The concept of variables in a science experiment can be confusing for fifth graders. Think of the independent variable as what you change in an experiment, the dependent variable as the response you observe because of what you changed, and the controlled variable as the things you keep the same so they don’t interfere with your results. The independent variable must be something measurable that you can change in the experiment. The dependent variables must be able to be measured and caused by the independent variable. The controlled variable must not change during the experiment. Try some easy projects that use three variables to understand the importance of each variable in an experiment.

## Do Seeds Germinate More Quickly in Fertilized Soil?

Plant seeds in identical seedling trays, using two trays of unfertilized soil and two seedling trays of fertilized soil, to see which soil helps the seeds germinate faster. Label the unfertilized seedling trays “A” and “B” and the fertilized seedling trays “C” and “D.” The controlled variables are: same kind of seed, same type of soil, same amount of water from the same source applied at the same frequency, same amount of exposure to the sun, same room temperature and same dew point. The fertilizer added to trays C and D is the independent variable. The time for germination to take place and the height of the seedlings are dependent variables.

## Does More Sugar Dissolve in Heated Water?

Compare how much sugar dissolves in containers of one cup of water, each at different temperatures. When sugar dissolves in water, you cannot see any sugar crystals floating in the water or settling on the bottom of the cup when you stop stirring; you'll use these visual indicators to compare how much has dissolved in each cup. You will change the temperature of the water, so this is the independent variable. The dependent variable is the amount of sugar that dissolves in each cup of water. The controlled variables are stirring each container the same amount and using sugar from the same bag.

## Does Changing the Mass on the End of a Pendulum Affect the Period?

Tie a weight to the end of a 3 1/2-foot string, leaving a 5-inch tail of string so you can add additional weights later in the experiment. Hang the string from a dowel rod taped to the top of a cabinet. Mark the angle from which you will swing the pendulum, then release the weight. Time how long it takes to swing back and forth five times. One swing is called a period. Divide the time by five to get the average period for the first trial. Conduct two more trials and average the period for the three trials. Repeat the procedure with two weights and three weights. The varying weights are the independent variable, while the number of swings, or periods, is the dependent variable. The length of the string and the angle of the swing are controlled variables.

## Does the Type of Surface Affect the Speed of a Toy Car?

Make a ramp with sides to make sure the car stays on the ramp. The ramp can be as simple as a board with modeling clay guard rails. You will test different surfaces, such as sand paper, floor tile or bare wood, on top of the ramp and measure the time and distance that a toy car travels using at least three trials each. The various surfaces on the ramp are the independent variables. The speed of the car, measured in distance traveled over a length of time, is the dependent variable. The controlled variables are using the same car, using the same ramp at the same angle, and letting go of the car without pushing at the same starting point.