States of Matter

SOLID	LIQUID	GAS

Matter, anything that has mass and takes up space, can be classified by its chemical composition. Mixtures are separable by physical process into substances, and likewise chemical substances can be combined physically into mixtures. Mixtures are homogeneous in some instances, like solutions, and some are heterogeneous like iron filings in dust. Compounds are separable into their elements by chemical reaction, and elements are combinable by chemical reactions. Compounds are usually held together by ionic bonds (where electrons are “borrowed”) or by covalent bonds (where electrons are shared).

Ionic bonds are formed when an element of lower ionization energy (metal) gives up one or more electrons to an element of high electron affinity (non-metal). The resulting cation(s) and anion(s) are held in a solid lattice by coulombic forces of attraction. Covalent bonds are formed when elements of similar electron affinity (non-metals) share one or more electron pairs between bonded nuclei. Usually there is unequal electron sharing and covalent bonded molecules show some polarity. Many organic molecules have polar interactions, which are characteristically weak. Hence, sucrose has strong covalent bonds holding a given sucrose molecule together but weak polar bonds between adjacent molecules. The polar interactions contribute to building solid sugar and these bonds are readily overcome by heat. It is often found that organic solids have lower melting points than ionic solids because polar/polar interactions are weaker than ionic bonds.

A second distinguishing feature for an ionic solid is the ability to form ions when dissolved in water. An ionic solid which dissolves in water to yield a solution which conducts electricity is termed an electrolyte. Sodium chloride is an electrolyte because nearly 100% of solid Na⁺Cl^- will separate in water solution into individual aquated sodium ions and chloride ions, Na⁺ _(aq) and Cl^- _(aq). These ions have the ability to conduct electricity as revealed by conductivity measurements. In comparison, sucrose is a nonelectrolyte because, when dissolved in water, it forms aquated polar molecules (not individual ions) which do not significantly conduct electricity.