1. A heating device comprising:a heat source material which is a phase change material and which undergoes an exothermic phase change upon activation by an activating agent to generate heat;
a heat source chamber within which the heat source material is held;
an activation chamber providing the activating agent;
a barrier separating the interior of the heat source chamber from the interior of the activation chamber, wherein the activation chamber is deformable and wherein deformation of the activation chamber causes rupture of the barrier to allow contact between the activating agent and the heat source material, the contact activating the exothermic phase change of the phase change material, and
a housing to receive a nicotine source arranged to release nicotine upon being heated by the generated heat.