Scientists are interested in investigating how limestone can be used to remediate the effects of acid rain. They collect water from a lake that has recently been exposed to acid rain. They divide the water into 10 different tanks and add different amounts of limestone to nine of the tanks. No limestone is added to one of the tanks. The pH of the water in the tanks is measured before the limestone is added and once every hour for 24 hours.

The independent variable is the amount of limestone placed inside the tanks.
The dependent variable is the ph of the water in the tanks after liming has been added.
The control group is the blood that received water, but did not receive limestone.
The amount of water could be an additional variable that was not mentioned.
A modification that would affect the results would be to place different amounts of limestone in each tank.
Explanation:
In an experiment, the dependent variable is one that influences a result that can be observed. As in the experiment above, the researchers want to see the influence of limestone in neutralizing the pH of acid rainwater, we can say that the amount of calcarop is the independent variable.
A dependent variable, in turn, is one that is influenced by the dependent variable in generating results. As the experiment above, wants to check the influence of limestone on the pH of water from acid rain, we can say that the pH of the water is the dependent variable.
The control group, is the one that does not receive the factor that is being tested in the experiment. As the influence of the amount of limestone on the pH change is being tested, we can say that the water lang that did not receive limestone is the control group.
The amount of water in each blood is a variable that was not mentioned, but that could modify the results, since larger amounts of water can dilute the limestone by modifying its effect on the pH of the water.
A modification that could alter the results is to place different amounts of limestone in each tank, which would provide totally different results.