批改

Introduce different quantities of salt in four beakers.

(For instance, ten spatulas in the first one, five spatulas in the second one, two spatulas in the third one, none in the last one) Pour the same volume of distilled water in each of them.

Thanks to a glass rod, stir up the water until the salt is completely dissolved.

Drop food colouring in the beakers and mix.

(Use a different colour in each of them) Fill test tubes with the different solutions.

Introduce a pipette in the test tube without salt, with 2 cm in depth  Put the finger on the top of the pipette in order to draw up the liquid.

Then, introduce the pipette in the less salt-concentrated tube, introduce it deeper than previously Take off the finger and put it again in order to draw up the second layer of liquid Do the same handling for the last two test tubes.