Do you know the freezing point of water? Is the freezing point the same as the melting point? Here is a look at the temperature of the freezing point, the factors that affect it, and whether it’s identical to the melting point.
Temperature of Normal Freezing Point of Water
The temperature of the freezing point of water is 32 °F, 0 °C, or 273.15 K. Note, the Kelvin temperature lacks a degree symbol because the Kelvin scale is an absolute temperature scale. This is the temperature at which liquid water undergoes a phase transition to become solid ice at 1 atmosphere of pressure.
Difference Between Freezing Point and Melting Point
The freezing point is the temperature at which a liquid changes into a solid, while the melting point is the temperature at which a solid changes into a liquid. For most practical purposes, the two temperatures are the same. So, the melting point of water is also 32 °F, 0 °C, or 273.15 K.
Sometimes the freezing point of pure water can be much lower than the normal freezing point or melting point. The reason is that it’s easy to supercool water. Supercooled water is water that lacks impurities, air bubbles, or surface defects that allow crystal formation. Very pure water in a smooth container may reach a temperature as low as -40 to -42 °F (-40 °C) before freezing solid!
Factors That Can Change the Freezing Point
If you look at a phase diagram, you’ll see the freezing point temperature depends on pressure. For most substances, reducing pressure below 1 atmosphere lowers the freezing point. However, the opposite occurs with water. Increasing the pressure initially gives a lower freezing point. The reason is that hydrogen bonding between water molecules makes the liquid more dense than the solid and highly stable. A very low pressure, water changes directly from water vapor into ice without ever becoming a liquid.
Impurities also affect the freezing point of water. In nearly all cases, dissolving a substance (e.g., sugar, salt, alcohol) lowers the freezing point. This is called freezing point depression. It is a colligative property of matter, which means it depends on the number of particles added to the water and not the chemical nature of the particles. Scientists at the University of Leeds found an exception to freezing point depression. Ammonium sulfate, a salt, actually raises the freezing point of water.
Particles that don’t dissolve in water, such as dust or pollen, also raise the freezing point of water. The particles act as nucleation points. Basically, they give water molecules an attachment point to start the crystallization process into ice. Ski resorts use this property to make snow at temperatures above freezing.
References
- Atkins, P.W. (2017). Elements of Physical Chemistry. ISBN 978-0-19-879670-1.
- Pedersen, U.R.; et al. (August 2016). “Thermodynamics of freezing and melting”. Nature Communications. 7 (1): 12386. doi:10.1038/ncomms12386
- Zachariassen, K.E.; Kristiansen, E. (December 2000). “Ice nucleation and antinucleation in nature”. Cryobiology. 41 (4): 257–79. doi:10.1006/cryo.2000.228