SCIE 4001
Unit 1
Introduction
1.1 About Matter
1.1 Nature of Matter
1.1 Properties of Matter
1.1 States of Matter
1.1 Kinetic Theory of Gases
1.1 Brownian Motion
1.2 Hard and Soft Water
 
 
 

 

 

Unit 1.2 Hard and Soft Water

Objectives.

Explain the terms

  • Temporary hard water
    Permanent hard water
  • List the factors responsible for the hardness of water
  • Use word equations to illustrate the formation of hard water
  • Use word equations to illustrate the softening of hard water by a chemical method
  • Explain the action of soap on hard water
What is hard Water? No hard water is not Ice!...

Graph of Height of lather (cm) vs tube

Fig 1.2.1 Soap vs detergent

Fist let me add some important foundation ideas and assumptions to this discussion...

  • The action of soap on water to produce lather (lathering of soap). We all are familiar that when we wash or bathe with soap, as the soap is exposed to water and massaged over the skin it make a fine bubbly foam (often called soap froth in our culture). This foam is called lather and the process lathering...
  • Soap is basically made from lye (sodium hydroxide) and oils or fats... Detergents are much more complex substances with many much harsher chemicals and NOT the same as soap. In our culture we often refer to detergents as 'powdered soap'.For this discussion, we are concerned with soap NOT detergents, so please understand that detergents (even if called 'powdered soap') are excluded as they way they act with water may be different...
  • Why do we use soap? Soap helps water to 'mix with oils' and wash away dirt embedded in body oils on our skin or clothes. Soap removes grease and dirt because it acts as a link between water and the dirt particles. On one side of the soap is the dirt and the other water. The water can perform a tug-of-war with the fabric fibers or surfaces to be cleaned using soap as the rope and pulling away the dirt and grease. The dirt which now adheres to the soap is washed away with the water.

So getting back to hard water...

Graph of Height of lather (cm) vs tube

Fig 1.2.2 Home test for Hardness

If the lather if difficult to make, that water is considered to be hard water...
So if we try to bathe with rain water the soap lathers very easily and the lather is difficult to wash off. Experiences with soap and water from different sources or geographical areas leads us to understand that the hardness of the water can vary from no resistance to lathering with rain water, various resistances with river water to extreme resistance with sea water. No one tries to soap while having a sea bath!

So hardness of water is really a range... In the activity section we'll look at an investigation to compare re hardness from various sources.

 

Exercise 1.2.1

Write out your understanding of 'hard water'

Graph of Height of lather (cm) vs tube

Fig 1.2.3 Temporary hard water

Temporary hard water

When the hardness of water can be removed by boiling, we say the hardness of water is temporary...

Graph of Height of lather (cm) vs tube

Fig 1.2.4 Permanent hard water

Permanent hard water

And guess what? When the hardness of water CANNOT be removed by boiling, we say the hardness of water is permanent...

What causes hardness in water? Brain storming...
Graph of Height of lather (cm) vs tube

Hypothesis: Chemicals in the water increases its hardness...
Based on the observations that

  • Water cooled from rain water is soft and not hard
  • Rain water is as close as we can easily come to a natural source of water being pure water we can obtain by distillation
  • Water from the river becomes harder as it passes over rocks and soil
  • Water from the sea is very hard...
Investigative Research

Research Question: What chemicals are present in sea water?

Answer after research: Sea water has almost every natural chemical compound present on earth and many manmade chemicals...

What chemical(s) are most abundant in sea water?

Brain storming...

New hypothesis: Salt (cooking salt - sodium chloride) in the water increases its hardness...

Investigation Plan and Design an investigation to test the hypothesis: Salt (cooking salt - sodium chloride) in the water increases its hardness...

Graph of Height of lather (cm) vs tube

Fig 1.2.5 Apparatus

Aim: investigation to test the hypothesis: Salt (cooking salt - sodium chloride) in the water increases its hardness...
Apparatus: 6 boiling tubes labeled A to E, sodium chloride (common salt), distilled water, liquid soap, beaker, stirrer, measuring cylinder
Manipulate variable: amount of salt in the water
Responding variable: Volume of lather produced
Variables held constant: amount of soap, volume of solutions, sizes of boiling tubes, temperature of solutions, amount of shaking of each tube

Method:

