Why is the temperature needed to freeze ocean water lower than the temperature needed to freeze the surface of a freshwater lake?
The salt and other materials dissolved in ocean water lowers the freezing point is one reason. The fact that the density of cold water is higher than the density of warmer water affects both but not to the same degree.
The temperature needed to freeze ocean water is lower than the temperature needed to freeze freshwater because of the presence of dissolved salts in seawater. When water freezes, its molecules form a solid crystal lattice structure, which requires energy to break the existing bonds between water molecules and form new ones in the crystal lattice.
In freshwater, the freezing point is commonly around 0 degrees Celsius (32 degrees Fahrenheit), as the water molecules need to lose enough thermal energy to overcome the forces holding them together as a liquid. However, seawater contains various salts, such as sodium chloride, magnesium sulfate, and potassium chloride, which are dissolved in it. These dissolved salts alter the properties of the water and lower its freezing point.
The presence of dissolved salts lowers the freezing point of seawater because the salts disrupt the formation of the crystal lattice structure. The dissolved ions from the salts interfere with the arrangement of water molecules and make it more difficult for the water molecules to come together and form ice. As a result, the freezing point of seawater is lower than that of freshwater.
To find the exact freezing point of seawater, you could use a phase diagram or a table that provides the freezing point as a function of salinity and temperature. These resources take into consideration the concentration of different salts in the water to determine the freezing point. It's important to note that other factors such as pressure and impurities can also affect the freezing point, but the presence of dissolved salts is the primary reason why the freezing point of seawater is lower than that of freshwater.