuric acid. The word and symbol equations for this reaction are:
copper oxide + sulfuric acid → copper sulfate + water
CuO + H₂SO₄ → CuSO₄ + H₂O
Think Like a Scientist
Investigation: Making the salt copper sulfate
Objective: Prepare copper sulfate crystals by reacting black copper oxide with dilute sulfuric acid.
Safety:
Do not boil the acid. Use low heat. Be careful when heating the evaporating basin as the solution may spit and burn you.
Method:
Step 1: Pour about 100 cm³ of dilute sulfuric acid into a 250 cm³ beaker. Add black copper oxide powder to the acid.
Step 2: Heat the mixture very gently, stirring all the time.
Step 3: When the mixture changes colour to blue, turn off the heat. Allow the mixture to cool.
Step 4: Filter the mixture. The filtrate is a solution of copper sulfate. Pour this into an evaporating basin.
Step 5: Place the evaporating basin on the pipeclay triangle on the tripod and heat very gently until you see crystals forming at the edge of the solution. Remove from the heat and leave for a few days to crystallise.
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Beaker, dilute sulfuric acid, black copper oxide, Bunsen burner, tripod, gauze, stirring rod, filter paper, funnel, evaporating basin, pipeclay triangle, heatproof mat, safety goggles.
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- Use safety goggles to protect eyes from splashes.
- Handle acid with care to avoid skin or clothing contact.
- Use low heat to prevent spitting or boiling over.
- Use tongs or heatproof mat for hot equipment.
- Dispose of waste materials safely.
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To remove the excess, unreacted copper oxide and obtain a clear copper sulfate solution.
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React black copper oxide with dilute hydrochloric acid instead of sulfuric acid, then follow the same filtering and crystallisation steps.
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copper oxide + hydrochloric acid → copper chloride + water
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copper oxide + nitric acid → copper nitrate + water
Metal carbonates and acids
Carbonates – such as calcium carbonate – are salts. Carbonates can be formed by the reaction of a metal with carbonic acid.
We can use carbonates to form other salts by reacting them with an acid.
For example:
sulfuric acid + calcium carbonate → calcium sulfate + water + carbon dioxide
H₂SO₄ + CaCO₃ → CaSO₄ + H₂O + CO₂
hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
acid + carbonate → salt + water + carbon dioxide
The line above shows the general equation for these reactions. The rock limestone is made of calcium carbonate. It is damaged when it reacts with acid rain and erodes.
The skeletons of coral are made from calcium carbonate and react with acids. This happens when the oceans become slightly more acidic as more carbon dioxide dissolves in the water.
Questions
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Magnesium carbonate + nitric acid → magnesium nitrate + carbon dioxide + water
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MgCO₃ + H₂SO₄ → MgSO₄ + CO₂ + H₂O
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Bubble the gas through limewater. If it turns milky, the gas is carbon dioxide.
Think Like a Scientist
Investigation: Preparing a salt from acid and a carbonate
Objective: To prepare copper chloride by reacting copper carbonate with hydrochloric acid.
Safety:
Take care when heating the solution (step 5), as it may spit and burn you.
Method:
Step 1: Place 25 cm³ of hydrochloric acid in a small beaker.
Step 2: Add a spatula of copper carbonate.
Step 3: Add more copper carbonate until it stops reacting. A small amount of unreacted copper carbonate should remain (excess ensures all acid reacts).
Step 4: Filter the mixture to remove excess copper carbonate. The filtrate is copper chloride solution.
Step 5: Pour the filtrate into an evaporating basin and heat gently to remove water.
Step 6: Stop heating when crystals begin forming at the edge.
Step 7: Leave the solution to cool and evaporate slowly to crystallise.
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Small beaker, hydrochloric acid, copper carbonate, spatula, filter funnel, filter paper, evaporating basin, tripod, gauze, pipeclay triangle, Bunsen burner, heatproof mat, tongs, safety goggles.
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- Wear goggles to protect eyes from acid.
- Use low heat to avoid spitting during evaporation.
- Handle glassware with care to avoid spills or burns.
- Do not inhale copper compounds.
- Dispose of waste copper compounds properly.
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Effervescence (fizzing) occurred as the copper carbonate reacted with the acid, forming a light blue solution.
