In your own words describe three key features of a fair test

In a fair test two or more things are compared. In order for a test to be fair or scientifically sound, children are required to ensure only one thing (this is called a variable) is changed.

For example, if testing which material is the most waterproof by pouring liquid onto a selection of different materials, in a fair test only the type of material (the component you are testing) should be changed. Therefore all other elements (the type of liquid used, the amount of liquid used, the distance the liquid is poured from, the speed at which it is poured and the size of the material) must remain the same each time the test is carried out. It would be unfair or an unscientific test if, for example, 1000ml of water were poured onto a piece of foil and 2ml of milk poured onto a piece of foam – you would not be able to fairly compare the materials. 

hope this will help you!!!

A fair test is a test which controls all but one variable when attempting to answer a scientific question. Only changing one variable allows the person conducting the test to know that no other variable has affected the results of the test.

Hope it helps

Have a great day : )

answer; both surfaces of the membrane are nonpermeable