Boyle's law: For a given quantity of gas, the volume of a gas is inversely proportional to the absolute pressure applied to it when the temperature is kept constant. Charles's law: For a given quantity of gas, the volume of a gas is directly proportional to the absolute temperature when the pressure is kept constant. Gay-Lussac's law: At constant volume, the absolute pressure of a gas is directly proportional to the absolute temperature. For any gas, the volume and temperature graph is a straight line and always projects back to -273°C, at zero volume. this temperature is called the absolute zero Absolute zero forms the basis of a temperature scale known as absolute scale or Kelvin scale