Before heating: Ice and air present in the tin, the ice particles vibrate about fixed positions, they do not exert a pressure on the lid at this time, air particles inside the tin hit the lid but exert the same pressure as the surrounding air outside.Once heat is applied, the ice begins to melt, the ice (solid) becomes water (liquid), the water molecules move about within the liquid, sliding over each other. This does not exert a pressure on the lid, air particles move faster/have more kinetic energy, this exerts more force/pressure on the lidVaporisation then occurs, water (liquid) becomes steam (gas), particles of water vapour move randomly and exert a force/ pressure on the lid by colliding with it. Air and water vapour particles gain more energy as they are heated further and more collisions occur. Force/pressure on the lid increases as more heat is supplied, where the force/pressure becomes great enough to blow the lid off of the tin