Water By Miss Buicke

OC14 Use cobalt chloride paper or anhydrous copper sulfate to test for water OC30 Conduct a qualitative experiment to detect the presence of dissolved solids in water samples, and test water for hardness (soap test) OC31 Understand that some dissolved compounds, including compounds of calcium, cause hardness in water, and that water hardness can be removed using an ion-exchanger

What we must know from the Syllabus:

OC33 Describe the processes involved in the treatment of water supplied to domestic consumers OC34 Investigate the de-composition of water by electrolysis; recall the composition of water.

Composition of water:1. Water is composed of hydrogen and oxygen.2. There are two parts hydrogen for every one

part oxygen.3. The chemical symbol for water is H2O.

The water molecule:

Tests for water1.Water turns blue cobalt-chloride paper pink.2.Water turns anhydrous copper sulphate from

white to blue (anhydrous means containing no water).

Blue cobalt-chloride paper turning pink in the presence of water

Anhydrous copperSulphate turningfrom white to blue in thePresence of water

The water cycle:

The amount of water on Earth remains constant. This is because water is continuously changing state and being recycled.

Pure water is difficult to find. This is because water is an excellent solvent. Many substances dissolve in it. Watertypically contains dissolved solids such as calcium and magnesium.

The water cycle

Experiment: to show the presence of dissolved substances in a water sample:

Equipment: retort stand, filter funnel, filter paper, beaker, tripod, wire gauze, bunsen burner, clock glass.

Chemicals: water samples ( tap water, lake water, sea water and well water)

Method: 1. Filter the water to remove any insolube particles.2. Pour some of the filtrate onto a clock glass.3. Heat the clock glass using a bunsen burner.4. When all the water has evaporated examine the clock

glass.

Result: There will be solid material on the clock glass.

Conclusion:The water sample contained dissolved solids.

Water treatment:Untreated water is the cause of millions of deaths every year. Diseases such as typhoidand cholera are widespread in countries where water is untreated.

Water is treated at a water works to make it fit to drink. There are five stages to the cleaning process. We must be able to name and describe these five stages.

Stages in water treatment:1. ScreeningThe first stage of screening involves passing water through a wire mesh or screen to remove any floating material such as leaves and rubbish.

2. SettlingWater flows into large tanks and a chemical called aluminium sulfate is added. This causes small insoluble particles to come together and settle to the bottom of the tanks.

3. FiltrationThe water is passed through beds of sand. Any remaining insoluble particles are removed. The water is now clear in colour but may contain bacteria.

.

4. ChlorinationChlorine is added to the water to kill any bacteria present. The amount of chlorine addedis carefully monitored.

5. FluoridationFluorine is added to the water in some countries to help prevent tooth decay.

Water treatment factory

Electrolysis is the splitting up of a compound by passing electricity through it.

Splitting water by electrolysis:

Water has the chemical formula H2O. When it is split each molecule releases two hydrogen atoms for every one oxygen atom. As a result, twice as much hydrogen gas is produced as oxygen gas.

Chemical equation:2H2O 2H2 +O2

To investigate the de-composition of water by electrolysis 1. Set up as shown (the apparatus is known as

a Hoffman Voltameter)2. Pure water is a poor conductor of

electricity so a little dilute sulphuric acid can be added.

3. Notice that the gases formed are in the ratio 2:1

Test for Hydrogen (it burns with a ‘pop’).Test for oxygen (it relights a glowing splint).

Desalination:

About 97% of all water on Earth is saltwater. In countries where fresh water is scarce, saltwater is desalinised i.e has the salt removed from it.

Properties of hard water

Boiling point 100 degrees

Excellent solvent

Colourless odourless

liquidTurns

anhydrous copper

sulfate from white to

blue

Turns cobalt

chloride paper

from blue to pink

Freezing point zero

degrees

Hard water does not easily form a lather with soap.

Water Hardness:

Soft water lathers easily with soap.

Hardness in water is caused by dissolved compounds, particularly compounds of calcium and magnesium.

problems associated with hard water

One of the main problems with hard water is that when heated in a kettle a white insolube solid called limescale is deposited. This substance sticks to heating elements and pipes making them less effective.Because of the problems caused by hard water it is often necessary to soften it before it can be used.

Softening water involves removing the calcium and magnesium ions.

This can be done in a number of ways:1. Boiling2. Distillation3. Ion exchange

Ion exchangeOne of the easiest and cheapest ways to remove hardnessfrom water is to pass it through an ion exchanger.As water passes through the ion exchanger, calciumand magnesium ions are swapped from sodium ions. Once the calcium and magnesium ions are removed, the water has been softened.

Eventually the exchanger will fill with calcium ions. This can be reversed by adding salt to the column and washing out the calcium ions.

Ion exchange resin

Advantages Disadvantages

Good source of calcium which is needed for strong bones and teeth

Limescale blocks pipes and leaves a scale on kettles and pipes

Most people prefer the taste of hard water

It wastes soap

It is good for brewing and tanning

It produces a scum with soap

The advantages and disadvantages of hard water

[2010 OL] [2006 OL] [2012 OL]Water is a compound composed of two elements.1. Name these two elements. 2. Name a chemical that can be used in a laboratory to test for the presence of water. 3. What colour change is noticed in this test for water?

Exam paper questions.

[2009][2012 OL]State how to test water to confirm the presence of hardness? Name a metallic element some of whose compounds cause hardness in wate

[2008 OL]Water supplied to domestic consumers is treated.

1. What name is given to the treatment used to remove large floating debris from the water?2. What name is given to the treatment used to help prevent tooth decay?

[2010 OL]Water is treated before it is supplied to our homes.Complete the statements below.Germs and bacteria are killed by adding _______________ .Floating materials are removed by ____________________

2006 OL] [2009 OL]The apparatus on the right can be used to decompose water by electrolysis.Acid is added to the water to allow an electric current to flow through the water.1. Name gas A and state a test for this gas.2. Name gas B and state a test for this gas.

What is water made from? Give four characteristics of water. Describe a laboratory experiment to detect the presence of dissolved solids in water samples. Give one advantage and one disadvantage of hard water.

Test questions

What dissolved compounds cause hardness in water? Name one method used to remove ‘hardness’ from water.