Freezing is a physical process where a liquid transforms into a solid state due to a decrease in temperature. This change involves molecules in the liquid losing kinetic energy, which causes them to slow down. As their movement diminishes, attractive forces between them allow the molecules to arrange themselves into fixed, ordered positions, forming a solid structure.
Everyday Items That Freeze
Many common household liquids and food items undergo freezing, each at its specific temperature. For instance, milk and soda have slightly lower freezing points than pure water, often due to dissolved solutes. Fruit juices, which contain sugars and other compounds, also freeze at temperatures below water’s 0°C (32°F) freezing point. Even certain alcoholic beverages can freeze, though their higher alcohol content significantly lowers their freezing points.
Other substances like corn oil freeze at much lower temperatures, approximately -20 °C (-4 °F). While many foods can be frozen for preservation, some, particularly those with high water content and delicate cellular structures, do not fare well. Items like lettuce, celery, and raw tomatoes become limp and mushy after thawing because their cell walls rupture during the freezing process. Dairy products such as cream cheese and yogurt can be frozen, but their texture may change, becoming less smooth.
The Science of Phase Change
Each substance has a characteristic freezing point, the temperature at which this phase change occurs under typical atmospheric pressure. The formation of a solid from a liquid involves nucleation, where initial clusters of molecules form a periodic structure, followed by crystal growth as more molecules attach to these nuclei. This process is exothermic, meaning that heat is released as the liquid transforms into a solid, even though the substance’s temperature may not rise.
Consequences of Freezing
For water, a notable consequence is expansion. When water freezes, it increases in volume by about 9%, which is an unusual property compared to most substances that contract upon solidification. This expansion can exert considerable force, leading to practical problems such as burst pipes or cracked containers if liquids are frozen in sealed vessels. The expansion of water upon freezing also results in ice being less dense than liquid water, explaining why ice floats.
For food, freezing can drastically alter texture and quality. The water within food cells forms ice crystals that can puncture and damage cell walls. This cellular damage leads to a loss of turgor and firmness, causing fruits and vegetables to become mushy or waterlogged after thawing. While freezing preserves food by inhibiting microbial growth, the physical changes at the cellular level can impact the overall palatability and visual appeal of thawed items.
Substances That Resist Freezing
Some substances inherently resist freezing, or are engineered to do so, by altering their molecular behavior at low temperatures. Antifreeze, commonly used in car radiators, exemplifies this principle. These solutions, often containing glycols like ethylene glycol or propylene glycol, lower the freezing point of water. They work by interfering with the attractive forces between water molecules, preventing them from forming the rigid crystalline structures characteristic of ice.
The addition of dissolved substances, or solutes, to a liquid also lowers its freezing point, a phenomenon known as freezing point depression. Salt, for instance, is widely used to de-ice roads because its ions disrupt the ability of water molecules to bond together into ice, requiring a much colder temperature for freezing to occur. The more particles a solute dissociates into, the greater its effect on lowering the freezing point. Certain organisms also produce biological antifreezes, such as antifreeze proteins, which bind to small ice crystals and inhibit their growth, allowing the organisms to survive in sub-zero environments.