The isobaric process
A change of state of a gas at constant mass and pressure is called an isobaric process (“baros” means pressure). Writing the equation of state for two states with p = const and dividing term by term gives V₁/T₁ = V₂/T₂. This is the law found experimentally in 1802 by the French physicist Gay-Lussac: at constant pressure the ratio of the volume of a given mass of gas to its absolute temperature is constant, i.e. V ∝ T. So if T doubles, the volume doubles too. On a V–T graph an isobar is a straight line through the origin; a higher pressure gives a less steep isobar, lying closer to the temperature axis, because the volume at a given T is smaller. Heating a gas under a freely moving piston, or a hot-air balloon, are examples of isobaric processes.
Put a soft balloon filled with air (inflated, tied firmly) from the room into a cold place (a fridge or by an open door) and watch how its volume changes; then return it to the room. Never put the balloon near a heat source or heat it; ask an adult.