0.8 Reactions in aqueous solutions  (Page 4/10)

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. ${\mathrm{Al}}^{3+}$ . 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 ${\mathrm{CaCO}}_3$ (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.

Investigation : acid rain

You are going to test the effect of 'acid rain' on a number of substances.

Materials needed:

Method:

1. Place a small sample of each of the following substances in a separate test tube: chalk, marble, zinc, iron and lead
2. To each test tube, add a few drops of dilute sulphuric acid.
3. Observe what happens and record your results.

Discussion questions:

• In which of the test tubes did reactions take place? What happened to the sample substances?
• What do your results tell you about the effect that acid rain could have on each of the following: buildings, soils, rocks and geology, water ecosystems?
• What precautions could be taken to reduce the potential impact of acid rain?

Electrolytes, ionisation and conductivity

Conductivity in aqueous solutions, is a measure of the ability of water to conduct an electric current. The more ions there are in the solution, the higher its conductivity.

Conductivity

Conductivity is a measure of a solution's ability to conduct an electric current.

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 .

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.

1. Strong electrolytes A strong electrolyte is a material that ionises completely when it is dissolved in water:
   $$\mathrm{AB\; (s,\; l,\; g)}\to {\mathrm{A}}^{+}\mathrm{(aq)}+{\mathrm{B}}^{-}\mathrm{(aq)}$$
   This is a chemical change because the original compound has been split into its component ions and bonds have been broken. In a strong electrolyte, we say that the extent of ionisation is high. In other words, the original material dissociates completely so that there is a high concentration of ions in the solution. An example is a solution of potassium nitrate:
   $${\mathrm{KNO}}_3\mathrm{(s)}\to {\mathrm{K}}^{+}\mathrm{(aq)}+{\mathrm{NO}}_3^{-}\mathrm{(aq)}$$
2. Weak electrolytes A weak electrolyte is a material that goes into solution and will be surrounded by water molecules when it is added to water. However, not all of the molecules will dissociate into ions. The extent of ionisation of a weak electrolyte is low and therefore the concentration of ions in the solution is also low.
   $$AB(s,l,g)\to AB\left(aq\right)\rightleftharpoons A^{+}\left(\mathrm{aq}\right)+B^{-}\left(\mathrm{aq}\right)$$
   The following example shows that in the final solution of a weak electrolyte, some of the original compound plus some dissolved ions are present.
   $$C_2{H}_3{O}_{2}H\left(l\right)\to C_2{H}_3{O}_{2}H\rightleftharpoons C_2{H}_3{O}_2^{-}\left(\mathrm{aq}\right)+H^{+}\left(\mathrm{aq}\right)$$