  • 5 ml water is added to tube A
  • A concentrate salt solution is prepared in a beaker
  • 1 ml of the concentrate salt solution and 4 mls of distilled water is added to the tube B to produce a 20% salt solution
  • 2 ml of the concentrate salt solution and 3 mls of distilled water is added to the tube C to produce a 40% salt solution
  • 3 ml of the concentrate salt solution and 2 mls of distilled water is added to the tube D to produce a 60% salt solution
  • 4 mls of the concentrate salt solution and 1 ml of distilled water is added to the tube EB to produce a 80% salt solution
  • 5 ml concentrate salt solution is added to tube A
  • Two drops of liquid soap is added to each tube
  • Each tube is shaken vigorously 20 times and then allowed to stand
  • The height of later in each tube is recorded with a ruler in cm

Graph of Height of lather (cm) vs tube

Fig 1.2.6 Results: The lather produced in each tube

Results:

Table 1.2.1 - The lather produced in each tube
Tube
A
B
C
D
E
Height of lather (cm)
12
12
12
12
12

All tubes had the same amount of lather.

Analysis of Results:

  • The presence of sodium chloride in the water did not affect the amount of lather produced.

Conclusion:

  • Sodium chloride (cooking salt) does not produce hardness in water.

Discussion of Results:

  • It is difficult to control the shaking of the tubes equally so it is a source of error that can be minimized by doing each trial multiple time and finding the average.
  • Measuring the volume of the soap added was difficult as a dropper was used. It can be minimized by doing each trial multiple time and finding the average.

Answer after Investigation: The salt in sea water does not cause hardness in water...
Modify Hypothesis: Other chemicals (not sodium chloride) in the water increases its hardness...

Additional research to find out which chemicals in our natural water sources causes hardness.

Using text books and the Internet research which chemicals cause hardness

After research we find the there are in the main four chemical substances responsible for hardness in water. They are:

  • Calcium hydrogen carbonate
  • Magnesium hydrogen carbonate
  • Calcium sulphate
  • Magnesium sulphate

Interesting... calcium and magnesium (metals) and hydrogen carbonate and sulphate...
They are present in sea water!

Investigation

We would like to plan and design an investigation to test what we learned above:

Graph of Height of lather (cm) vs tube

Fig 1.2.7 Apparatus

It is easy to modify your investigation above to do this... you might be familiar with magnesium sulphate... it called epsom salts in our culture and used by some parents to purge children generally before the new term term starts...

Aim: investigation to test the hypothesis: Magnesium sulphate in the water increases its hardness...
Apparatus: 6 boiling tubes labeled A to E, magnesium sulphate, distiller water, liquid soap, beaker, stirrer, measuring cylinder
Manipulate variable: amount of magnesium sulphate in the water
Responding variable: Volume of lather produced
Variables held constant: amount of soap, volume of solutions, sizes of boiling tubes, temperature of solutions, amount of shaking of each tube
Method:

  • 5 ml water is added to tube A
  • A concentrate magnesium sulphate solution is prepared in a beaker
  • 1 ml of the concentrate magnesium sulphate solution and 4 mls of distilled water is added to the tube B to produce a 20% salt solution
  • 2 ml of the concentrate magnesium sulphate solution and 3 mls of distilled water is added to the tube C to produce a 40% salt solution
  • 3 ml of the concentrate magnesium sulphate solution and 2 mls of distilled water is added to the tube D to produce a 60% salt solution
  • 4 mls of the concentrate magnesium sulphate solution and 1 ml of distilled water is added to the tube EB to produce a 80% salt solution
  • 5 ml concentrate magnesium sulphate solution is added to tube A
  • Two drops of liquid soap is added to each tube
  • Each tube is shaken vigorously 20 times and then allowed to stand
  • The height of later in each tube is recorded with a ruler in cm

Graph of Height of lather (cm) vs tube

Fig 1.2.8 Results: The lather produced in each tube

Results:

Table 1.2.2 - The lather produced in each tube
Tube
A
B
C
D
E
Height of lather (cm)
12
9
6
3
0

 

 

Graph of Height of lather (cm) vs tube  
Fig 1.2.9 Results: The graph of results  

Analysis of Results:

  • The higher presence of magnesium sulphate in the water causes a reduction in the amount of lather produced.

Conclusion:

  • Magnesium sulphate does produce hardness in water.

Discussion of Results:

  • It is difficult to control the shaking of the tubes equally so it is a source of error that can be minimized by doing each trial multiple time and finding the average.
  • Measuring the volume of the soap added was difficult as a dropper was used. It can be minimized by doing each trial multiple time and finding the average.

Answer after Investigation: Magnesium sulphate in the water increases its hardness...

Further Questions: Which chemicals in our natural water sources causes temporary hardness.
Which chemicals in our natural water sources causes permanent hardness.

Plan and Design an investigation to test the question: Which chemicals in our natural water sources causes temporary hardness. ...

