Freezing is a phase change where a substance transitions from a liquid to a solid state due to the removal of thermal energy. The inherent properties of the molecules determine the temperature at which this transformation occurs. While many people associate freezing only with water, the temperature at which substances solidify varies dramatically, spanning thousands of degrees across the physical world. This wide range of freezing points reveals that everything from the air we breathe to the metals in a ring can freeze.
The Physics of Freezing
The mechanism behind freezing is the reduction of molecular motion, which is directly related to a substance’s temperature. As a liquid cools, its molecules lose kinetic energy and slow down, decreasing the forces of collision that keep them apart. When the temperature drops sufficiently, the intermolecular attractive forces begin to dominate the weakened kinetic energy, allowing the molecules to settle into fixed positions.
This settling process results in the formation of a rigid, ordered, three-dimensional structure known as a crystal lattice. The freezing point is the specific temperature at which the liquid and solid phases of a substance can exist simultaneously in equilibrium. External factors like pressure only slightly alter this temperature, typically shifting the freezing temperature by only a few hundredths of a degree.
Freezing Common Liquids
The solidification point of everyday liquids demonstrates how molecular structure influences a substance’s thermal behavior. Water, for instance, freezes at 0°C (32°F) at standard pressure, but uniquely expands upon freezing due to the formation of a hexagonal lattice that spaces the molecules farther apart in the solid phase than in the liquid. This density anomaly is why ice floats.
In contrast, the freezing behavior of household fats and oils is governed by their mixture of fatty acids. Butter, a saturated fat, remains solid at room temperature and melts over a range, typically between 32°C and 35°C (90°F to 95°F), due to the tight packing of its molecules. Olive oil, which is mostly unsaturated, has a much lower solidification point, often beginning to cloud around 10°C (50°F) and fully freezing closer to -6°C (21°F).
Alcohols require significantly colder temperatures to solidify because their molecular structure does not favor the formation of a crystalline lattice near 0°C. For example, ethanol must be cooled to -114°C (-173°F) before it transitions from a liquid to a solid. This difference in freezing points illustrates how small chemical variations drastically change the temperature required for solidification.
Freezing Gases and Metals
Freezing extends beyond common liquids, encompassing both the lightest gases and the densest metals, demonstrating a massive range of temperatures. Gaseous elements must be subjected to temperatures hundreds of degrees below zero to solidify. Nitrogen, which makes up nearly 78% of the air, has a freezing point of approximately -210°C (-346°F), while oxygen freezes at -218.79°C (-361.82°F).
These low temperatures are necessary to overcome the weak intermolecular forces, known as van der Waals forces, that hold these simple molecules together. Once cooled sufficiently, these gases condense into liquids and then solidify, forming pale, ordered crystals. At the opposite end of the temperature spectrum are metals, which also freeze.
Metals like gold and iron have high freezing points because their atoms are bound by strong metallic bonds that require significant thermal energy to break. Pure gold solidifies from a liquid at 1,064°C (1,947°F), while iron requires a temperature of 1,538°C (2,800°F).
Why Some Things Resist Freezing
Some liquids can remain in a liquid state even when cooled below their theoretical freezing point, a phenomenon called supercooling. This occurs when the liquid lacks nucleation sites, which are microscopic irregularities or impurities that act as starting points for crystal formation. Without these initial seeds, the liquid cannot form a stable crystalline lattice and remains supercooled until a sudden disturbance triggers rapid solidification.
Materials like glass resist traditional freezing because they do not form an ordered crystal structure when cooled. Instead of freezing at a sharp temperature, the liquid becomes progressively more viscous until it reaches a glass transition temperature, solidifying into an amorphous solid. In this state, the molecules are frozen in a random, disordered arrangement, behaving as a rigid solid.