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Carbon dioxide (CO₂) is released during the reaction between copper carbonate and hydrochloric acid.
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copper carbonate + hydrochloric acid → copper chloride + carbon dioxide + water
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It appears as blue-green crystals after evaporation and crystallisation.
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Copper chloride is the salt formed in this reaction.
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Copper carbonate is insoluble but reacts with acid, while copper chloride is soluble and passes through the filter into the solution.
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React copper carbonate with dilute sulfuric acid instead of hydrochloric acid, then follow the same filtration and crystallisation steps.
Salts in rocks
The blue-green colours in these rocks in the Atacama Desert in Chile tell you that they contain copper salts. This bright blue-green mineral in the rock is called malachite. It is made from copper carbonate.
Forming salts by neutralisation
Alkalis react with acids to neutralise them. When an acid is neutralised by an alkali, a salt is produced. For example, when sodium hydroxide reacts with hydrochloric acid, the salt sodium chloride is formed. The other product is water.
sodium hydroxide + hydrochloric acid → sodium chloride + water
NaOH + HCl → NaCl + H₂O
The general equation for neutralisation reactions is:
acid + alkali → salt + water
Questions
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Use an indicator such as litmus paper or universal indicator. Acids turn litmus red, alkalis turn it blue.
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Neutral
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Alkalis have a pH above 7, feel soapy, and can neutralise acids. They turn litmus blue.
Think Like a Scientist
Investigation: Preparing a salt by neutralisation
Objective: To make a salt by neutralising an acid with an alkali and crystallising the resulting solution.
Safety:
Take care when heating the filtrate (step 8), as it may spit and burn you.
Method:
Step 1: Place hydrochloric acid in a burette.
Step 2: Measure out 20 cm³ of sodium hydroxide into a small flask.
Step 3: Add a few drops of universal indicator solution.
Step 4: Add the acid from the burette slowly, swirling the flask as you add the acid.
Step 5: When the indicator turns green, the solution is neutral.
Step 6: Add a spatula of charcoal powder to the green solution and stir with a glass rod to remove colour.
Step 7: Filter the mixture to remove the charcoal.
Step 8: Place the filtrate in an evaporating basin and heat gently.
Step 9: Stop heating when crystals begin to form at the edge.
Step 10: Leave for a few days to allow full crystallisation of the salt.
Show Answer
Burette, clamp stand, hydrochloric acid, sodium hydroxide, measuring cylinder, small conical flask, universal indicator, charcoal powder, glass rod, filter funnel, filter paper, evaporating basin, tripod, gauze, Bunsen burner, tongs, safety goggles.
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- Wear goggles to protect from acid or alkali splashes.
- Handle hydrochloric acid and sodium hydroxide with care.
- Use low heat when evaporating to avoid burns or spitting.
- Avoid breathing charcoal powder or indicator vapours.
- Clean up any spills immediately.
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The indicator turns purple or dark blue in sodium hydroxide (an alkali).
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Green
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To mix the acid and alkali evenly and allow the neutralisation to occur smoothly without splashing.
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Add more sodium hydroxide drop by drop until the indicator returns to green, showing a neutral solution.
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Sodium chloride
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hydrochloric acid + sodium hydroxide → sodium chloride + water
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White crystals of sodium chloride were formed after evaporation.
Alkalis and bases
When a metal oxide dissolves in water, it forms an alkaline solution.
Metal oxides are called bases. Soluble metal bases form alkalis when they dissolve in water.
For example:
sodium oxide + water → sodium hydroxide
Sodium oxide is a base. The sodium hydroxide is an alkali.
Some metal oxides are not soluble in water, for example iron and copper oxide. So they do not form alkalis. But they can still react with acids to form salts.
copper oxide + sulfuric acid → copper sulfate + water
This is a reaction between a base and an acid, forming a salt and water.
Important Concept
A base is a substance that can react with an acid to neutralise it. If the base dissolves in water, it is called an alkali.
Questions
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A base is a substance that reacts with acids to form salts. An alkali is a base that is soluble in water.
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Magnesium oxide + sulfuric acid → magnesium sulfate + water
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MgO + H₂SO₄ → MgSO₄ + H₂O
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React iron oxide with hydrochloric acid. This will produce iron chloride and water.