Freezing and Crystallization
– Most liquids freeze by crystallization, forming a crystalline solid from a uniform liquid.
– Crystallization is a first-order thermodynamic phase transition.
– The temperature of the system remains nearly equal to the melting point during coexistence of solid and liquid.
– Crystallization involves two major events: nucleation and crystal growth.
– Nucleation is the clustering of molecules on a nanometer scale to form a defined crystal structure.
– Freezing is the process of changing a substance from a liquid to a solid state.
– It occurs when the temperature of the substance reaches its freezing point.
– During freezing, the particles in the substance slow down and arrange themselves into a rigid structure.
– Freezing is an exothermic process, meaning it releases heat.
– Common examples of freezing include the formation of ice cubes and the solidification of water.
Supercooling
– Crystallization of pure liquids usually begins at a lower temperature than the melting point.
– Homogeneous nucleation requires high activation energy, leading to a lower starting temperature for freezing.
– Nucleation can be hindered if the hypothetical nucleus is too small.
– Heterogeneous nucleation can occur in the presence of impurities or nucleators, raising the supercooling point.
– Water can supercool to -40°C before freezing in the absence of nucleators.
Exothermicity
– Freezing is an exothermic process, releasing heat and pressure.
– The temperature of the material does not rise during freezing, except in the case of supercooling.
– Energy is continuously removed from the freezing liquid to sustain the process.
– Enthalpy of fusion is the energy released upon freezing and is equal to the energy required for melting.
– Low-temperature helium is an exception to the general rule of exothermic freezing.
Vitrification
– Glass and glycerol can harden without crystallizing, forming amorphous solids.
– Amorphous materials undergo a gradual change in viscoelastic properties over a range of temperatures.
– Vitrification is a non-equilibrium process and does not qualify as freezing.
– Glass transition temperature is the knee point in the density vs. temperature graph.
– Vitrification does not involve an abrupt phase change between crystalline and liquid states.
Expansion
– Substances expand when heated, a phenomenon known as thermal expansion.
– Different substances have different rates of expansion for the same temperature rise.
– Thermal expansion occurs in all objects and states of matter.
– Expansion is a result of increased molecular motion with higher temperature.
– Expansion is a reversible process that occurs in both solids and liquids.
