1M hydrochloric acid
Place some of the absorbent paper on each of the saucers.
One should be soaked in the acid, one from a 50/50 solution of acid and water, one from a 10% solution of acid and water and one with just water.
Place 20 cress seeds on each saucer.
Leave for one week for the seeds to grow ensuring that the paper does not dry out.
Ensure the saucers are as close together as possible to avoid variance in temperature and light received.
Measure the height of each of the plants that has germinated, and also record how many of the plants germinated.
You may also record any other pertinent information about the plants such as leaf colour or shape.
Ensure goggles are used when working with acidic solutions, also ensure that the saucers are left to germinate in an area where they will not be knocked onto the floor and break.