Gas Laws · 6 min read
Boyle's law
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Introduction to Gas Behavior
Have you noticed what happens when you try to squeeze an inflated balloon? As you press it harder, the space inside gets smaller, and you feel the air pushing back with more force. This is the basic idea of how gas pressure and volume work together.
When we talk about gases, we look at three main things: Pressure, Volume, and Temperature. Boyle's Law focuses on only two of these: Pressure and Volume. For this law to work, the Temperature must not change at all. It must stay 'constant' just like when the weather stays the same all day.
What is Boyle's Law
Imagine you are pumping your bicycle tyre with a hand pump. When you push the handle down, you are reducing the volume (space) inside the pump. Because that space is smaller, the air molecules become 'angry' and hit the walls of the pump harder. This increases the pressure.
In simple terms, this means when you squeeze a gas to make it small, the pressure goes up, and when you let it expand, the pressure goes down.
Mathematical Expression
We write this relationship as P is proportional to 1/V. When we introduce a constant (k), it becomes P = k/V or simply: P1V1 = P2V2.
In this formula:
P1 is the initial pressure (the starting pressure)
V1 is the initial volume (the starting space)
P2 is the final pressure (the new pressure)
V2 is the final volume (the new space)
Real Life Examples
1. Squeezing a syringe: When you block the tip of a syringe and push the plunger, the air inside gets compressed and the pressure increases.
2. Deep sea divers: As bubbles rise from a diver under water, the pressure from the sea decreases, so the bubbles grow bigger (larger volume) before they hit the surface.
Worked Example
A gas occupies a volume of 200 cm3 at a pressure of 760 mmHg. What will be its volume if the pressure is increased to 1000 mmHg, assuming temperature is constant?
Step 1: Write down what you know. P1 = 760 mmHg, V1 = 200 cm3, P2 = 1000 mmHg, V2 = ?
Step 2: Use the formula P1V1 = P2V2. So, 760 x 200 = 1000 x V2.
Step 3: Solve for V2. V2 = (760 x 200) / 1000 = 152000 / 1000 = 152 cm3. The volume decreased as pressure increased, which follows the law.
Key points
- •Temperature must remain constant for Boyle's law to apply.
- •Pressure and Volume have an inverse relationship.
- •The graph of P against V is a curve called an hyperbola.
- •The graph of P against 1/V is a straight line passing through the origin.
- •P1V1 = P2V2 is the standard calculation formula.