Final answer:
The statement that the volume of a gas is directly proportional to its temperature if the pressure and amount of the gas are constant is known as Charles's law.
Step-by-step explanation:
After careful studies and measurements in a laboratory, scientists concluded that the volume of a gas is directly proportional to its temperature if the pressure and amount of the gas are kept constant. This statement is best described as Charles's law. Charles's law is a fundamental principle in thermodynamics stating that the volume of a given amount of gas is directly proportional to its absolute temperature (measured in kelvins) when the pressure is maintained constant. This behavior of gases was determined through experiments that demonstrated that at constant pressure, the volume of a fixed amount of gas increases with an increase in temperature and decreases with a decrease in temperature.