Elements, compounds and mixtures Study Guide

Chromatography is a powerful laboratory technique used to separate mixtures. It relies on the principle that different substances in a mixture will have varying affinities for two phases: a stationary phase and a mobile phase. In both paper chromatography and thin layer chromatography (TLC), separation occurs because substances travel across the stationary phase at different speeds. The stationary phase does not move, while the mobile phase flows through it, carrying the components of the mixture. Substances that are more soluble in the mobile phase and less adsorbed by the stationary phase will travel further and faster. Conversely, substances that are less soluble in the mobile phase and more adsorbed by the stationary phase will move slower and not travel as far. The result is that the components of the mixture separate into distinct spots or bands on the stationary phase, allowing for their identification or isolation. A key measure in chromatography is the Rf value, which is the ratio of the distance travelled by the spot to the distance travelled by the solvent front, providing a quantitative way to compare substances.

Metals and non-metals are two fundamental categories of elements, each possessing distinct physical and chemical properties that dictate their uses and behaviours. Physically, metals are generally lustrous (shiny), malleable (can be hammered into shapes), ductile (can be drawn into wires), and are good conductors of both heat and electricity. They typically have high melting and boiling points and high densities. In contrast, non-metals tend to be dull, brittle (break easily when hammered), are poor conductors of heat and electricity (insulators, with graphite as a notable exception), and often have low melting and boiling points and low densities. Chemically, metals tend to lose electrons to form positive ions (cations) in chemical reactions, especially when reacting with non-metals to form ionic compounds. They readily react with oxygen to form basic oxides. Non-metals, conversely, tend to gain or share electrons. When reacting with metals, they gain electrons to form negative ions (anions). They react with oxygen to form acidic or neutral oxides. Understanding these characteristic differences is crucial for predicting how elements will behave and for selecting appropriate materials for various applications.