The vapor pressure of a chemical is the pressure its vapor exerts in equilibrium with its liquid or solid phase. The vapor pressure’s importance in environmental work results from its effects on the transport and partitioning of chemicals among the environmental compartments (air, water, and soil). The vapor pressure expresses and controls the chemical’s volatility. The volatilization of a chemical from the water surface is determined by its Henry’s law constant (see Chapter 4), which can be estimated from the ratio of a chemical’s vapor pressure to its water solubility. The volatilization of a chemical from the soil surface is determined largely by its vapor pressure, although this is tempered by its sorption to soil solids and its Henry’s law constant between soil, water, and air. A substance’s vapor pres-

sure determines whether it will occur as a free molecule in the atmosphere or will be associated with particulate matter (see Chapter 10).