Pressure & Temperature in Gases
Section: 1. States of Matter | Syllabus: Cambridge AS Level Physics 9702
Gas Pressure Gas pressure is caused by gas particles colliding with the walls of their container. Understanding how pressure, temperature, and volume are related is fundamental to chemistry. Gas Pressure The force exerted by gas particles per unit area when they collide with the walls of their container.
How Gas Pressure is Created According to kinetic theory, gas pressure results from countless particle collisions: Gas particles move randomly in all directions at high speeds When particles hit the container walls, they exert a force Each individual collision produces a tiny force Billions of collisions per second create a constant pressure The total pressure depends on the frequency and force of collisions Key Concept Gas pressure is not caused by the weight of the gas, but by the constant bombardment of particles against container walls.
Relationship Between Pressure and Temperature When temperature increases, gas pressure increases (if volume is kept constant). Why Does Pressure Increase with Temperature? Higher temperature → Particles gain kinetic energy More kinetic energy → Particles move faster Faster particles → More frequent collisions with walls Faster particles → Harder collisions (greater force per collision) More frequent and harder collisions → Higher pressure Important This relationship assumes the volume and amount of gas remain constant.
Temperature must be measured in Kelvin (K) for gas law calculations. Mathematical Relationship (Gay-Lussac's Law) For a fixed amount of gas at constant volume: P ∝ T (in Kelvin) or P₁/T₁ = P₂/T₂ Where: P = pressure T = temperature in Kelvin Subscripts 1 and 2 represent initial and final conditions Relationship Between Pressure and Volume When volume decreases, gas pressure increases (if temperature is kept constant).
Why Does Pressure Increase as Volume Decreases? Smaller volume → Particles are confined to a smaller space Same number of particles in smaller space → Higher particle density Smaller container → Particles hit the walls more frequently More frequent collisions → Higher pressure Key Point The particles themselves don't change speed (temperature is constant), but they hit the walls more often because they have less distance to travel between collisions.
Mathematical Relationship (Boyle's Law) For a fixed amount of gas at constant temperature: P ∝ 1/V or P₁V₁ = P₂V₂ Where: P = pressure V = volume This is an inverse relationship - as one increases, the other decreases Relationship Between Volume and Temperature When temperature increases, gas volume increases (if pressure is kept constant).
Why Does Volume Increase with Temperature? Higher temperature → Particles move faster Faster particles → Hit walls harder and more frequently To maintain constant pressure → Container must expand Larger volume → Reduces collision frequency back to original pressure Mathematical Relationship (Charles's Law) For a fixed amount of gas at constant pressure: V ∝ T (in Kelvin) or V₁/T₁ = V₂/T₂ The Combined Gas Law All three relationships can be combined into a single equation: P₁V₁/T₁ = P₂V₂/T₂ This equation works when any or all of pressure, volume, and temperature change for a fixed amount of gas.
Exam Tip Temperature must ALWAYS be in Kelvin for gas law calculations. Convert from Celsius using: K = °C + 273 The Kelvin Temperature Scale The Kelvin scale is essential for gas law calculations. 0 K (absolute zero) = -273°C - the lowest possible temperature 273 K = 0°C (freezing point of water) 373 K = 100°C (boiling point of water) Conversion: K = °C + 273 Why Kelvin?
It's an absolute scale starting at absolute zero where particle motion is minimal Remember At absolute zero (0 K), particles have minimum kinetic energy. They don't stop moving completely due to quantum effects, but their motion is minimal.
Explaining Everyday Observations Why do aerosol cans explode if heated? The can has a fixed volume. When heated, the gas particles inside move faster and hit the walls harder and more frequently, increasing pressure.
If pressure becomes too high, the can explodes. Why do car tires need more air in winter? Cold temperatures cause gas particles to move slower, reducing pressure. Lower pressure makes tires look deflated, so more air needs to be added.
Why does a balloon shrink in a freezer? At lower temperatures, gas particles move slower and exert less pressure. The balloon's flexible walls contract until the internal pressure matches the external atmospheric pressure.
Why does a sealed crisp packet expand on a plane? At high altitude, external air pressure is lower. The higher pressure gas inside the packet pushes outward more than the lower external pressure pushes inward, causing expansion.
Practical Applications Hot Air Balloons Heating air increases particle speed, causing the air to expand (volume increases). The expanded hot air is less dense than surrounding cool air, creating lift.
Pressure Cookers Sealed container prevents steam from escaping. As water boils …
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