Answer:
C. hydration number
Explanation:
When we dissolve an ionic compound (a charged species) the charges can interact with the water molecule. In the case of cations (positive charges) the negative dipole of water (generated in the oxygen) will interact with the positive charge at the same time the anions (negative charges) the positive dipole of water (generated in the hydrogen).
The amount of water molecules that can interact with a single ion (cation or anion) is called hydration number. In the example, we have a hydration number of "4" for the sodium cation.
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