Water is a colorless, odorless, and tasteless substance which exists naturally as either solid, liquid, or gas. Water is a very stable compound which can withstand extreme temperatures before decomposing into individual atoms. In the universe there are two prime ingredients: matter and energy. All matter can be classified as compounds, elements, or mixtures. An element can be defined as a substance which cannot be decomposed chemically into a simpler substance. To understand water chemistry, it is important to know how atoms are held together to form compounds.
A molecule may consist of one atom or two or more similar atoms. An atom is matter in its simplest form and is inconceivably small. Atoms consist of protons, neutrons, and electrons. Protons are positively charged particles, neutrons have no charge, and electrons are negatively charged and considerably smaller. Protons and electrons determine chemical properties of an atom, while neutrons merely add mass or weight to the atomic unit.
Most atoms have a nucleus formed by neutrons and protons. The number of protons in an atom equals the number of electrons. The number of neutrons equals the mass number minus the atomic number. An atom is electrically neutral. Hydrogen is the smallest and simplest atom, consisting of one proton and one electron orbiting around it. Other atoms have two or more orbiting electrons; the most complex natural element, uranium, has 92.
ELEMENTS AND ISOTOPES
The number of protons in an atom’s nucleus is the atomic number, and determines which element it is. Atomic mass units relate all atoms to each other by mass and were adopted to simplify measurement of the atom’s particles.
Not all atoms of a particular element are identical. Atoms may vary in their atomic weight (the number of protons and neutrons in the nucleus). Since atoms must remain electrically neutral, their atomic weight can change only by adding neutrons. Atoms of the same element, but with different atomic weights are called isotopes. All known elements exist in two or more isotopic forms. Hydrogen, for example, has three natural isotopes: protium, deuterium, and tritium. All three are common in water.
COMPOUNDS AND MIXTURES
A compound is a substance cornposed of two or more chemically bonded atoms or elements. It has a definite and unvarying composition. The chemical properties of a given compound are different from those of its constituent elements. Water, a typical compound, is composed of two elements, hydrogen and oxygen, in definite proportions. It is a uniform substance whether one is considering a drop, a glass, or a lake of it. Thus, a compound is homogenous.
In sharp contrast, a mixture will vary in the amounts of the ingredients it contains. No exact ratio of substances is necessary to constitute a mixture. At the same time, the ingredients in a mixture continue to
maintain their essential properties. A mixture may have varying proportions of its ingredients in different parts of the sample and is therefore heterogeneous.
Inert elements are those which may not combine with others. Their outermost orbits contain eight electrons, except for helium, which has two. These orbits are considered full. Atoms with “unfilled” orbits can form compounds, those with “filled” cannot. Helium and neon are examples of inert elements.
Atoms tend to form stable compounds by shifting their electronic structure. This is called valence bonding. An element’s valance number tells how many of its electrons participate in forming a compound. An atom’s valence electrons are those occurring beyond the last filled orbit (free electrons). These may participate in a chemical reaction. In a compound which is composed of two elements, one element will “give up” a free electron to another element and develop a positive charge. The element accepting an electron will develop a negative charge. These charged atoms are called ions.
toms are held together to form compounds by many types of chemical bonds. When atoms form compounds through electron transfer, they are held together by ionic bonds. Covalent bonding occurs when atoms “share” electrons. For example, oxygen needs two electrons to complete its outermost orbit, while hydrogen needs only one. Water, whose chemical formula is H2O is an example of covalent bonding. Common gases, such as oxygen, hydrogen, and nitrogen, form molecules by sharing electrons.
OXIDATION AND REDUCTION
Chemical reactions occur when atoms combine to form new elements or when elements break down into individual atoms. Originally, the term oxidation signified a chemical reaction involving the addition of oxygen to a compound. Similarly, the removal of oxygen was defined as reduction. Oxidation now means an increase in positive valence, a decrease in negative valence. Reduction, which goes on simultaneously, refers to a decrease in positive valence and an increase in negative valence. The substance that loses electrons is oxidized and is referred to as a reducing agent; the substance that gains electrons is reduced and is called the oxidizing agent. Oxidation and reduction always occur together and in equal amounts.
Water’s chemical nature causes the splitting of many molecules. Certain substances called electrolytes, which dissolve in distilled water, increase its conductivity and form electrically charged particles called ions. An ion can be defined as an electrically charged atom or group of atoms in solution.
Because the solution as a whole is electrically neutral, there are two types of ions, one charged negative
ly, the other positively. Positively charged ions are called cations, negatively charged ions are called anions.
When a molecule dissociates (ionizes) in water, the total charge before and after the reaction must be the same. This condition is called electroneutrality. The sum of the positive charges equals the sum of the negative charges. A reaction not in equilibrium will adjust itself under stress until equilibrium is reestablished. Changing the temperature, pressure or concentration of elements in a compound will upset its equilibrium and set off a chemical reaction. Water treatment uses this principle to change the chemical nature of a particular water supply.
Many impurities exist as ions in natural waters (calcium, magnesium, sodium, iron, and manganese as cations, and bicarbonate, chloride, sulfate, nitrate, and carbonate as anions). These electrically charged dissolved particles make water a good conductor of electricity. Conversely, pure water has a high electrical resistance, which is frequently used as a measure of its purity.
A number of methods have been developed for the reduction of these ionic impurities from water because many of them interfere with the beneficial uses of water. These are: (a) distillation, (b) precipitation and separation, (c) ion exchange, and (d) membrane separation.
ACIDS, BASES, AND SALTS
Briefly, acids can be defined as compounds which release hydrogen ions in solution. All acids contain hydrogen. Bases are substances which can release hydroxide ions. Salts are substances containing both metallic and non-metallic ions, and are classified as normal, acid, and basic.
When equal weights of an acid are combined with a base, the free hydrogen and hydroxide ions will combine to form water until an equilibrium is achieved. This process, called neutralization, is important in water chemistry. Rain, an acid solution, is quickly neutralized as it reaches limestone in soil and bedrock. If rain were not neutralized by bases in the soil and rock, all water obtained from precipitation would be acidic.
Another important characteristic of a solution is its pH, which indicates the relative concentration of hydrogen ions; and that determines whether a substance is acidic or basic. The pH also gives the solution’s relative acidity or alkalinity.
A solution consists of a solvent and a solute. A solvent promotes dissolving; the solute is the substance being dissolved. Water is the universal solvent, and an almost infinite variety of solute substances and concentrations can exist in water. Among the various contaminants to be found are hardness compounds, chlorides, sulfates, fluorides, iron, sodium, silica, and others. Each has unique properties, and will be discussed in further detail.