Chemical vs. Physical Properties
Everything that has mass and volume is made of matter. Physical and chemical properties are both used to describe matter. Chemical properties are those that can only be measured by attempting to change the chemical identity of a material through a chemical reaction. Physical properties are those that can be measured without having to change a material’s chemical identity. Physical properties are dependent upon the chemical structures and features of substances, i.e., how their molecules and/or atoms are arranged in space and how much energy is available to them. Physical properties include: magnetic attraction, density, boiling point, freezing point, melting point, solubility, color, and odor. Chemical properties include: flammability, pH, and reactivity with water.
Some properties depend upon how much material is present in a sample, while other properties do not. Intrinsic properties are not dependent upon how much material is present. Melting point, boiling point, density, odor, and color are all considered intrinsic properties. Extrinsic properties do depend on the size of a sample. For example, mass, volume, and heat content are all considered extrinsic properties. Forensic scientists measure both types of properties, but intrinsic properties are most useful in terms of identifying substances at a crime scene.7
Atoms, Compounds, and Mixtures
All matter can be broken down into atoms, which are the simplest units of chemical importance. Elements are composed of all atoms with the same atomic number (number of protons) and are the smallest units that still retain their characteristic chemical properties. Elements cannot be broken down by normal chemical means, so they are considered the most basic building blocks for creating compounds, or molecules. The periodic table organizes the 118 known elements by increasing atomic number and bears its name because the elements in the vertical columns share many physical and chemical properties—which is to say that the properties repeat periodically.
Atoms combine to form more complex units, called molecules. Two or more atoms can combine in ways that form compounds with very different properties from those of its individual atoms. For example, sodium chloride (NaCl), otherwise known as table salt, is formed from the combination of chlorine (Cl2), a toxic gas, and sodium (Na), a highly reactive metal. Furthermore, the specific arrangements and interconnections between atoms within a molecule determine its properties. The same atoms can be rearranged in various ways to form a number of molecules that each have distinct physical and chemical properties. When the same atoms are arranged in different ways, they produce new substances called isomers. For example, ethanol (See Figure 1.) and dimethyl ether (See Figure 2.) are two isomers with the molecular formula C2H6O but they have very different chemical and physical properties. Ethanol is found in alcoholic beverages; it has a boiling point of 78°C and it is soluble in water. Dimethyl ether, on the other hand, has a boiling point of -23.6°C and is only partly soluble in water.
Figure 1. Ball-and-stick model of ethanol (C2H6O)
Retrieved: https://commons.wikimedia.org/wiki/File:Ethanol-3D-balls.png
Figure 2. Ball-and-stick model of dimethyl ether (C2H6O)
Retrieved: https://en.wikipedia.org/wiki/Dimethyl_ether#/media/File:Dimethyl-ether-3D-balls.png
Mixtures occur when two or more compounds are mixed together but retain their own individual properties and do not chemically react. Most of what we encounter in the world, besides water, is a mixture of substances rather than pure elements or compounds. hom*ogenous mixtures, such as air, appear uniform throughout a sample. Individual components of the mixture cannot be distinguished physically, even though they still retain their individual identities. Heterogeneous mixtures are non-uniform throughout a sample. For example, a mixture of iron filings, sand, and salt is considered heterogeneous, as we can still see the individual components.7
Separating Mixtures
There are various methods for separating mixtures into their individual compounds, and they are generally divided into physical and chemical separation methods. Physical separation methods utilize the differences in physical (intrinsic) properties of each of the different components in the mixture. Individual components of a mixture retain their individual properties, so these differences—including density, boiling point, freezing point, and magnetic attraction—can be used to cause a separation.
Density is defined as the mass of material per unit of volume. If two substances have different densities, they can be separable using a liquid of intermediate density. For example, if a substance in a mixture has a density greater than that of the liquid, it will sink, and if the other substance in the mixture has a density less than that of the liquid, it will float. This method is commonly used to separate mineral and biological samples found at crime scenes, including the separation of blood cells from serum and the separation of glass samples.
Differences in solubility can also be used to separate mixtures. If one component of a mixture is soluble in a liquid while the other is not, then when that liquid is added to the mixture it will cause one to dissolve and leave the other behind. The liquid is filtered to remove the insoluble component of the mixture. The remaining liquid can be evaporated to leave behind the soluble component.
Boiling point can be used to separate a mixture of liquids. As a liquid mixture is slowly heated, the components of the liquid boil and change from liquid to gas when their unique boiling point is reached. The vapor is collected and condensed to obtain the pure liquid sample. Melting point is similarly used, except the temperature of the liquid mixture is lowered. As the temperature becomes low enough, one component may crystallize out as a solid.
Chromatography is the most powerful and commonly used separation method, and it is central in forensic chemistry. Chromatography is based upon how two substances interact with each other; substances can be attracted, repelled, or they can display an intermediate of the two. In paper chromatography, molecules that have a strong interaction with the paper bind tightly and move only slightly when the edge of the paper is placed in a solvent. Molecules that have a weak interaction are easily displaced and move rapidly across the paper in a solvent. The paper in this example is known as the stationary phase, while the solvent containing the mixture of substances is called the mobile phase. In gas chromatography, the components of a mixture are pushed through a tube (stationary phase) in a gas (mobile phase); because each component of the mixture interacts differently with the material inside the tube, they are separated similarly to paper chromatography.7
