There's two forms of latent heat, there's latent heat of fusion which is melting, and there's latent heat of vaporization which is boiling. They describe the direction of energy flow when changing from one thing to the next like solid to liquid, or liquid to gas.
If you have a lump of solid at its melting point ... like ice at 32°F ... you have to put a certain amount of heat into it just to change it to water at 32°F. That amount of heat, that's used just to change a solid lump into liquid without changing its temperature, is called the heat of fusion for that substance.
The number is different for every substance.
For water, it takes 336 joules of heat to melt 1 gram of ice into 1 gram of water, all at 32°F (0°C). That's an enormous latent heat of fusion ... more than almost any other known substance. That's why ice is such a good choice when you need something to put in your drink to cool it down. Ice absorbs a huge amount of heat before it melts and the drink gets watered down.
Are you referring to the fact that water is a compound while hydrogen is an element? If I'm wrong just comment and clarify and I can edit it, I don't even know what kind of unit you're in. :)
