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K 2 SO 4 (s) 2 K + (aq) + SO 4 2 - (aq)

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

C 6 H 12 O 6 (s) C 6 H 12 O 6 (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)

Interesting fact

The ability of ionic compounds to dissolve in water is extremely important in the human body! The body is made up of cells , each of which is surrounded by a membrane . Dissolved ions are found inside and outside of body cells in different concentrations. Some of these ions are positive (e.g. Mg 2 + ) and some are negative (e.g. Cl - ). If there is a difference in the charge that is inside and outside the cell, then there is a potential difference across the cell membrane. This is called the membrane potential of the cell. The membrane potential acts like a battery and affects the movement of all charged substances across the membrane. Membrane potentials play a role in muscle functioning, digestion, excretion and in maintaining blood pH to name just a few. The movement of ions across the membrane can also be converted into an electric signal that can be transferred along neurons (nerve cells), which control body processes. If ionic substances were not able to dissociate in water, then none of these processes would be possible! It is also important to realise that our bodies can lose ions such as Na + , K + , Ca 2 + , Mg 2 + , and Cl - , for example when we sweat during exercise. Sports drinks such as Lucozade and Powerade are designed to replace these lost ions so that the body's normal functioning is not affected.

Write a balanced equation to show how silver nitrate ( AgNO 3 ) dissociates in water.

  1. The cation is: Ag + and the anion is: NO 3 -
  2. Since we know both the anion and the cation that silver nitrate dissociates into we can write the following equation:
    AgNO 3 (s) Ag + (aq) + NO 3 - (aq)

Ions in solution

  1. For each of the following, say whether the substance is ionic or molecular.
    1. potassium nitrate ( KNO 3 )
    2. ethanol ( C 2 H 5 OH )
    3. sucrose (a type of sugar) ( C 12 H 22 O 11 )
    4. sodium bromide ( NaBr )
  2. Write a balanced equation to show how each of the following ionic compounds dissociate in water.
    1. sodium sulphate ( Na 2 SO 4 )
    2. potassium bromide ( KBr )
    3. potassium permanganate ( KMnO 4 )
    4. sodium phosphate ( Na 3 PO 4 )

Ions and water hardness

This section is not examinable and is included as an example of ions in aqueous solution.

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 CaCO 3 (limestone or chalk) or from mineral deposits of CaSO 4 . The main source of magnesium is a sedimentary rock called dolomite, CaMg(CO 3 ) 2 . Hard water may also contain other metals as well as bicarbonates and sulphates.

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Source:  OpenStax, Chemistry grade 10 [caps]. OpenStax CNX. Jun 13, 2011 Download for free at http://cnx.org/content/col11303/1.4
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