An experiment is considered a controlled experiment if it contains tests with chemicals.

http://en.wikipedia.org/wiki/Scientific_control

Actually, an experiment is considered a controlled experiment based on the control group and the manipulation of variables, not necessarily the use of chemicals.

A controlled experiment refers to a scientific investigation in which an individual or group of variables are carefully controlled or manipulated while others are held constant. The purpose of a controlled experiment is to determine the cause-and-effect relationship between independent variables (the ones being manipulated) and dependent variables (the ones being measured or observed).

In a controlled experiment, researchers divide the subjects or samples into two groups: the experimental group and the control group. The control group serves as a baseline or reference point against which the experimental group is compared. The experimental group is exposed to the variable being tested, while the control group is not.

For example, let's say a researcher wants to test the effects of a new drug on human subjects. They would randomly assign participants to two groups: the experimental group, which receives the new drug, and the control group, which receives a placebo or an existing standard treatment. By comparing the outcomes of the two groups, the researcher can determine if the new drug has any significant impact.

While chemicals may be used in various scientific experiments, they are not a defining characteristic of a controlled experiment. The essence of a controlled experiment lies in the careful control of variables to isolate and measure the effects of specific factors or treatments.