When you add sodium chloride (salt) to icy water, the temperature of the system generally decreases, reaching a value below 0 degrees Celsius. This phenomenon is known as the freezing point depression.
To understand why this happens, let's consider the intermolecular forces at play. In pure water, the molecules are held together by hydrogen bonding, which is a relatively strong intermolecular force. These hydrogen bonds give water its unique properties, including a high boiling point and a melting point of 0 degrees Celsius.
When you introduce sodium chloride into the water, the sodium and chloride ions dissociate from each other due to the polar nature of water molecules. The water molecules surround the individual ions, forming a hydration shell through the attractive forces between the partial positive charges on the hydrogen atoms of water and the negative chloride ions, as well as the partial negative charges on the oxygen atoms of water and the positive sodium ions.
The introduction of the sodium and chloride ions disrupts the hydrogen bonding network between water molecules. As a result, the effective intermolecular forces between water molecules are weakened, reducing the overall strength of the intermolecular forces in the system. We can say that the addition of salt increases the disorder or randomness within the system, leading to an increase in entropy.
Lowering the temperature below 0 degrees Celsius means that the weakened intermolecular forces are not strong enough to hold the water molecules together as a solid. Consequently, the water molecules remain in the liquid state, even at temperatures below their normal freezing point.
In summary, when sodium chloride is added to icy water, the freezing point of the water is depressed due to the disruption of the hydrogen bonding network. The weakened intermolecular forces allow the water to remain in the liquid state at temperatures below 0 degrees Celsius.