2. Cláusulas Generales
2.6. Publicación y repositorios
Tap water contains many impurities. If it has a lot of calcium and magnesium salts dissolved in it, then it is hard. Soap reacts with these salts to produce calcium
and magnesium precipitates. These are left behind as a dirty grey substance called scum, which deposits as a dirty ring around basins and baths, or as scale in pipes and kettles. Soft water has less dissolved salts and soap produces less scum. Soap lathers better, feels smoother and more slippery
in soft water, and less of it is required to get clean.
Scum-free and bubbles galore!
Many New South Wales cities have excellent soft water: it lathers well and leaves very little scum. In other areas, ‘water softener’
systems are attached to each home’s water supply.
Beads of zeolite replace the offending calcium and magnesium ions with sodium. Soap doesn’t react
with sodium.
Soap is made when natural fatty acids found in materials like vegetable oils and animal fats react with an alkaline (basic) solution such as sodium hydroxide. The process is called saponification and is summarised by the reaction:
fat + alkaline solution
→
soap + glycerolPrac 3 p. CD12
Skin soap
Bases such as caustic soda (sodium hydroxide) and their alkaline solutions are
extremely dangerous if they
come in contact with skin. The
skin becomes slippery as its
fats undergo saponification
and form soap!
Fig 2.5.4 Lather (bubbles) keeps
the dirt and grease from re-depositing on the hair.
2.5
>>>
2.5
UNIT
[Questions
]
Soaps
Soaps
c Research the dry-cleaning process. Describe how
it cleans clothes, making reference to the chemistry involved. If necessary, use diagrams to assist your explanation.
d Explain why soap films are often coloured. e Describe the machine that can make three-storey-
high soap bubbles.
Action
3 Design a survey of soaps. Record your results in a
table showing the first six ingredients of at least three different brands of soap, hair shampoos and shower gels. Identify and discuss any trends you find.
Checkpoint
Water
1 Modify the following statements to make them
correct:
a Water is a non-polar molecule. b Sodium chloride is a polar molecule. c Water is able to dissolve grease.
2 State the types of substances that normally dissolve
in water.
Making grease soluble
3 Identify the type of compound that grease is
made of.
4 Some liquids are able to dissolve grease. List three
such liquids.
5 List three ways in which grease is prevented from
re-depositing on a surface.
6 State the reactants in saponification.
Hard and soft water
7 Lathering results in ‘scum’ forming when water is hard. List the chemicals that cause water to be hard. 8 Clarify what is meant by ‘soft water’.
9 State the advantages of soft water.
[Extension]
Investigate
1 a Use a dictionary to define the term ‘phobia’ and
include some examples.
b One end of a surfactant molecule is hydrophobic
and the other end is hydrophilic. Clarify the meaning of these terms and identify which end is which.
2 Conduct research on the Internet to answer the
following questions:
a List what is in a soap-free cleanser like Dove. b Scotch, 3M and ENJO all make cloths that clean
without the use of chemicals. Describe how they do this.
Think
10 Explain how soap is able to dissolve both in water and
in grease.
11 Identify as many factors as you can that will affect the
cleaning of a piece of fabric.
12 If lather doesn’t help to dissolve grease, explain how it
helps to remove grease from a fabric.
13 If shaving cream did not lather, state where the cut
whiskers would end up.
14 Identify three vegetable oils that could be used for the
production of soap.
15 If animal fat is needed to produce soap, propose some
sources of the fat.
Skills
16 Contrast detergent with soap. 17 Compare soap molecules with:
a plastics
b ionic compounds
18 Construct a word equation for the production of soap. 19 Construct a diagram showing how soap helps grease
to dissolve in water.
Create
20 Construct a three- to four-frame cartoon/diagram
showing how shampoo-conditioners work. Whale soap?
In the past, whale blubber was commonly the fat from which soap was made. Whales are now protected, however, and the fat used in soap manufacture comes mostly from cows slaughtered for their meat. Just about
any fat or oil can be used and many soaps are now made with vegetable or plant oils. Palmolive soap is named because it is made with palm oil and olive oil.
Detergents are produced from chemicals in crude oil. The big advantage of detergents is that they don’t produce scum.
UNIT
UNIT
2.5
2.5
[
Practical activities]
2.5
UNIT
Prac 1 Unit 2.5Make soap!
WARNING: The soap made here uses and contains very corrosive sodium hydroxide. Do not get any sodium hydroxide on your skin or in your eyes. Do not use the soap produced.
Aim To produce a sample of soap
Equipment
Olive oil or coconut oil, 1 M sodium hydroxide solution, saturated solution of sodium chloride, kerosene, 3 test tubes, rubber stopper, 400 mL beaker, 100 mL beaker, 250 mL beaker, hot plate (preferably) or a Bunsen burner, bench mat, tripod, gauze mat, matches, filter paper or paper towel
Method
1 Pour about 5 mL of oil into a test tube.
2 Carefully add 10 mL of sodium hydroxide solution. 3 Place the test tube in a boiling water bath for
30 minutes. Shake the tube every few minutes to mix the contents.
4 Place 50 mL of the sodium chloride solution in the
100 mL beaker, then pour the hot oil mix in. The soap formed should float to the top.
5 Scoop up the soap and place it in the 250 mL beaker.
Rinse a few times with a little water.
6 Let the soap dry on filter paper/paper towel.
7 Two-thirds fill the other test tube with water and add a
little soap.
8 Stopper and shake. Does it lather?
test tube 250 mL beaker water yellow flame bench mat 5 mL oil 10 mL sodium hydroxide solution Fig 2.5.5 Prac 2 Unit 2.5