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8.1 Ions in aqueous solution

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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

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.

Interesting fact

The simplest way to check whether water is hard or soft is to use the lather/froth test. If the water is very soft, soap will lather more easily when it is rubbed against the skin. With hard water this won't happen. Toothpaste will also not froth well in hard water.

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 H 3 O + ) 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).

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:

p H = - l o g [ H + ]

or

p H = - l o g [ H 3 O + ]

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

Calculate the pH of a solution where the concentration of hydrogen ions is
1 × 10 - 7 mol · dm - 3 .

  1. In this example, the concentration has been given and is 1 × 10 - 7 mol · dm - 3

  2. pH = -log[H + ]

    = -log(1 × 10 - 7 )

    = 7

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Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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