Re: why are some salts dry when they actually contain water e.g. barium chloride?

Good question!

Many solids, including most salts (such as barium chloride) are found as crystals. This means that the atoms of the salt are arranged together in a regular, repeating structure that is more stable than just randomly connecting the pieces together. In these crystals there are regularly spaced "gaps", too small to see, but large enough and shaped just right to fit a certain number of water molecules.

Imagine it as stacking marbles together: the round marbles have spaces in between them, no matter how tightly packed they are. If you had a smaller size marble, it could fit into those gaps without disrupting the way the original marbles are stacked.

So if you have a "dry" crystal (anhydrous), it has no water present in those gaps. Crystals that DO have water in them (hydrous) can hold only a certain amount of water in the spaces available. Using your example, for every one molecule of barium chloride in the crystal (BaCl2), there is enough space to bind 2 water molecules of water (barium and chlorine atoms are both bigger than the hydrogen and oxygen atoms that make up water [barium is a LOT bigger], so it makes sense that there are gaps present that are big enough to fit water into).

If you only have less water present than the crystal can hold, it is "partially hydrated". This water is chemically bound to the crystals (weakly, but it IS bound). It will not feel "wet" because the water is part of the crystal and unavailable in liquid form. This is the case even when the crystal is fully hydrated (2 water molecules for every 1 barium chloride, for example).

If more water is present than the crystals are capable of holding, then the water can start to act as a solvent and begin to dissolve the crystals. Even if only a little extra water is present, it can be in liquid form and will thus be "wet".

Salt crystals that can bind water have a strong enough attraction for water that they will actually bind water from the air! If you have anhydrous crystals, you have to store them in a zero-humidity container.

The water in most hydrous crystals can be removed by heating gently. The temperature at which the water vaporizes is often lower than the melting point of the crystal (using barium chloride as the example again, its melting point is 960 degrees Celsius, but it loses all water at 120 degrees Celsius).

I hope this answers your questions!

-Jason

References:

Practical information on barium chloride: http://www.luminet.net/~wenonah/hydro/bacl.htm

Material safety data sheet on barium chloride (see section 9 for melting point) http://www.jtbaker.com/msds/englishhtml/b0372.htm

Information on crystal structure: Any introductory chemistry text - but I use Chemistry: 6th Edition by Raymond Chang (McGraw-Hill publishing, 1998) There is also some further information regarding hydrated salts from a previous MadSci question: 831499414.Ch