Why Does Blue Copper Sulfate Turn White When You Heat It? The Water Leaves
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Some crystals contain water locked into their structure in fixed proportions, and that water is part of the compound rather than dampness clinging to it.
What is actually there
In these substances water molecules occupy definite positions within the crystal structure, usually arranged around a metal ion or filling regular gaps in the lattice. The number is fixed and characteristic, so a given compound takes five water molecules per unit or two or ten rather than an arbitrary amount, which is why the formula is written with that number stated. The crystal is perfectly dry to the touch. Removing the water is a chemical change that alters the structure, the colour and the properties, not a matter of drying something off.
How they behave
The presence of structural water produces several distinctive behaviours:
- •Heating drives the water off in stages at definite temperatures
- •Colour frequently changes sharply when it leaves
- •The dried form usually reabsorbs water from damp air
- •Some forms lose water to dry air on their own and crumble
- •Others absorb so much they dissolve in the water they collect
- •The change is reversible in most cases
The classroom demonstration
Copper sulfate is the standard example because the change is unmissable. The ordinary crystals are a deep blue, and the colour comes from water molecules arranged around each copper ion, four in a square and one more held between the sulfate groups. Heating drives them off and leaves a pale grey white powder, which is the same compound without its water. Adding a drop of water to that powder restores the blue instantly and releases noticeable heat. The reaction is used as a test for the presence of water precisely because it is so visible and so specific.
The words for the behaviours
Three related terms describe what these substances do in air and they are easy to mix up. A substance that loses its structural water spontaneously to dry air, crumbling to powder in the process, is described as effloresced, and washing soda left in a warm room does exactly that. A substance that absorbs water from the air is hygroscopic, which is why some powders cake. A substance that absorbs so much that it dissolves in the water it has collected is deliquescent, and calcium chloride left open will eventually form a puddle of its own solution.
Where it matters industrially
Water held in a structure is doing quiet work in several large industries. Plaster is made by heating gypsum to remove most of its water, and adding water back causes it to set as interlocking crystals reform, which is why a plaster cast warms slightly as it hardens. Cement sets by a related process in which water is incorporated into new crystal structures rather than evaporating, so concrete cures underwater. Drying agents in packaging are substances that absorb water into a structure. And a fire-resistant building board works partly because the water in its crystals absorbs heat as it is driven off.
The takeaway
Water molecules sitting at definite positions in a crystal, in fixed proportions, are part of the compound rather than dampness on it, and the crystal is dry to the touch. Heating drives them off at definite temperatures, usually changing colour, and adding water back reverses it. Plaster setting and concrete curing are the same chemistry running in the direction of taking water in.