It takes a few days, but by growing the crystals, you’ll see how solutions are made and how the crystalline shapes of salt is formed.
What You’ll Need
To grow your own salt crystals you’ll need:
- Table salt – sodium chloride
- Distilled Water
- A clean, clear glass container – a jam jar is perfect
- A spoon for stirring.
What To Do
Here’s what to do:
- Stir salt into boiling hot water until no more salt will dissolve (crystals start to appear at the bottom of the container). Be sure the water is as close to boiling as possible.
- Carefully pour the solution into your jar. (putting a spoon into the jar before adding the water should prevent the jar breaking.
- Suspend your string into the jar from the spoon laid across the top of the jar.
- Leave your jar somewhere it will not be disturbed and wait for your crystal to grow!
What To Look For
Once your crystals have grown, here are some things for you to look for in them:
- Any impurities in the salt or the water will change the shape and colour of the crystals you grow. What shape and colour are yours?
- Try using different types of table salt – try iodized salt, un-iodized salt, sea salt, or even salt substitutes. Is any difference in the appearance of the crystals?
- Try using different types of water, such as tap water compared with distilled water. Is there any difference in the appearance of the crystals?
The teacher should demonstrate this experiment which uses boiling water. Children should not be permitted to handle the jars until the water has cooled to room temperature.
The Science Behind It
Using Chemical Reactions To Make Salt
The reaction between an acid and a base is called Neutralisation. This is exactly how indigestion medicines works – it contain chemicals that react with and neutralise excess stomach acid. Industry uses this same method to produce a wide range of salts and products.
Here’s how neutralisation works:
Acidic solutions contain hydrogen (H+) ions.
Alkaline solutions contain hydroxide (OH–) ions.
Here’s the word equation for a reaction between an acid and an alkali:
Acid + alkali → salt + water
The ionic equation for all neutralisation reactions is:
H+(aq) + OH–(aq) → H2O(l)
The type of salt that is produced during the reaction depends on the acid and alkali used.
Acids, Alkalis and the Salts they Produce
Neutralising hydrochloric acid produces chloride salts.
Hydrochloric acid + sodium hydroxide → sodium chloride + water
Neutralising nitric acids produces nitrate salts.
Nitric acid + potassium hydroxide → potassium nitrate + water.
Neutralising sulphuric acid produces sulphate salts.
Sulphuric acid + sodium hydroxide → sodium sulphate + water.
Making Salts from Metal Oxides
Metal Oxides can also be used as bases and be reacted with acids to make salts and water.
Here’s word equation for a reaction between an acid and a metal base:
Metal oxide + acid → salt + water
Copper Oxide (CuO) + hydrochloric acid (2HCl) → copper chloride (CuCl2) + water (H20)
While fairly reactive metals can be reacted with acids to form salt and hydrogen, salts of very unreactive metals, such as copper, cannot be made this way because these metals do not react with acids.
And salts of very reactive metals, such as sodium, cannot be made this way because the reaction between the metal and the acid is too vigorous to be carried out safely.
Making Salt from Precipitation Reactions
Some insoluble salts can be made from the reaction between two solutions. Barium sulphate is an insoluble salt. It can be made by the reaction between solutions of barium chloride and sodium sulphate.
Barium chloride + Sodium Sulphate → barium sulphate + sodium chloride
Precipitation reactions can be used to remove unwanted ions from solutions. This technique is used to treat drinking water and treat effluent.
Making Salts from Metal Carbonates
Acids can be neutralised by metal carbonates to form salts. Most metal carbonates are insoluble, so they are bases, but they are not alkalis.
When acids are neutralised by metal carbonates, a salt, water and carbon dioxide are produced. This means that rocks, such as limestone, that contain carbonate compounds are damaged by acid rain.
Here’s the word equation of the reaction:
Metal carbonate + acid → salt + water + carbon dioxide