Final answer:
In an experiment to find out whether the rate at which water freezes depends on the shape of its container, the independent variable is the shape of the container, and the dependent variable is the rate of freezing. One controlled variable would be the environmental temperature.
Step-by-step explanation:
An experiment is a scientific procedure carried out to test a hypothesis by varying one or more factors and observing the effect of this change on some aspect of the system being investigated.
In the given scenario, the independent variable is the shape of the container because it is the factor that you are changing in the experiment to test its effect on the rate at which water freezes. The dependent variable is the rate at which water freezes because it is what you are measuring in response to the changes in the independent variable.
One variable that should be controlled would be the temperature of the environment where the containers are placed, as temperature is a critical factor affecting the rate of freezing and should be kept constant to ensure that any observed changes are solely due to the shape of the container.