Unit 3 - Soaps, Detergents and Emulsions

Question 1

Soap Formation

Answer 1

Soaps are produce by the alkaline hydrolysis of edible fats and edible oils. The alkali used is usually either potassium or sodium hydroxide

The fatty acid molecules are neutralised by the alkali, forming water-soluble, ionic salts called soaps. Glycerol is also formed in this hydrolysed reaction. The soluble salts can be extracted from the hydrolysis mixture by adding a large excess of sodium chloride. After this, the soap can be filtered off

Question 2

Cleansing Actions of Soap

Answer 2

Soaps can be used to remove non-polar substances such as oil and grease. Soaps are useful for cleaning because of the ion formed when dissolved in water.

Question 3

Cleansing Actions of Soap - Features of Soap Ions

Answer 3

Soap ions have:
* a long non-polar tail, which is readily soluble in non-polar compounds (hydrophobic)
a negatively charged ionic head that is water soluble (hydrophilic)

Question 4

Cleansing Actions of Soaps - How Its Done

Answer 4

The hydrophobic tails dissolve in the oil or grease (non-polar). The negatively-charged hydrophilic heads remain in the surrounding water (polar).

Agitation causes ball-like structure to form. The ball-like structures have hydrophobic tails on the inside and the negative hydrophilic head on the outside. The negatively-charged ball-like structures repel each other and the oil or grease is kept suspended in the water

Question 5

Effect of Hard Water on Soap

Answer 5

In many parts of the UK, tap water contains a higher concentration of Ca2+ and Mg2+ ions. This is called hard water. Hard water is water that contains high levels of dissolved metal ions.

When soap is used in hard water, scum, an insoluble precipitate, is formed. If sodium stearate soap is used, the stearate ions (St-) react with calcium ions to form an insoluble grey scum, calcium stearate (insoluble also)

Question 6

Soapless Detergents

Answer 6

In hard water areas, soapless detergents are used instead of soap because they don’t form scum. Soapless detergents are formed when a fat or oil is treated with concentrated sulfuric acid. Soapless detergents have non-polar hydrophilic tails and ionic hydrophilic heads.

Soapless detergents remove oil and grease the same way as soap. The ionic head is attracted to polar water molecules but doesn’t form a precipitate with Ca2+ or Mg2+ ions.

Question 7

Emulsions

Answer 7

An emulsion contains small droplets of one liquid dispersed in another liquid. A mixture of oil (non-polar) and water (polar) is an example of an unstable emulsion in which the layers quickly separate.

Question 8

Emulsifiers

Answer 8

An emulsifier can be used to prevent non-polar and polar liquids separating into layers. For example, when an emulsifier is mixed with oil and water, a stable emulsion is formed. Some substances act as emulsifiers naturally. Emulsifiers are similar in structure to soap and detergents.

Question 9

Making Emulsifiers

Answer 9

Emulsifiers for use in food can be made by reactiing edible oils with glycerol. In the molecules formed, only one or two fatty acid groups are linked to each glycerol backbone.

The OH groups present in the emulsifier are hydrophilic whilst the fatty acid chains are hydrophobic. The hydrophobic fatty acid chains dissolve in oil whilst the hydrophilic hydroxyl groups dissolve in water, forming a stable emulsifier.

Question 10

Emulsifiers IN Foods

Answer 10

Emulsifiers are used in a wide range of foods including:
* sauces
* low-fat spreads
* ice cream