(Primary Science) Variables in Experiments

To make sure that experiments are fair, we must change only one variable and fix other variables (factors). There are 3 types of variables.

• Independent variable is the variable that you change.
  
• Dependent variable is the variable that is affected by the independent variable. You measure this variable every time you change the independent variable.
  
• Controlled variables are those that you fix or keep constant.
  

Decide what variable to change and what variables to fix in experiments...

Let's try to think of the factors that affect plant growth. I shall list only 4 factors. They are:

• intensity of light, 
• amount of carbon dioxide available, 
• amount of water available, and 
• the type of soil. 

Suppose there are 4 students who're curious about how each of the factors affect plant growth.



We find that all the 4 problems have different independent variables but the same dependent variable.

Try these questions before you refer to the answers below.

1. Plant growth is the increase in height in a certain time interval. How do you measure and calculate plant growth?
   
2. How can the intensity of light be changed in John's experiment?
   
3. Predict the outcome of John's experiment. Explain your answer.
   
4. Predict the outcome of Alan's experiment. Explain your answer.
   

Answers: 1. A metre rule / measuring tape can be used to measure the initial height and final height. The difference in the heights is the plant growth. 2. By changing the distance between the light source and the plant / By changing the number of batteries connected in series to the lamp. 3. More intense light should increase plant growth. This is because more intensity of light increases the rate of photosynthesis. 4. More carbon dioxide available will increase plant growth. This is because carbon dioxide is needed in photosynthesis to make food.