Video: Selecting the Type of Substance Whose Crystal Structure Include Water

Which of the following is a substance whose crystal structure includes water? [A] Hydride [B] Halide [C] Hydroxide [D] Hydrate [E] Hydrocarbon


Video Transcript

Which of the following is a substance whose crystal structure includes water? A) Hydride, B) Halide, C) Hydroxide, D) Hydrate, or E) Hydrocarbon?

The question refers to a substance that includes water in its crystal structure. Water has the chemical symbol H₂O. And the phrase “includes water” suggests that the H₂O is present in its entirety. It’s not broken up into other substances. So we’re not dealing with substances that might have been produced by a reaction with water, like sodium hydroxide. Let’s think of a substance like copper sulfate.

Copper sulfate is naturally a crystal being an ionic solid. A sample of copper sulfate will probably look quite a pale blue, if not white. But the form of copper sulfate most of us are familiar with is very, very bright blue. So what’s the difference? If you use or buy copper sulfate, the chances are it will actually be copper sulfate pentahydrate. This chemical formula says that for each unit of copper sulfate, we have five water molecules. These water molecules aren’t attached to the copper in the same way hydroxide would be. They’re part of the crystal structure. And the key part of the name that tells us that is the word hydrate.

Pentahydrate means five water molecules per unit in the crystal structure. So our answer is D, hydrate. A hydride is a substance containing H⁻ ions. A substance that is a halide, like a metal halide, contains X⁻ ions, where the X is a halogen. For instance, F⁻, Cr⁻, Br⁻, and so on. A substance that is a hydroxide contains hydroxide ions, OH⁻. And a hydrocarbon is a substance that contains only hydrogen and carbon. None of these other types of substance even get close to describing a crystal structure that includes water. So hydrate is definitely our correct answer.

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