 

Graph of Height of lather (cm) vs tube

Temporary hardness is caused by
Calcium hydrogen carbonate
Magnesium hydrogen carbonate

Permanent hardness is caused by
Calcium sulphate
Magnesium sulphate

 

 

How do calcium hydrogen carbonate, magnesium hydrogen carbonate, calcium sulphate and magnesium sulphate get into our natural water sources?

 

Calcium sulphate is slightly soluble in water and occurs in rocks as the minerals:

  • chalk
  • gypsum
  • alabaster

Magnesium sulphate also occurs as a mineral in rocks. It is quite soluble in water as those who have been forced to drink epsom salts know!

Calcium carbonate occurs naturally in limestone and coral rocks. Both calcium and magnesium carbonate occurs in Dolomite rocks. They are almost insoluble in water. Limestone which occurs commonly in our hills is really calcium carbonate. In occurs as white rocks which look like corral or may occur in a bluish form called “metal” and used for road repair, making cement and concrete. Calcium is often associated with magnesium in many rocks.

So how do they get into the water?

When water contains carbon dioxide, it reacts with the calcium carbonate to form calcium hydrogen carbonate. Similarly magnesium carbonate forms magnesium hydrogen carbonate. These hydrogen carbonates are very soluble but not very stable so sometimes they decompose back to the carbonate.

Do you understand why the calcium hydrogen carbonate and magnesium hydrogen carbonate cause only temporary hardness?
Yes! they are easily decomposed back to the carbonate which is insoluble and precipitates out of the water! On the other hand calcium and magnesium sulphates are very stable so are unaffected by heating!
Graph of Height of lather (cm) vs tube

Exercise 1.2.2

On a scale of 0 to 10 the hardness of the rain water is considered as 0 and that of the lake in the picture as 10.
(a) Determine whether the water at points A to G is soft (S) or hard - temporary (T) or permanent (P) or both (B).
(b) Complete the table by writing the hardness of water (on a scale of 1 to 10 with 1 being soft and 10) at points being maximum hardness in the table below:

Table 1.2.3
The Hardness of water at various points in a region

Region
A B C D E F G
Hardness of ground water
             
Type of Hardness              

Put your mouse pointer over the pictures on the left for the answer.

Graph of Height of lather (cm) vs tube
Use word equations to illustrate the action of hard water on soap
Recall from above: Soap is basically made from lye (sodium hydroxide) and oils or fats... It is really a chemical substance called sodium stearate which reacts with calcium hydrogen carbonate, magnesium hydrogen carbonate, calcium sulphate and magnesium sulphate.

X Y + sodium stearate (soap) = X stearate (insoluble scum) + sodium Y
where X = calcium or magnesium and Y = hydrogen carbonate or sulphate

you can write the equation like:
calcium sulphate + sodium stearate (soap) = calcium stearate (insoluble scum) + sodium sulphate
calcium hydrogen carbonate + sodium stearate (soap) = calcium stearate (insoluble scum) + sodium hydrogen carbonate
magnesium sulphate + sodium stearate (soap) = magnesium stearate (insoluble scum) + sodium sulphate
magnesium hydrogen carbonate + sodium stearate (soap) = magnesium stearate (insoluble scum) + sodium hydrogen carbonate

Problems with Using hard water

  • More soap is needed to form lather. The soap has to react with all the dissolved calcium hydrogen carbonate and/or magnesium hydrogen carbonate and/or calcium sulphate and/or Magnesium sulphate to remove it from the water. You can imagine that this would be difficult with running water like in the shower!
  • The scum produced adheres to fixtures and can be difficult to remove from bathtubs or sinks etc.
  • Some industrial processes like dying cannot use hard water.
  • Causes “kettle fur” Over a period of time deposits of calcium/magnesium compounds remain behind when hard water is boiled.
  • Leaves unsightly film on dishes when washed (especially in a dishwasher)... Vinegar is used to remove the film in the washing process if added to the dishwasher...
  • May leave clothes feeling stiff after washing with soap. The scum formed when the calcium/magnesium reacts with the soap may remain in the fabric rather than float to the water surface, When it dries it will impart stiffness to the fabric.

Advantages of hard water

  • Has a taste many people prefer. Dissolved materials cause water to have a taste.
  • A source of calcium for strong bones and teeth. Bones and teeth have large quantities of calcium for strength.
  • Less likely to react with metallic pipes. A coat of material prevents the water from attacking the inner metallic surface of the pipe.
  • Good for brewing beer. Ask yourself why the beer will taste better.

