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activity: Investigating
Hard Water
Is your
water hard or soft?
If soap lathers easily, it means your water contains very little
calcium and magnesium compounds. It is soft water. If you get a
greyish scum forming on the water surface, or hardly any lather,
then you have hard water.
The scum forms because the calcium and magnesium compounds react
with soap to give an insoluble product.
A word equation for the reaction is:
calcium sulphate + sodium
stearate (soap) = calcium stearate (scum) + sodium sulphate
What
you will need:
- 3 clean transparent jars
- rain water
- tap water
- detergent
- teaspoon or spatula
What
you will do:
- You'll need to collect some rain water, so pick a wet day to
do this experiment.
- When it rains, take two of your clean jars and collect your rain
water. Collect enough to fill the jars half way. Label one jar
as soft water.
- Take the second jar of rain water and add half a teaspoon, or
a spatula, of epsom salts. Label this as hard water.
- Take the third jar and fill it half full with tap water.
- Add 1 ml of detergent (liquid soap) to each jar.
What
do you predict?
- Make a prediction about what you think will happen when you
shake your jars twenty times each. You can record your results
in a table like this:
| Water
sample |
Prediction |
Actual
Results |
|
soft water
|
|
|
|
hard
water
|
|
|
|
tap
water
|
|
|
What
do you observe?
- Shake each jar of water and detergent twenty times. Write down
your observations. Did you predict correctly?
Questions
- Is your water hard or soft?
- Which two ions make water hard?
- Why won't any scum (calcium stearate) form if you've got soft
water?
- Why did you use rain water to prepare samples of soft and hard
water? Could you have used tap water?
- Is rain water hard or soft water?
- Write a word equation for the reaction of magnesium sulphate
and soap (sodium sulphate).
- Is the scum that forms soluble
or insoluble?
Explain your answer.
Click
here to check your answers.
Click here to
plan your own water hardness experiment.
Year 10s at work |
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