Why does sodium chloride solution freeze at a lower temperature than water?

[ahtutor]

Why does sodium chloride solution freeze at a lower temperature than water?

[ahtutor]

Upon adding a non-volatile solute like NaCl to water, the vapor pressure is lowered. The vapor pressure of this solution becomes equal to that of pure solid solvent which is the freezing point of the solution occurs at a lower temperature. Thus freezing point of NaCl solution is lower than that of pure solvent.

[CHLOEE KONSAM]

Sodium chloride (NaCl) solution freezes at a lower temperature than pure water due to a phenomenon known as freezing point depression, which is a colligative property. This property depends on the concentration of solute particles in the solution.

In a pure water molecule, the water molecules are held together by hydrogen bonds. When the temperature decreases, these hydrogen bonds become stronger, making it more difficult for the water molecules to move freely and transition into a solid state (ice). This is why water freezes at 0 degrees Celsius (32 degrees Fahrenheit) under normal atmospheric pressure.

When you add sodium chloride (table salt) to water, it dissociates into sodium ions (Na⁺) and chloride ions (Cl⁻) in the solution. These ions disrupt the hydrogen bonding between water molecules. The positively charged sodium ions are attracted to the negatively charged oxygen atoms in water molecules, and the negatively charged chloride ions are attracted to the positively charged hydrogen atoms in water molecules. This interference weakens the hydrogen bonding network in the solution, making it easier for water molecules to move and form solid ice.

As a result, you need to lower the temperature below 0 degrees Celsius (32 degrees Fahrenheit) to freeze a sodium chloride solution because the disrupted hydrogen bonding network requires more thermal energy (heat) to transition from a liquid to a solid compared to pure water.

The extent of freezing point depression depends on the concentration of the solute particles. The more solute particles you add to the solution, the greater the depression of the freezing point. This phenomenon has practical applications, such as in antifreeze solutions for cars, where substances like ethylene glycol are added to lower the freezing point of water, preventing engine coolant from freezing in cold temperatures.

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