A characteristic property is a chemical or physical property that can be used to identify a substance. Mass and volume cannot be used to identify a substance because they vary depending on how much of the substance you have, while density can be used to identify a substance no matter how much of the substance you have. In general, characteristic properties refer to “pure” substances. A pure substance is a substance that is made up of only one type of molecule, such as water (H20), isopropyl alcohol (C3H8O), carbon dioxide (CO2), or gold (Au). Pure substances have a specific chemical composition. Mixtures, on the other hand, do not have specific chemical compositions. The amount of sodium (Na) in salt water can vary depending on the concentration of the salt (NaCl) in the salt water. And as you have seen, the density of salt water depends on the concentration of the salt.
Other characteristic properties include the melting point, boiling point, heats of fusion and vaporization, heat capacity, magnetism, electrical conductivity, and solubility of a substance. (You will learn much more about solubility later in this unit.) In addition to identifying a substance, characteristic properties can also be used to separate mixtures. For example, a mixture of water and oil can be separated because water and oil have different densities, and a mixture of water and alcohol can be separated because water and alcohol have different boiling points. Centrifugation and sedimentation use differences in density to separate substances; distillation uses differences in boiling point; magnetic separation uses differences in magnetism; and leaching and precipitation use differences in solubility. A common laboratory technique used to separate substances is known as chromatography. One type of chromatography uses the difference in polarity to separate more polar molecules from less polar molecules. (The “polarity” of a molecule refers to how much the molecule behaves like a small magnet. Water is a very polar molecule. Polarity is extremely important in biology, and you will learn much more about it later in this unit.) Because fresh water is in such short supply around the world, there is a tremendous amount of research in finding cost effective ways to separate salt from salt water (turning seawater into fresh water).
Melting and boiling points are easy concepts for us to understand when it comes to water. We are familiar with water in all three states (solid, liquid, and gas) and have seen water transition from one state to another. However, we tend to think of many other pure substances as existing in only one state since that is the only state we encounter it in. Aluminum is a solid, carbon dioxide is a gas, and ethanol is a liquid, right? Well, yes and no. All three of those substances can exist in all three states depending on the temperature and pressure.
|substance||density (g/cm3)*||melting point (°C)**||boiling point (°C)**|
|carbon dioxide (CO2)||00.0020||00-78-||00-57-|
|isopropyl alcohol (C3H8O)||00.7860||00-89-||0083|
|sodium chloride (NaCl)||02.1650||0801||1413|
|silicon dioxide (SiO2)||02.6480||1650||2230|
Liquid helium is used in cryogenic applications, such as cooling superconductor magnets. Liquid oxygen is a common propellant in rocketry. Dry ice is solid carbon dioxide; it is used to refrigerate portable coolers, carbonate beverages, and make ice cream, and in fog machines. Propane is a gas that is often used as a fuel for barbecue grills and portable stoves; it liquefies under moderate pressure and can be stored easily in steel cylinders as a liquid. Acetone is a solvent and the primary component in cleaning agents such as nail polish remover and some paint thinners. Glycerin is used as a sweetener and thickener in foods, and in many personal care products such as soap. Sodium chloride is table salt. And silicon dioxide (or silica) is a common mineral found in sand and quartz, and used in the production of glass.