Gas laws (Boyle, Charles, combined)

Gas laws describe how pressure, volume, and temperature relate. Boyle's law says that at constant temperature, pressure and volume are inversely related: raise the pressure and the volume shrinks. P1V1 = P2V2. A gas at 2.0 L and 1.0 atm, squeezed to 2.0 atm, occupies 1.0 L, not 4.0 L. The squeezing and the shrinking happen in opposite directions, like a see-saw about the constant held in the middle.

Charles's law says that at constant pressure, volume and temperature move together: V1/T1 = V2/T2. Warm a gas and it expands. The common mistake is treating both pairs the same way. Pressure and volume move opposite; volume and kelvin temperature move together. Test the direction on the equation before plugging numbers in. The temperature scale, not just the numbers, decides whether the ratio is correct.

Temperatures must be in kelvins before any gas law is used. The conversion is T(K) = T(°C) + 273. Celsius adds another trap: using 25 and 75 in a ratio would distort the answer, because kelvin is the only scale that starts at absolute zero and makes the ratios honest. Kelvins start at absolute zero, so 300 K is genuinely double 150 K. Celsius values are offsets, not true ratios.

The combined gas law, P1V1/T1 = P2V2/T2, folds both into one equation. Whatever is held constant cancels automatically. Plug in the values, solve for the one unknown, and keep the units with the answer. Check the direction once more at the end against the trends. If the two temperatures cancel and the pressure halves, expect the volume to expand. That direction check catches inverted equations before they cost you the question.