Water is a polar molecule (Figure 1). This means that one part of the molecule has a slightly positive charge (positive pole) and the other part has a slightly negative charge (negative pole).

Figure 1: Water is a polar molecule

It is the polar nature of water that allows ionic compounds to dissolve in it. In the case of sodium chloride (NaCl) for example, the positive sodium ions (Na⁺) will be attracted to the negative pole of the water molecule, while the negative chloride ions (Cl^--) will be attracted to the positive pole of the water molecule. In the process, the ionic bonds between the sodium and chloride ions are weakened and the water molecules are able to work their way between the individual ions, surrounding them and slowly dissolving the compound. This process is called dissociation. A simplified representation of this is shown in Figure 2.

Definition 1: Dissociation

Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller molecules or ions, usually in a reversible manner.

Figure 2: Sodium chloride dissolves in water

The dissolution of sodium chloride can be represented by the following equation:

NaCl(s) →→ Na⁺(aq) + Cl^--(aq)

The symbols s (solid), l (liquid), g (gas) and aq (material is dissolved in water) are written after the chemical formula to show the state or phase of the material. The dissolution of potassium sulphate into potassium and sulphate ions is shown below as another example:

K₂SO₄(s) →→ 2K⁺(aq) + SO₄^2--(aq)

Remember that molecular substances (e.g. covalent compounds) may also dissolve, but most will not form ions. One example is sugar.

C₆H₁₂O₆(s) ⇌⇌ C₆H₁₂O₆(aq)

There are exceptions to this and some molecular substances will form ions when they dissolve. Hydrogen chloride for example can ionise to form hydrogen and chloride ions.

HCl(g) →→ H⁺(aq) + Cl^--(aq)

Definition 2: Water hardness

Water hardness is a measure of the mineral content of water. Minerals are substances such as calcite, quartz and mica that occur naturally as a result of geological processes.

Hard water is water that has a high mineral content. Water that has a low mineral content is known as soft water. If water has a high mineral content, it usually contains high levels of metal ions, mainly calcium (Ca) and magnesium (Mg). The calcium enters the water from either CaCO₃ (limestone or chalk) or from mineral deposits of CaSO₄. The main source of magnesium is a sedimentary rock called dolomite, CaMg(CO₃)₂. Hard water may also contain other metals as well as bicarbonates and sulphates.

A water softener works on the principle of ion exchange. Hard water passes through a media bed, usually made of resin beads that are supersaturated with sodium. As the water passes through the beads, the hardness minerals (e.g. calcium and magnesium) attach themselves to the beads. The sodium that was originally on the beads is released into the water. When the resin becomes saturated with calcium and magnesium, it must be recharged. A salt solution is passed through the resin. The sodium replaces the calcium and magnesium and these ions are released into the waste water and discharged.

The concentration of specific ions in solution affects whether the solution is acidic or basic. You will learn about acids and bases in Grade 11. Acids and bases can be described as substances that either increase or decrease the concentration of hydrogen (H⁺ or H₃O⁺) ions in a solution. An acid increases the hydrogen ion concentration in a solution, while a base decreases the hydrogen ion concentration. pH is used to measure whether a substance is acidic or basic (alkaline).

Definition 3: pH

pH is a measure of the acidity or alkalinity of a solution. The pH scale ranges from 0 to 14. Solutions with a pH less than seven are acidic, while those with a pH greater than seven are basic (alkaline). pH 7 is considered neutral.

pH can be calculated using the following equation:

or

The brackets in the above equation are used to show concentration in mol··dm-3-3.

Understanding pH is very important. In living organisms, it is necessary to maintain a constant pH so that chemical reactions can occur under optimal conditions.

In agriculture, farmers need to know the pH of their soils so that they are able to plant the right kinds of crops. The pH of soils can vary depending on a number of factors such as rainwater, the kinds of rocks and materials from which the soil was formed and also human influences such as pollution and fertilisers. The pH of rain water can also vary and this too has an effect on agriculture, buildings, water courses, animals and plants. Rainwater is naturally acidic because carbon dioxide in the atmosphere combines with water to form carbonic acid. Unpolluted rainwater has a pH of approximately 5,6. However, human activities can alter the acidity of rain and this can cause serious problems such as acid rain.

The following simulation allows you to test the pH of various substances.

Figure 3

The acidity of rainwater comes from the natural presence of three substances (CO₂, NO, and SO₂) in the lowest layer of the atmosphere. These gases are able to dissolve in water and therefore make rain more acidic than it would otherwise be. Of these gases, carbon dioxide (CO₂) has the highest concentration and therefore contributes the most to the natural acidity of rainwater. We will look at each of these gases in turn.

Definition 4: Acid rain

Acid rain refers to the deposition of acidic components in rain, snow and dew. Acid rain occurs when sulphur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, mist, dry dust, hail, or sleet. This increases the acidity of the soil and affects the chemical balance of lakes and streams.

Although these reactions do take place naturally, human activities can greatly increase the concentration of these gases in the atmosphere, so that rain becomes far more acidic than it would otherwise be. The burning of fossil fuels in industries, vehicles etc is one of the biggest culprits. If the acidity of the rain drops to below 5, it is referred to as acid rain.

Acid rain can have a very damaging effect on the environment. In rivers, dams and lakes, increased acidity can mean that some species of animals and plants will not survive. Acid rain can also degrade soil minerals, producing metal ions that are washed into water systems. Some of these ions may be toxic e.g. Al³⁺. From an economic perspective, altered soil pH can drastically affect agricultural productivity.

Acid rain can also affect buildings and monuments, many of which are made from marble and limestone. A chemical reaction takes place between CaCO₃ (limestone) and sulphuric acid to produce aqueous ions which can be easily washed away. The same reaction can occur in the lithosphere where limestone rocks are present e.g. limestone caves can be eroded by acidic rainwater.

H₂SO₄ + CaCO₃ →→ CaSO₄ .. H₂O + CO₂