Chemical Analysis: Purity, Chromatography and Gas Tests
Pure Substances and Mixtures
In chemistry, a pure substance consists of a single element or compound. It has a sharp, fixed melting point and boiling point. A mixture contains two or more substances not chemically combined. Mixtures melt over a range of temperatures rather than at a sharp point.
A pure substance has a sharp, specific melting point. An impure substance / mixture melts over a range of temperatures. You can test purity by checking the melting point against the known value.
Chromatography
Chromatography is used to separate and identify substances in a mixture. In paper chromatography, a spot of the mixture is placed on a pencil line on chromatography paper. The paper is placed in a solvent. As the solvent moves up the paper, it carries the dissolved substances with it. Different substances travel different distances depending on their solubility.
The R\(_f\) value is calculated as:
$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$$
Each substance has a unique \(R_f\) value in a given solvent. A pure substance produces a single spot; a mixture produces multiple spots.
You can identify substances by comparing their \(R_f\) values with known reference values, or by running known substances alongside the unknown on the same chromatogram. If a spot in the unknown lines up exactly with a spot from a known substance, they are the same compound.
Tests for Common Gases
You need to know the tests for four gases:
Gas tests: Hydrogen — squeaky pop with a lighted splint. Oxygen — relights a glowing splint. Carbon dioxide — turns limewater milky/cloudy. Chlorine — bleaches damp litmus paper white.
A formulation is a mixture designed as a useful product, where each component is present in a measured quantity to give it specific properties. Examples include medicines (active ingredient + binder + coating), paints (pigment + solvent + binder), and alloys (metals mixed in specific ratios for desired properties).
Formulations are found everywhere in everyday life. Each ingredient has a specific purpose, and changing the proportions changes the properties of the product.