Water’s transformation from liquid to solid is a common phenomenon. It is widely known that pure water freezes at 32 degrees Fahrenheit (0 degrees Celsius), which serves as a common reference point. The question of whether water can freeze at a slightly warmer temperature, such as 35°F, explores the subtle physics behind this phase change. Understanding the conditions required for water to solidify explains why 35°F is almost always a temperature of safety for liquid water.
The Standard Freezing Point of Pure Water
The temperature at which a liquid turns into a solid is its freezing point. For pure water under standard atmospheric pressure, this point is precisely 32°F (0°C). This temperature is a constant physical property of pure water and acts as the threshold for the phase transition. Standard atmospheric pressure is defined as one atmosphere.
The process of freezing is a thermodynamic event where water molecules lose internal energy, slowing their random motion. As the molecules slow, they begin to arrange themselves into the highly ordered, crystalline structure of ice. For this transition to occur, a specific amount of heat, known as the latent heat of fusion, must be removed from the liquid water.
At exactly 32°F, liquid water and solid ice can coexist in equilibrium, where the rate of freezing equals the rate of melting. A sample of pure water will only begin to solidify once its temperature reaches this exact point and energy is continuously withdrawn. If the surrounding air is also at 32°F, the water will not freeze because there is no temperature difference to drive the necessary heat transfer away from the liquid.
How Solutes Change the Freezing Point
The freezing point of water is affected by dissolved substances, a phenomenon known as freezing point depression. Solutes, such as salt, sugar, or alcohol, interfere with the water molecules’ ability to organize and form the rigid crystalline structure required for ice. These particles dilute the water, making the liquid state more stable, which means it requires a lower temperature to freeze.
Adding a substance to water always lowers its freezing point below 32°F, never raises it. For instance, salt spread on icy roads melts the ice because it forms a brine solution with a freezing point significantly below 0°C. Antifreeze, containing ethylene glycol, is added to car radiators to prevent the water from freezing at typical cold-weather temperatures.
The extent to which the freezing point is lowered depends directly on the concentration of the solute particles, not the type of particle itself. This principle explains why saltwater, such as in the ocean, has a freezing point slightly lower than that of fresh water. Impurities change the freezing temperature by pushing the threshold further away from 35°F, making the liquid state more stable at colder temperatures.
Why 35 Degrees Fahrenheit is Always Liquid
Under nearly all conditions encountered in everyday life, water at 35°F (1.7°C) will remain in its liquid state. This temperature is three degrees above the standard freezing point of pure water, placing it safely outside the range of phase transition. The physical laws governing the change from liquid to solid require the temperature to be at or below 32°F for solidification to begin.
Confusion about freezing at 35°F may stem from phenomena that make a surface feel colder than the surrounding air, such as rapid evaporative cooling. Water can be supercooled, remaining liquid even slightly below 32°F, but this requires highly pure water and a complete absence of nucleation sites. Supercooling is a temporary state below the standard freezing point, not a condition that allows freezing to occur above it.
The presence of dissolved gases or minor impurities in typical tap water might cause a minuscule variation in the exact freezing temperature, but this effect is always depression, meaning it lowers the point. Consequently, water at 35°F contains too much internal energy and is too far above the phase transition threshold to form a stable ice structure. At this temperature, the water molecules are moving too energetically to lock into the orderly arrangement of ice.