Why is the freezing point of seawater lower than that of pure water?

The freezing point of seawater is lower than that of pure water primarily due to the presence of dissolved solutes, primarily salts such as sodium chloride. When these solutes are added to water, they disrupt the formation of the structured ice crystals that occur when water freezes. This phenomenon is known as freezing point depression, which occurs because the dissolved particles interfere with the ability of water molecules to arrange themselves into a solid crystalline structure.

In pure water, molecules can easily form hydrogen bonds and create an organized ice lattice at 0 degrees Celsius. However, with the addition of salts or other solutes, the system becomes more complex. The dissolved ions take up space and reduce the number of water molecules that can align correctly to form ice. As a result, the freezing point of seawater is lowered below that of pure water, which has a freezing point of 0 degrees Celsius.

The presence of solutes thus has a significant impact, making it the key factor that leads to the freezing point depression observed in seawater.