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W08 Water has a mass - Learn how to handle a two pan scale, make weights(.
In your first experiment you learned how to measure the volume of water. Beside the volume each substance has a characteristic mass. For weighing two pan scales (Photo 1) were used in Egypt more than 4000 years ago. Modern versions of such scales you still find in most schools nowadays (Photo 2). You will learn quickly how to use it and how to make weights using square paper from your exercise book.
On a digital pocket scale (Photo 3) you find that 1/4 of a DIN A 4 paper sheet weighs a little bid more than 1 gramm. (60 squares = 1000 mg).
Material
Two pan scale with a stand (Photo 1), ruler, 1 sheet of DIN A 4 square paper,  aluminium foil, scissors, insulin syringe 1 ml (only from a pharmacy!), water samples.
Experiment
1. Fix the scale at a stand.
2. Look whether it is in equilibrium like seen in Photo 2. If one of the pans is higher than the other, add a piece of aluminium foil of the right size to it.
3. Cut the square paper into 4 parts of equal size. Fold one of it and place it in the left pan.
4. Add 1.0 ml water to the right pan.
5. Cut off the number of squares needed for the paper weight to balance the scale again.
6. Count / calculate, how many squares equal 10 ml ´water.
7. Cheque how many sqares are needed on one pan to unbalance the scale.
8. Leave the scale with water and paper weight until it is no longer balanced.
Results
a) 1 ml tap water balance a paper weight witht ........... squares.
b) The scale can be unbalanced by ........mg of paper weight.
c) After ..... hours the pan with the paper weight is deeper.
Explanationg
1 ml freshwater weighs exactly 1.0 g. So the paper weight is also 1.0 g (= 1000 mg). To get lighter weights you can cut it.
The weight of the water decreases due to evaporation. This means that water is of limited use as weight.