Compounds often, but not always, appear greatly different from any of the elements that make them up. At room temperature and pressure, both hydrogen and oxygen are gases. But water under the same conditions is a liquid. If it gets a few tens of degrees colder, water turns solid at standard pressure. If it gets hot enough, water becomes a gas, odorless and colorless, just like hydrogen or oxygen. Another common example of a compound is rust. This forms when iron joins with | 10 Basic physical concepts A compound is different than a simple mixture of elements. If hydrogen and oxygen are mixed the result is a colorless odorless gas just like either element is a gas separately. A spark however will cause the molecules to join together this will liberate energy in the form of light and heat. Under the right conditions there will be a violent explosion because the two elements join eagerly. Water is chemically illustrated in Fig. 1-3. 1-3 Simplified diagram of a water molecule. Compounds often but not always appear greatly different from any of the elements that make them up. At room temperature and pressure both hydrogen and oxygen are gases. But water under the same conditions is a liquid. If it gets a few tens of degrees colder water turns solid at standard pressure. If it gets hot enough water becomes a gas odorless and colorless just like hydrogen or oxygen. Another common example of a compound is rust. This forms when iron joins with oxygen. While iron is a dull gray solid and oxygen is a gas rust is a maroon-red or brownish powder completely unlike either of the elements from which it is formed. Molecules When atoms of elements join together to form a compound the resulting particles are molecules. Figure 1-3 is an example of a molecule of water consisting of three atoms put together. The natural form of an element is also known as its molecule. Oxygen tends to occur in pairs most of the time in the earth s atmosphere. Thus an oxygen molecule is sometimes denoted by the symbol O2. The O represents oxygen and the subscript 2 indicates that there are two atoms per molecule. The water molecule is symbolized H2O because there are two atoms of hydrogen and one atom of oxygen in each molecule. Insulators 11 Sometimes oxygen atoms are by themselves then we denote the molecule simply as O. Sometimes there are three atoms of oxygen grouped together. This is the gas called ozone that has received much attention lately in environmental news. It
