 Water can be hot or cold; these extremes of temperature can be neutralized by mixing hot and cold, to even out the temperature. In a similar manner, acidic and basic are two ways to describe chemicals. Mixing acids and bases can cancel out their extreme effects; a substance that is neither acidic nor basic is neutral. The pH scale measures how acidic or basic a substance is. pH can range from 0 to 14. Each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times more acidic than a pH of 6. The same holds true for pH values above 7, each of which is ten times more basic than the next lower whole value. For example, a pH of 9 is ten times more basic than a pH of 8. Pure water is neutral, with a pH of 7.0. When chemicals are mixed with water, the mixture can become either acidic or basic. Vinegar and lemon juice are acidic substances, while laundry detergents and ammonia are basic. (Another word for basic is 'alkaline')  Acids, like vinegar (a dilute solution of acetic acid), taste sour. Bases, or alkaline substances, have a bitter taste and slippery feel. Both strong acids and strong bases share one feature: they are 'reactive'. These chemicals can cause severe burns. Automobile battery acid, for example, is a strongly acidic chemical that is reactive. Household drain cleaners, on the other hand, often contain lye, a very alkaline chemical that is also reactive. Certain chemicals have the special property of appearing in different colours depending on the pH of the solution they are in. Such chemicals are known as acid-base indicators, or simply 'indicators', because when a few drops of indicator are added to a solution, the colour of the solution indicates its pH. Some substances enable solutions to resist pH changes when an acid or base is added. Such substances are called buffers. Buffers are very important in helping organisms maintain a constant pH. All living things are water-based, which means that they must maintain liquids within themselves at a precise pH level. Your blood, for example, must be maintained at a pH of about 7.4 for you to survive. Similarly, enzymes, which are proteins that act as catalysts for important biological reactions, will only work within a certain pH range. The electromagnetic forces from a concentration of positively charged hydrogen ions in the tissues (its acidity) affects the shape of proteins, pushing and pulling on these large molecules; a protein's shape determines its function. Changing the shape of a protein molecule will make it do something different! Acid rain is rainwater with a lower than normal pH value. Strangely, even 'normal' rain is slightly acidic, with a pH of about 5.6 This is because it contains dissolved carbon dioxide; normal rainwater is actually weak carbonic acid. Acid rain then is rain which has a pH less than 5.6. The rain becomes more acidic when water molecules react with other gases in the air, usually sulphur dioxide and several forms of nitrogen oxides. These gases are created by industrial factories, coal-fired power plants, and vehicle emissions. So acid rain is actually a mixture of weak carbonic, sulphuric, and nitric acids. Also, areas where the soil has been disturbed (mines, forest fires, landslides, or construction sites) may be making the water in streams and lakes more acidic, as rainwater and snowmelt react with acidic minerals in the ground. A more detailed look at pH:  A water molecule has three parts: two hydrogen nuclei joined to an oxygen nucleus. Electrons surround the molecule, which has no overall charge. Water can ionize other substances. It can also ionize itself, turning itself into an acid or base. What happens when a water molecule is pulled apart?
When an acid is poured into water, it gives up H+ ions (hydrogen) to the water. When a base is poured into water, it gives up OH- ions (hydroxide) to the water. Here is a description of what happens when hydrochloric acid (HCl) is mixed with water: HCl is a strong acid, and is very soluble in water. It dissociates into its component ions like this:   The hydrogen ion that is produced interacts strongly with the oxygen of a water molecule. The resulting ion, H3O+ is called a
hydronium ion (shown at right). Substances which give up a proton easily when dissolved in water are strong acids. Alternately, substances which don't easily give up hydrogen ions in water are called weak acids.(Note that 'strong' and 'weak' here do not describe the concentration of the acid in water ... it is possible to have a dilute (weak) solution of a strong acid, or vice versa). Similarly, strong bases dissociate almost completely in water. The most common soluble strong bases are the hydroxides of the alkali metals and alkaline earth metals. Some examples are NaOH, KOH, and Ca(OH)2. Like acids, not all bases dissociate completely; if only a fraction of the molecules react , the substance is called a weak base. H2O can act as both a proton donor and acceptor for itself. A proton can be transferred from one water molecule to another, resulting in the formation of one hyroxide ion (OH-) and one hydronium ion (H3O+). This is called the autoionization or dissociation of water. This equilibrium can be expressed as:  Water acts as both an acid and a base. The ability of a substance to act as either is known as amphoterism. |
A hydrogen ion is a single hydrogen nucleus with a positive charge (it is missing one electron). Acids are substances which ionize in water, producing a surplus of hydrogen ions. The more of these ions that are produced, the more acidic the solution, and the lower the pH.
A hydroxide ion is the part of the water molecule that is left when a hydrogen ion is removed. Hydroxide ions have an extra electron, and so are negatively charged. Bases are substances which increase the numbers of hydroxide ions, and thus remove hydrogen ions. By reducing the concentration of hydrogen atoms, bases raise the pH.