When ionic compounds dissolve in water they go through a process called dissociation, splitting into the ions that make them up. However, when you place covalent compounds in water, they typically do not dissolve but form a layer on top of the water.
Explanation: To dissolve an ionic compound, the water molecules must be able to stabilize the ions that result from breaking the ionic bond. They do this by hydrating the ions. When you place an ionic substance in water, the water molecules attract the positive and negative ions from the crystal.
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