Investigation : Investigating the hydrosphere

Dissociation in water

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)

Ions and water hardness

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 pH scale

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

Acid rain

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

Electrolytes

An electrolyte is a material that increases the conductivity of water when dissolved in it. Electrolytes can be further divided into strong electrolytes and weak electrolytes.

Definition 6: Electrolyte

An electrolyte is a substance that contains free ions and behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions.

Non-electrolytes

A non-electrolyte is a material that does not increase the conductivity of water when dissolved in it. The substance goes into solution and becomes surrounded by water molecules, so that the molecules of the chemical become separated from each other. However, although the substance does dissolve, it is not changed in any way and no chemical bonds are broken. The change is a physical change. In the oxygen example below, the reaction is shown to be reversible because oxygen is only partially soluble in water and comes out of solution very easily.

Factors that affect the conductivity of water

The conductivity of water is therefore affected by the following factors:

Experiment : Electrical conductivity

Aim:

To investigate the electrical conductivities of different substances and solutions.

Apparatus:

Solid salt (NaCl) crystals; different liquids such as distilled water, tap water, seawater, benzene and alcohol; solutions of salts e.g. NaCl, KBr; a solution of an acid (e.g. HCl) and a solution of a base (e.g. NaOH); torch cells; ammeter; conducting wire, crocodile clips and 2 carbon rods.

Method:

Set up the experiment by connecting the circuit as shown in the diagram below. In the diagram, 'X' represents the substance or solution that you will be testing. When you are using the solid crystals, the crocodile clips can be attached directly to each end of the crystal. When you are using solutions, two carbon rods are placed into the liquid and the clips are attached to each of the rods. In each case, complete the circuit and allow the current to flow for about 30 seconds. Observe whether the ammeter shows a reading.

Figure 4

Results:

Record your observations in a table similar to the one below:

Table 2

Test substance	Ammeter reading

What do you notice? Can you explain these observations?

Remember that for electricity to flow, there needs to be a movement of charged particles e.g. ions. With the solid NaCl crystals, there was no flow of electricity recorded on the ammeter. Although the solid is made up of ions, they are held together very tightly within the crystal lattice and therefore no current will flow. Distilled water, benzene and alcohol also don't conduct a current because they are covalent compounds and therefore do not contain ions.

The ammeter should have recorded a current when the salt solutions and the acid and base solutions were connected in the circuit. In solution, salts dissociate into their ions, so that these are free to move in the solution. Acids and bases behave in a similar way and dissociate to form hydronium and oxonium ions. Look at the following examples:

KBr → K + + Br - KBr → K + + Br -

NaCl → Na + + Cl - NaCl → Na + + Cl -

HCl + H 2 O → H 3 O + + Cl - HCl + H 2 O → H 3 O + + Cl -

NaOH → Na + + OH - NaOH → Na + + OH -

Conclusions:

Solutions that contain free-moving ions are able to conduct electricity because of the movement of charged particles. Solutions that do not contain free-moving ions do not conduct electricity.

Demonstration : The reaction of ions in solution

Apparatus and materials:

4 test tubes; copper(II) chloride solution; sodium carbonate solution; sodium sulphate solution

Figure 5

Method:

Results:

Test for a chloride

Prepare a solution of the unknown salt using distilled water and add a small amount of silver nitrate solution. If a white precipitate forms, the salt is either a chloride or a carbonate.

Cl^-- + Ag⁺ + NO₃^--→→ AgCl + NO₃^-- (AgCl is white precipitate)

CO₃^2-- + 2Ag⁺ + 2NO₃^--→→ Ag₂CO₃ + 2NO₃^-- (Ag₂CO₃ is white precipitate)

The next step is to treat the precipitate with a small amount of concentrated nitric acid. If the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed, and the precipitate disappears, the salt is a carbonate.

AgCl + HNO₃→→ (no reaction; precipitate is unchanged)

Ag 2 CO 3+2 HNO 3→2 AgNO 3+H2O+ CO 2 Ag 2 CO 3+2 HNO 3→2 AgNO 3+H2O+ CO 2 (precipitate disappears)

Test for a sulphate

Add a small amount of barium chloride solution to a solution of the test salt. If a white precipitate forms, the salt is either a sulphate or a carbonate.

SO 42-+ Ba 2++ Cl -→ BaSO 4+ Cl - SO 42-+ Ba 2++ Cl -→ BaSO 4+ Cl - (BaSO44 is a white precipitate)

CO 32-+ Ba 2++ Cl -→ BaCO 3+ Cl - CO 32-+ Ba 2++ Cl -→ BaCO 3+ Cl - (BaCO33 is a white precipitate)

If the precipitate is treated with nitric acid, it is possible to distinguish whether the salt is a sulphate or a carbonate (as in the test for a chloride).

BaSO 4+ HNO 3→ BaSO 4+ HNO 3→ (no reaction; precipitate is unchanged)

BaCO 3+2 HNO 3→ Ba ( NO 3)2+H2O+ CO 2 BaCO 3+2 HNO 3→ Ba ( NO 3)2+H2O+ CO 2 (precipitate disappears)

Test for a carbonate

If a sample of the dry salt is treated with a small amount of acid, the production of carbon dioxide is a positive test for a carbonate.

Acid + CO₃^2--→→ CO₂

If the gas is passed through limewater and the solution becomes milky, the gas is carbon dioxide.

Ca(OH)₂ + CO₂→→ CaCO₃ + H₂O (It is the insoluble CaCO₃ precipitate that makes the limewater go milky)

Test for bromides and iodides

As was the case with the chlorides, the bromides and iodides also form precipitates when they are reacted with silver nitrate. Silver chloride is a white precipitate, but the silver bromide and silver iodide precipitates are both pale yellow. To determine whether the precipitate is a bromide or an iodide, we use chlorine water and carbon tetrachloride (CCl44).

Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a reddish brown.

Chlorine water frees iodine gas from an iodide and colours the carbon tetrachloride purple.

Discussion : Creative water conservation

As populations grow, so do the demands that are placed on dwindling water resources. While many people argue that building dams helps to solve this water-shortage problem, the reality is that dams are only a temporary solution and that they often end up doing far more ecological damage than good. The only sustainable solution is to reduce the demand for water, so that water supplies are sufficient to meet this. The more important question then is how to do this.

Discussion:
Divide the class into groups, so that there are about five people in each. Each group is going to represent a different sector within society. Your teacher will tell you which sector you belong to from the following: Farming, industry, city management or civil society (i.e. you will represent the ordinary 'man on the street'). In your groups, discuss the following questions as they relate to the group of people you represent: (Remember to take notes during your discussions, and nominate a spokesperson to give feedback to the rest of the class on behalf of your group)

Group Project : School Action Project

There is a lot that can be done within a school to save water. As a class, discuss what actions could be taken by your class to make people more aware of how important it is to conserve water.

Summary Exercise