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Hydrates refer to compounds containing water, which are quite broad in scope. The water may be linked to other moieties by a coordination bond, such as a hydrated metal ion, or a covalent bond, such as hydrated trichloroacetaldehyde. It can also be referred to as white crystals formed by certain components of natural gas and water under certain temperature and pressure conditions. The appearance is similar to dense ice and snow, and the density is 0.88~0.90 g/cm3. Studies have shown that hydrate is a clathrate crystal inclusion, water molecules are combined by hydrogen bonding to form cage crystals, and gas molecules are enclosed in the crystal lattice.

The hydrate of the low molecular gas is a body-centered cubic lattice, and the larger gas molecules are crystal structures similar to diamond.

When a compound crystallizes from an aqueous solution of its constituent ions, the resulting crystal tends to be a hydrate. So hydrates are not simply mixed with water, and hydrates are also pure.

The water in the hydrate is present in a defined amount, for example, the composition of the hydrate of anhydrous copper sulfate CuSO4 is CuSO4·5H2O. The water in the hydrate has several different combinations: one is a ligand, which is coordinated to a metal ion, called a coordinated crystal water; the other is bound to an anion, called an anionic water.

Water may also be present in the crystal in a certain proportion without being directly combined with a cation or an anion, occupying a certain portion in the crystal lattice. This combined form of water is called lattice water and typically contains 12 water molecules. Some crystalline compounds also contain water but do not contain a certain proportion. For example zeolites and other silicate minerals. Some poorly soluble metal hydroxides are actually hydrates as well.