When a piece of plant tissue such as a potato cylinder is left in a solution, water moves by osmosis. In a dilute solution water enters the cylinder and its mass increases. In a concentrated solution water leaves and its mass decreases.
Cylinders rarely start with exactly the same mass, so we compare them using a percentage change, which is fair even when cylinders start with different masses. Percentage change in mass = (final mass minus original mass) divided by original mass, multiplied by 100. A positive answer is a percentage gain and a negative answer is a percentage loss.
Worked example 1: a cylinder goes from 5.0 g to 5.6 g. The change is 0.6 g, and 0.6 divided by 5.0 is 0.12, so the percentage gain is 12%.
Worked example 2: a cylinder goes from 4.0 g to 3.4 g. It has lost 0.6 g, and 0.6 divided by 4.0 is 0.15, so the percentage loss is 15%. Always divide by the original mass, not the final mass.
Worked example 3: a cylinder goes from 8.0 g to 8.4 g. The change is 0.4 g, and 0.4 divided by 8.0 is 0.05, so the gain is 5%.