Fundamentals of Chemistry
INTRODUCTION: THE FUNDAMENTAL PROPOSITION OF CHEMISTRY AND MATTER
Chemistry is based on the premise that all matter is composed of some combination of 92 naturally occurring elements. Of these 92 elements, 11 are gases and two are known liquids at room temperature. Two others are solids but turn liquid at body temperature. The vast majority are solids and metals. The word “combination” is the key concept that gives chemistry its reputation of complexity. It may mean a single element or compound, or mixtures of elements, or mixtures of compounds, or mixtures of elements with compounds. Furthermore, mixtures may be subdivided into homogenous or heterogeneous. And to make matters even more complex, these systems of mixtures, if solutions, may exist as solids, liquid, or gases. Part of the challenge in chemistry, then, is to sort things out, organize, characterize, and identify them.
Chemistry is the branch of physical science that studies matter and the changes or “molecular rearrangements” it can undergo. These rearrangements, conventionally known as chemical reactions, involve the breaking of existing chemical bonds between atoms to form new bonds with other atoms. In the process, different molecules with different properties are formed. Electrons may be gained or lost, and energy changes generally accompany the reaction. Chemical reactions, as distinguished from nuclear reactions, involve the exchange of only electrons—never protons or neutrons. In a sense, chemistry is the recycling of atoms.
Visible evidence of chemical reactions includes the following:
- • Bubbling or fizzing, indicating the release of a gas
- • Color change
- • Temperature change (heat released or absorbed)
- • Formation of a precipitate
- • Emission of light (chemiluminescence) or sound
Examples of common chemical reactions include the following:
- • Rusting of iron or corrosion of any metal
- • Generation of a current by a battery
- • Combustion of fuel to produce energy
- • Neutralization of excess stomach acid by an antacid
- • Hardening of concrete
Chemistry also studies the structure of matter, including chemical and physical properties, correlating properties on the microscopic scale with behavior observed on the macroscopic scale. These can include properties such as vapor pressure, osmotic pressure, solubility, boiling and melting points, and energy and its transformations. Many of these properties may dictate or influence the outcome of a reaction.
There are actually four, conventional, physical states in chemistry: solid (s), liquid (l), gas (g), or aqueous solution (aq). The first three are regarded as pure, consisting of only a single substance. For example, CaCl2 (s) would be pure solid calcium chloride, while CaCh (l) would be molten calcium chloride (at a very high temperature). The fourth physical state, solution, is a homogenous mixture of uniform and constant composition, where a solute and a solvent can be identified and distinguished. [There are gaseous solutions, like air (oxygen dissolved in nitrogen), liquid solutions, like salt water (sodium chloride dissolved in water), and solid solutions, like gemstones (iron or chromium atoms regularly spaced in an aluminum oxide crystal).] Often referred to as the universal solvent, water is a ubiquitous substance on Earth and in the human body, and is the basis of all known life. Water-based (or aqueous) solutions are given the symbol (aq) inserted immediately after the chemical formula. Thus, an aqueous solution of calcium chloride would be written as CaCl2 (aq), indicating that a given amount of solid calcium chloride was dissolved in a sufficient volume of water to make an aqueous solution. Note that water alone is a solvent, not a solution. The presence of the symbol (aq) always indicates a solute (usually a solid, but may also be another liquid or gas) dissolved in water, the solvent, to form a solution.