Basically, when you melt something you turn it from a solid into a liquid. Most substances will melt under the right conditions, although some molecules will just disintegrate rather than melting when they are heated. Some substances, like carbon dioxide, will go directly from being a solid to being a gas at atmospheric pressure. This is called sublimation. You asked specifically about water and diamond. Liquid water, like you get out of a faucet or in a river, is melted water. Frozen water shows up as snow and ice. Steam would be an example of water in its gaseous form. Diamond is a little trickier. The temperature that a substance melts at depends on what pressure it is at. You can melt diamond if it you put it under a huge pressure (around 120,000 atmospheres) and at a very high temperature (4440 C). At higher pressures the melting temperature will be even higher. At lower pressures the diamond would go from being diamond to being graphite and then, as you continued to raise the temperature, would melt into liquid carbon. At atmospheric pressure and room temperature graphite is more thermodynamically stable than diamond.
Basically, when you melt something you turn it from a solid into a liquid. Most substances will melt under the right conditions, although some molecules will just disintegrate rather than melting when they are heated. Some substances, like carbon dioxide, will go directly from being a solid to being a gas at atmospheric pressure. This is called sublimation. You asked specifically about water and diamond. Liquid water, like you get out of a faucet or in a river, is melted water. Frozen water shows up as snow and ice. Steam would be an example of water in its gaseous form. Diamond is a little trickier. The temperature that a substance melts at depends on what pressure it is at. You can melt diamond if it you put it under a huge pressure (around 120,000 atmospheres) and at a very high temperature (4440 C). At higher pressures the melting temperature will be even higher. At lower pressures the diamond would go from being diamond to being graphite and then, as you continued to raise the temperature, would melt into liquid carbon. At atmospheric pressure and room temperature graphite is more thermodynamically stable than diamond.
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Basically, when you melt something you turn it from a solid into a liquid. Most substances will melt under the right conditions, although some molecules will just disintegrate rather than melting when they are heated. Some substances, like carbon dioxide, will go directly from being a solid to being a gas at atmospheric pressure. This is called sublimation. You asked specifically about water and diamond. Liquid water, like you get out of a faucet or in a river, is melted water. Frozen water shows up as snow and ice. Steam would be an example of water in its gaseous form. Diamond is a little trickier. The temperature that a substance melts at depends on what pressure it is at. You can melt diamond if it you put it under a huge pressure (around 120,000 atmospheres) and at a very high temperature (4440 C). At higher pressures the melting temperature will be even higher. At lower pressures the diamond would go from being diamond to being graphite and then, as you continued to raise the temperature, would melt into liquid carbon. At atmospheric pressure and room temperature graphite is more thermodynamically stable than diamond.