Gas Laws and Kinetic Theory · 8 min read
Boyle's and Charles' laws
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Introduction to Gas Laws
Gas particles are like people inside a very crowded Danfo bus. They move about, bump into the walls, and occupy space. To understand how they behave, we look at three things: Temperature, Pressure, and Volume. Gas laws show us how these three behave when we change one of them.
Boyle's Law explained
Think of a bicycle pump. When you push the handle down, you are reducing the space (Volume) inside the pump. Because the space is small, the air particles hit the walls harder and more often. This makes the Pressure go up. If you pull the handle up to give them more space, the pressure drops.
Boyle's Law states that the volume of a fixed mass of gas is inversely proportional to its pressure, provided the temperature remains constant.
Boyle's Law means if you double the pressure, the volume will become half, as long as the heat does not change.
Mathematical formula for Boyle's Law:
P1V1 = P2V2
Charles' Law explained
Think of a football left outside under the hot sun. The air inside gets hot, the particles move faster, and they push the leather out to make the ball bigger. If you put that same ball in a cold deep freezer, it will shrink and become 'flabby' because the volume reduces as the temperature drops.
Charles' Law states that the volume of a fixed mass of gas is directly proportional to its absolute temperature, provided the pressure remains constant.
Charles' Law means that gas gets bigger when it is hot and smaller when it is cold, provided the pressure is the same.
Important note on Temperature
In Gas Law calculations, never use Celsius (oC). You must always convert it to Kelvin (K) by adding 273 to the Celsius value. For example, 27oC is 27 + 273 = 300 K.
Mathematical formula for Charles' Law:
V1 / T1 = V2 / T2
Worked Example for Boyle's Law:
1. A gas occupies 500 cm3 at a pressure of 700 mmHg. What is the volume if the pressure is increased to 1000 mmHg at constant temperature?
2. Identify values: V1 = 500 cm3, P1 = 700 mmHg, P2 = 1000 mmHg, V2 = ?
3. Use formula: P1V1 = P2V2
4. Substitute: 700 x 500 = 1000 x V2
5. Calculate: 350,000 / 1000 = V2. Therefore, V2 = 350 cm3.
Key points
- •Boyle's Law deals with Pressure and Volume at constant Temperature.
- •Charles' Law deals with Volume and Temperature at constant Pressure.
- •In Boyle's Law, as Volume goes up, Pressure goes down (Inverse).
- •In Charles' Law, as Temperature goes up, Volume goes up (Direct).
- •Always convert temperature to Kelvin (K = oC + 273) before solving.