States of matter and mixtures Study Guide

Paper chromatography is a separation technique used to distinguish between pure and impure substances, and to identify unknown substances. The process involves a stationary phase (the paper) and a mobile phase (a solvent). As the solvent moves up the paper, it carries the components of a sample at different rates, depending on their solubility in the solvent and their adherence to the paper. A pure substance will ideally show only one spot on the chromatogram, whereas an impure substance will yield multiple spots. Unknown substances can be identified by comparing their spots' positions to those of known substances run on the same chromatogram. Additionally, the retention factor (R_f) value, which is the ratio of the distance travelled by the substance to the distance travelled by the solvent front, can be calculated. This value is characteristic for a given substance under specific chromatographic conditions and can be used for identification.

Potable water is water that is safe to drink. Untreated water from sources like rivers, lakes, and groundwater often contains impurities such as suspended solids, microorganisms, and dissolved salts, making it unsafe for consumption. To make waste and groundwater potable, a multi-stage purification process is typically employed, involving sedimentation, filtration, and chlorination. Sedimentation removes larger insoluble particles by allowing them to settle. Filtration then removes smaller suspended solids and some microorganisms. Finally, chlorination kills most remaining bacteria and other pathogens, making the water safe for human consumption. For seawater, which has a very high concentration of dissolved salts, a different method called distillation is often used to make it potable. Distillation involves boiling the water to produce steam and then condensing the steam back into liquid water, leaving the dissolved salts behind. Water used in scientific analysis requires a very high level of purity, often even purer than potable water. This is because dissolved salts or other impurities could interfere with chemical reactions, introduce errors in measurements, or contaminate samples, leading to inaccurate results.