A liquid is loosely defined as any substance that is not a gas and that can flow to fit the shape of a container. Liquids are in one of the three primary states of matter, which include the gaseous and solid states. The physical properties of liquids are those that are readily observable or measurable without subjecting the liquid to some type of chemical reaction. The chemical properties are those properties that are only measurable or detectable as a result of a of a chemical reaction. While every substance has unique properties, chemists use certain properties of liquids as tools to classify and identify them and to predict how they will behave under certain conditions and react with other substances.
The physical properties of liquids include a large number of characteristics. Odor and color are two simple examples. Certain characteristics are used only to describe the physical properties of liquids. Viscosity, or thickness, for instance, describes the resistance of a liquid to the tendency to flow. A liquid with high viscosity is very thick, a property that tends to increase as temperature decreases.
Adhesion and cohesion are also physical properties of liquids, which together produce the property known as surface tension. Adhesion is the capacity of a liquid to stick to solids. Cohesion is the tendency of the molecules of a liquid to stick to each other. These properties determine surface tension, which is the force at the surface of the liquid that causes it to act as a film. It is surface tension that allows water form droplets and it is the reason that certain insects can walk the surface of streams and puddles.
Temperature related properties of liquids include boiling point, which is the temperature at which the liquid begins to evaporate or convert to a gas. The freezing point is the temperature at which the liquid begins to turn to a solid. Each pure liquid will have its own specific boiling and freezing points. Density describes the amount of mass that is present in a given volume of the liquid. It is sometimes called specific gravity, which is a measure of the density of a substance compared to pure water.
While there are numerous other physical properties of liquids, only a few others are commonly used in science. Miscibility with water, or the ability to dissolve in solution, can be described as the capacity to dissolve in water or to absorb water, depending on which liquid is present in the greatest amount in a hypothetical mixture. Vapor pressure is the rate at which a liquid evaporates in air. Compressibility is the resistance of a liquid to compression, and expansion and contraction describe a liquid's tendency to increase or decrease in volume with changes in temperature.
The chemical properties of liquids vary greatly from liquid to liquid and are defined as the way a liquid behaves in a chemical reaction. Each liquid has its own set of chemical properties, including pH, ability to conduct electricity, and flammability. Heat of combustion describes the amount of heat given off when a liquid burns. Many other chemical properties can be used to describe liquids as well, including its reactivity with other substances, particularly water.