Fats and Oils – SS3 Chemistry Lesson Note (First Term)

Fats and oils are lipids made of esters of propane-1,2,3-triol (glycerol) with three fatty acid molecules.

• Solubility: both are insoluble in water.
• Solvents: both dissolve in organic solvents such as ether and chloroform.

Behavioural objectives

1. Define fats and oils and state their structure.
2. Identify the sources of fats and oils.
3. State the physical properties of fats and oils.
4. Describe the chemical properties of fats and oils: hydrolysis, saponification, hydrogenation and rancidity.
5. State the uses of fats and oils.

What are fats and oils?

Fats and oils are triglycerides, which are esters of glycerol and long-chain fatty acids.

• Lipids: a general group of compounds that includes fats, oils and waxes.
• Fats: lipids that are usually solid at room temperature and mostly of animal origin.
• Oils: lipids that are liquid at room temperature and mainly from plants.
• Triglyceride: the ester formed when one molecule of propane-1,2,3-triol joins with three fatty acid molecules.
• Fatty acid: a long-chain carboxylic acid, for example stearic acid (C₁₇H₃₅COOH) and oleic acid (C₁₇H₃₃COOH).

How is a triglyceride formed?

A triglyceride forms when one glycerol molecule undergoes esterification with three fatty acid molecules.

1. One molecule of propane-1,2,3-triol has three -OH groups.
2. Each -OH group reacts with the -COOH group of one fatty acid.
3. Three molecules of water are lost.
4. One triglyceride (an ester with three ester links) is formed.

What is the difference between saturated and unsaturated fatty acids?

• Saturated fatty acid: a fatty acid whose carbon chain has only single C-C bonds, for example stearic acid.
• Unsaturated fatty acid: a fatty acid whose carbon chain has one or more C=C double bonds, for example oleic acid.

What are the sources of fats and oils?

Fats and oils come from two main sources: plants and animals.

• Plant sources: seeds, nuts, olives and avocados.
• Common Nigerian plant examples: palm oil, palm kernel oil, groundnut oil, coconut oil, shea butter and cotton seed oil.
• Animal sources: meat, fish and dairy products.
• Common animal examples: butter, lard, beef fat (tallow), cod-liver oil and fish oils.
• Exceptions: shea butter and cocoa butter are plant fats, while fish oils are liquid animal oils.

How do fats differ from oils?

• Fats: triglycerides that are solid at room temperature, mostly from animals and rich in saturated fatty acids.
• Oils: triglycerides that are liquid at room temperature, mostly from plants and rich in unsaturated fatty acids.

What are the physical properties of fats and oils?

There are seven physical properties to know: state, melting point, solubility, density, odour and flavour, colour and feel.

1. State at room temperature

• Fats: solid at room temperature.
• Oils: liquid at room temperature.

2. Melting point

• Fats: higher melting point, so they stay solid.
• Oils: lower melting point, so they stay liquid.
• Note: they melt over a range of temperatures because they are mixtures.

3. Solubility

• In water: insoluble.
• In organic solvents: soluble in ether, chloroform and similar solvents.

4. Density

• Compared with water: both are less dense, so they float on water.

5. Odour and flavour

• Source: a fat or oil may carry a particular odour and flavour from where it came.
• Processing: refining and storage also change the odour and flavour.

6. Colour

• Range: from colourless to yellow, red or brown.
• Causes: natural pigments, impurities, source and processing. Red palm oil gets its colour from natural pigments.

7. Feel

• Greasy: both feel greasy and leave a greasy stain on paper.

What are the chemical properties of fats and oils?

The main chemical properties are hydrolysis, saponification, hydrogenation and rancidity. Polymerisation, isomerisation and the test for unsaturation are also covered.

1. What is hydrolysis of fats and oils?

Hydrolysis is the breaking of the ester links of a fat or oil by water.

• Conditions: an acid or an enzyme (lipase) is present.
• Products: glycerol and fatty acids.
• Equation: Triglyceride + 3H₂O → Glycerol + 3 Fatty acids

2. What is saponification?

Saponification is the reaction of a fat or oil with a strong base to give soap and glycerol.

• Bases used: sodium hydroxide or potassium hydroxide.
• Equation: Triglyceride + 3NaOH → Glycerol + 3 Soap molecules
• Soap: the sodium or potassium salt of a fatty acid, for example sodium stearate (C₁₇H₃₅COONa).
• Sodium hydroxide: gives hard soap.
• Potassium hydroxide: gives soft soap.

What are the steps in making soap in the laboratory?

1. Place palm oil or another oil in a beaker.
2. Add concentrated sodium hydroxide solution.
3. Heat and stir the mixture for some time.
4. Add sodium chloride (common salt) to make the soap separate out.
5. Filter off the soap and wash it with a little water.
6. Press and dry the soap.

3. What is the difference between hydrolysis and saponification?

4. What is hydrogenation?

Hydrogenation is the addition of hydrogen to the C=C bonds of unsaturated oils.

• Catalyst: finely divided nickel.
• Result: the liquid oil becomes a solid or semi-solid fat.
• Use: making margarine from vegetable oils.

5. What is rancidity?

Rancidity is the change that gives old fats and oils an unpleasant smell and taste.

• Causes: air (oxygen), moisture, light, heat and micro-organisms.
• Prevention: store in a cool, dark, airtight container.

6. What is polymerisation of oils?

Polymerisation is the joining of unsaturated oil molecules to form larger molecules.

• Condition: it happens under certain conditions.
• Effect: the oil thickens and its quality falls, which is usually unwanted.

7. What is isomerisation of unsaturated fatty acids?

Isomerisation is the change of a cis-isomer of an unsaturated fatty acid into a trans-isomer.

• Health: trans fats are linked with health concerns.

8. How do you test for unsaturation?

Bromine water is used to test whether an oil is unsaturated.

1. Shake a little of the oil with bromine water.
2. Observe the colour of the mixture.

• Unsaturated oil: the brown colour of bromine is removed.
• Saturated fat: the brown colour stays.

What is the simple test for fats and oils?

The grease spot test shows that a substance contains a fat or oil.

1. Rub a little of the sample on a piece of plain paper.
2. Hold the paper up to the light.

• Positive result: a translucent, greasy stain remains on the paper.

What are the uses of fats and oils?

Fats and oils are used as food, in cooking and in making soap, margarine, cosmetics and other products.

• Food: a source of energy, and they carry fat-soluble vitamins (A, D, E and K).
• Cooking: frying and baking.
• Soap making: saponification of oils and fats.
• Margarine: made by hydrogenation of vegetable oils.
• Cosmetics: creams, pomades and lotions, for example shea butter.
• Paints and candles: some oils and fats are used in them.
• Lubricants: some oils are used as lubricants.

Evaluation

1. Mention three sources of fats and oils.
2. What is saponification? Describe how saponification takes place.
3. Identify four uses of fats and oils.
4. State two differences between fats and oils.
5. How can you show that an oil is unsaturated?

Assignment

Write the word equation for the hydrolysis of a triglyceride, then state the products formed when the same triglyceride is heated with sodium hydroxide.