The independent variable is the factor you change on purpose. The dependent variable is what you measure to see the effect of the change. Control variables are all the other factors you keep the same so the test is fair.

Example: In the biscuit dunking experiment, the independent variable is the type of biscuit, the dependent variable is the time taken for the biscuit to break, and the control variables include the tea temperature, volume of tea, dunking depth and the person dunking.

Collect the gas in a test tube. Light a splint and carefully bring the lighted splint to the mouth of the test tube. If hydrogen is present, it will react quickly with oxygen in the air and make a distinctive squeaky pop sound. This sound confirms the presence of hydrogen gas.

In paper chromatography, a spot of dye mixture is placed on a pencil baseline. The bottom of the paper is placed in a solvent. As the solvent moves up the paper, it carries the dyes with it. Dyes that are more soluble in the solvent travel further, while less soluble dyes travel a shorter distance. Because different dyes move at different speeds, they separate into distinct spots on the paper.

Salt water is placed in a flask and heated. The water has a lower boiling point than the dissolved salt, so it boils and turns into steam while the salt remains in the flask. The steam travels into the condenser, where cold water circulating around the outside cools the vapour. The vapour condenses back into liquid water and is collected in a separate beaker as the distillate. The remaining salt stays behind in the original flask.

Chlorine gas is toxic and harmful to breathe in. It can irritate or damage the lungs, eyes and skin. For this reason, tests involving chlorine should only be carried out in a fume cupboard or by a teacher. To test for chlorine, hold damp blue litmus paper near the mouth of the test tube. The paper first turns red, because chlorine is acidic, and then is bleached white. This colour change confirms the presence of chlorine gas.

Filtration is used to separate an insoluble solid from a liquid. The mixture is poured through filter paper; the solid remains as residue, and the liquid that passes through is the filtrate.

Evaporation and crystallisation are used when the solid is soluble in the liquid. The solution is heated to evaporate some of the solvent, making it more concentrated. When enough solvent has evaporated, the solution is allowed to cool and crystals of the solute form. So filtration removes insoluble particles, while evaporation/crystallisation recovers dissolved solids.

In a solution, the solute particles are completely dissolved and spread evenly between the solvent particles. They are so small that they cannot be seen and do not scatter light, so the solution appears clear. In other mixtures, such as suspensions, the solid does not fully dissolve. The particles remain large enough to be seen or to scatter light, which makes the liquid look cloudy.

Control variables are all the factors that are kept the same during an experiment. They matter because if they change, you cannot be sure whether the independent variable or something else caused the change in the dependent variable. Keeping control variables constant makes the experiment a fair test and ensures that the results are valid and reliable.

First, choose several types of biscuits to test (e.g. Digestive, Rich Tea, Hobnob). The independent variable is the type of biscuit. Prepare cups of tea with the same temperature and volume and use the same dunking depth and person dunking as control variables. For each biscuit, dunk it into the tea and start a timer. Stop timing when the biscuit breaks. The dependent variable is the time taken to break. Repeat for each biscuit type and compare the average times. The biscuit that lasts longest before breaking is judged to be the best for dunking.

A solution containing a dissolved solid is gently heated so that some of the solvent evaporates. As more solvent is removed, the solution becomes more concentrated. When it reaches the point of crystallisation, small crystals begin to appear. Heating is then stopped and the solution is allowed to cool slowly. As it cools, more solute particles join the crystal structure, forming larger, regular crystals. Finally, the crystals can be removed and dried.