When water breaks apart an ionic compound into cations and anions:

Dissociation

Explanation:
The molecule of water is polar. This means that the hydrogen has a partial positive charge while the oxygen has a partial negative charge.
Ionic compounds have equal number of positive and negative charges.
When these compounds are placed in water, the partial positive hydrogen starts to attract the negative ion of the ionic compound while the partial negative oxygen starts to attract the positive ion of the ionic compound.

This process is called dissociation and leads to the breaking of ionic compound into cations and anions in water.

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