Use word equations to illustrate the softening of hard water by a chemical method

1. Using Sodium Carbonate (Washing soda) removes both types of hardness
XY + sodium carbonate = X carbonate + sodium Y.
where X = calcium or magnesium and Y = hydrogen carbonate or sulphate.
e.g. calcium sulphate + sodium carbonate = calcium carbonate (insoluble precipitate) + sodium sulphate.
calcium hydrogen carbonate + sodium carbonate = calcium carbonate (insoluble precipitate) + sodium hydrogen carbonate
magnesium sulphate + sodium carbonate = magnesium carbonate (insoluble precipitate) + sodium sulphate
magnesium hydrogen carbonate + sodium carbonate = magnesium carbonate (insoluble precipitate) + sodium hydrogen carbonate

2. Using Ion Exchange Resin removes both types of hardness
XY + sodium ions in resin = X (negative ions in resin) + sodium Y.
where X = calcium or magnesium and Y = negative ions in resin.
e.g. calcium sulphate + sodium ions (negative ions in resin) = calcium (negative ions in resin) + sodium sulphate
calcium hydrogen carbonate + sodium (negative ions in resin) = calcium (negative ions in resin) + sodium hydrogen carbonate
magnesium sulphate + sodium (negative ions in resin) = magnesium negative ions in resin) + sodium sulphate
magnesium hydrogen carbonate + sodium (negative ions in resin) = magnesium(negative ions in resin) + sodium hydrogen carbonate

3. Using calcium hydroxide (slaked lime)
calcium hydrogen carbonate + calcium hydroxide = calcium carbonate (insoluble precipitate) + water
Only temporary hardness of calcium hydrogen carbonate removed as magnesium hydrogen carbonate, calcium sulphate and magnesium sulphate are relatively unaffected.

Softening of hard water

These substances which cause hardness in water can be removed by physical and chemical means

Method Type of hardness removed How hardness is removed Comments
Physical Boiling Temporary The hydrogen carbonate is soluble in water but it decomposes to the carbonate, when heated, and this is insoluble Very expensive
Distillation Both types Removes all dissolved solids Very expensive
Chemical Sodium carbonate (washing soda) Both types Sodium carbonate reacts with the calcium and magnesium compounds in the water to form the insoluble calcium carbonate or magnesium carbonate Fairly cheap
Calcium hydroxide (slaked lime) Temporary Calcium hydroxide reacts with calcium/magnesium hydrogen carbonate to produce the insoluble carbonate Quite cheap
Ion – exchange resin Both types The resin “swaps” sodium ions for the calcium and magnesium present in the hard water. In recharging it would swap sodium ions in the flushing salt solution for the absorbed calcium/magnesium Expensive but the resin can be recharged by using common salt

Summary

  • soft water readily forms lather with soap
  • hard water reacts with soap to form scum and so more soap is needed to form lather.
  • soapless detergents do not form scum.
  • hardness of water can be tested by how much lather it makes with soap
  • hard water contains dissolved compounds, usually of calcium or magnesium
  • compounds are dissolved when water comes into contact with rocks
  • permanent hard water remains hard when it is boiled
  • temporary hard water is softened by boiling
  • temporary hard water contains hydrogen carbonate ions (HCO3-) that decompose on heating to produce carbonate ions which precipitates.
  • temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles
  • hard water has some benefits because calcium compounds are good for the development and maintenance of bones and teeth and also help to reduce heart disease
  • hard water can be made soft by removing the dissolved calcium and magnesium ions by adding sodium carbonate or using sing commercial water softeners such as ion exchange columns
Exercise 1.2.3

1. The amount of lather produced by soap indicated whether it is ________ or ________ water

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2. How is hard water formed?
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3. Write a word equation for the reaction between hard water and soap.

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4. Describe how one can test a water sample for hardness at home.

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5. Why is it incorrect to say that calcium or magnesium carbonate causes hardness in water?

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6. What substances cause temporary hardness in water?

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7. What substances cause permanent hardness in water?

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8. Why does the inside a kettle become covered in lime scale?

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9. Explain TWO disadvantages of having a hard water supply in your home.

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10 Explain TWO advantages of having a hard water supply in your home.

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11. Describe TWO chemical process which can be used to make hard water soft and state what kind of hardness each removes.

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Graph of Height of lather (cm) vs tube

12. A class collected water from 5 rivers on various field trips and used the same amount of water and soap in each tube. All tubes were shaken with a mechanical shaker in the laboratory for the same period of time. Rate the hardness of the waters from softest to hardest.

Put your mouse pointer over the pictures on the left for the answer.

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Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.