4. A student added 10 mL of a yeast suspension and 10
mL of 30°C water to each of two test tubes. Five
grams of sugar was then added to one of the test
tubes. Both test tubes were gently swirled to mix the
contents, and then the test tubes were placed in a
warm water bath for 15 minutes. The student made
observations of any bubbles that formed in the test
tubes and recorded the data in a table.
This experiment was most likely carried out to
investigate the
A) effect of sugar on a metabolic activity of yeast
B) effect of temperature on a metabolic activity of
yeast
C) solubility of yeast in water at 30°C
D) solubility of sugar in a yeast suspension

In this experiment, everything was kept the same except for adding sugar into one test tube and none on the other. The data that was recorded was the bubble formation. Bubble formation is an indicator for a chemical reaction and in this case it is the metabolic activity of the yeast.

Everything that is kept constant like the amount of yeast suspension, temperature and time it was placed in the warm water bath are what we call controlled variables. This is done to prevent any other variables from affecting what is being tested.

The amount of sugar is what is different and this is called the independent variable. This is what we are testing to see if it will have an effect.

Finally, bubble formation or metabolic activity is the dependent variable. This is what will be observed as a result of the changes in the independent variable.