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It is physically possible for any substance to be a solid, liquid or a gas although for small, light molecules, like hydrogen, it is hard to make the molecules cold enough to be a solid. For big molecules, like lead, it needs a lot of heat to make them into a gas. Whether a substance is a solid, liquid or gas i.e. its state of matter, depends on the temperature. When solids are heated, they melt into a liquid, when they are heated further the liquid boils to create a gas. A pure substance will always melt and boil at the same temperatures e.g. pure water has a melting point of 0^oC and a boiling point of 100^oC. Although pure water boils at 100^oC, it doesn’t have to be that hot for a liquid water molecule to become a gas molecule. A few molecules in the water will have enough energy below the boiling point to escape the liquid and turn into gas. This is why you can leave your washing-up to dry on a rack. The water on your plates and glasses slowly evaporates into water vapour without the water reaching 100^oC.