Activity: Hard water

The theme of this year's National Science Week is "Investigating Freshwater", so this week we will look at one aspect of water quality.

You will need

• Epsom salts (magnesium sulphate)
  • Epsom salts can be found in the supermarket.
  • If you can't find Epsom salts, try ordinary table salt instead.
• Dishwashing Detergent
• Water
• Two identical bottles or jars, with lids.
• A larger container
• Two teaspoons
• Adult

What to do

1. Place enough water to fill one of the bottles into the container.
2. Add some detergent to the water and stir slowly. We don't want to produce any bubbles yet.
3. Half-fill each bottle with the water from the large container. Make the water level the same in both bottles. This way we know the amount of water and detergent in the bottles is the same.
4. Place a teaspoon of Epsom salts into one of the bottles. Use a clean teaspoon.
5. Screw the lids onto the bottles.
6. Shake the bottles for ten seconds.
7. Compare the bubbles in the two bottles.

What's happening?

You should find that less bubbles formed in the bottle with the Epsom salts in it.

The chemical name for Epsom salts is magnesium sulfate. When it is dissolved in water, the magnesium in the Epsom salts reacts with the detergent to form a chemical that doesn't dissolve in water. With less detergent in the water, there were less bubbles.

Water containing dissolved magnesium or calcium compounds is often referred to as "hard water". (Hard water doesn't feel harder to touch, it just refers to having mineral salts dissolved in it.) Water hardness varies from place to place. Hard water is often found in towns and farms that use bore water or spring water, as underground minerals dissolve in the water.

If you found there was no difference between the two bottles, you may live in an area which already has very hard water. If you try the activity again using distilled or filtered water, you may find it works better.

Applications

• Hard water is harder to clean with than soft water. If you live in an area with hard water, you may need to use water softener for doing the laundry.
• Hard water can be a big problem for industry, especially any industry that boils water or uses water as a coolant. As hard water evaporates, it can leave behind the compounds dissolved in it, which can block up pipes and wear out mechanical appliances more quickly.
• Hard water doesn't make a good bubble mixture. Companies that make bubble mixture have been known to import distilled water when their local water supply was too hard.