3. Gas laws. Gay-Lussac's law In the 19th centur y, the French physicist Joseph Louis Gay-Lussac studied the variations in the pressure of a gas when its temperature changed , and the volume remained constant. We can perform similar experiments, applying the scientific method . Data analysis ⇒ The higher the temperature, the higher the pressure, and vice versa. ⇒ In each row, the quotient of p / T remains constant. Its value depends on the volume. ⇒ The pressure–temperature graph is a straight line that passes through the point (0, 0). ⇒ The gradient of the line depends on the volume. 3 Conclusion ⇒ Pressure and temperature are directly proportional quantities. 4 Experiment ⇒ Set the position of the piston so that the gas occupies a constant volume, VA. • Put the container in a thermostatic bath and write down the temperature on the thermometer, T1 (in K). • On the manometer, read the value of the pressure, p1. • Write down the values of T1 and p1 in a row in a table. • Heat the thermostatic bath until T2. Write down the value of p2. • Repeat until there are several rows of data. ⇒ Change the position of the piston so that the gas occupies the volume VB. • Repeat the steps to obtain another series of data. 1 VA p1 VA p2 p T p T p T = = constant ; 1 1 2 2 Gay-Lussac's law When a gas experiences transformations at a constant volume, the quotient of the pressure it exerts and its absolute temperature remains constant. Data collection 2 Table Graph dependent variable dependent variable independent variable independent variable Experiment A (constant VA) Experiment B (constant VB) T (K) p (atm) T (K) p (atm) 200 0.5 333 0.5 320 0.8 533 0.8 400 1.0 667 1.0 520 1.3 867 1.3 600 1.5 1000 1.5 7 Why do spray cans, such as those that contain cream, recommend that they are kept away from fire, even when they are empty? T1 T2 CHALLENGE 1 2 500 1000 p (atm) T (K) 0 0 VA < VB A B 14
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