Oils, fats, margarine, soaps and detergents

Doc Brown's GCSE/IGCSE/O Level KS4 science CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

14. 'Domestic' products – Oils, fats, margarine and soap organic molecules

This page describes the molecular structure of natural oils, fats and 'soapy' soaps. How do you make soaps from natural oils? How is margarine made? What is the composition of a typical margarine? The terms–names glycerol, triglycerides, long chain fatty acids, monounsaturates and polyunsaturates all explained. The uses of oils and fats is described and explained. There are extra sections on dry cleaning solvents and biological detergents. These revision notes on edible oil extraction, use in cooking, fats, margarine production and the molecules we use in soaps and detergents should prove useful for the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.

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Advanced Chemistry Page Index and Links


Naturally Occurring Molecules from plants and animals

14. Fats, Oils and Margarine

Natural vegetable oils and animal fats are important raw materials for the chemical industry wide ranging applications and uses e.g. soaps, detergents, cosmetics, lubricants, paints, many food industry products. It is possible that in future vegetable oils could be an important renewable and sustainable alternative to some of the products we derive from crude oil.

  • Plant Oils – Uses

    • Many plants produce useful oils that can be extracted and converted into consumer products including processed foods.

    • Emulsions can be made and have a number of uses.

    • Vegetable oils can be hardened to make margarine.

    • Biodiesel fuel can be produced from vegetable oils. See biomass and alternative fuels page.

    • Vegetable oils are an important source of energy and even vitamins like vitamin E in seed oils.

    • Vegetable oils contain essential fatty acids which are bodies need for certain metabolic processes.

  • Plant Oils – extraction and processing into useful products

    • The fruits and seeds of some plants contain appreciable and economically viable quantities of oil.

      • eg olives (olive oil), peanuts, walnuts (walnut oil), brazil nuts, rape seed, avocados, soya oil etc.

      • They are usually liquid at room temperature, whereas animal fats tend to be solids at room temperature.

    • The traditional way to extract the oil from plant material is to crush it between metal plates of a press, which literally squashes the oil out eg extracting olive oil from crushed olives.

    • You can also extract the oil from crushed plant material by using a centrifuge (a high spin 'barrel' with holes in the out surface), whose rapid rotation spins the oil out to out regions of the container from which the oil can be collected through the outer holes.

    • It is also possible to extract the plant oil with a solvent.

    • However, before any oil can be used it must be purified to remove impurities.

    • Plant oils can be highly refined by fractional distillation which removes any solvents, water and other dissolved impurities.

    • You can also use steam distillation to extract lavender oil or orange oil etc. from the crushed plant leaves or peel etc. but this more to do with the perfume industry than the food industry!

      • BUT you can do it with much simple apparatus eg boiling the plant material with water in a flask connected to a condenser and collection flask (see below).

        • , a simple experiment you could do in schools and colleges.

  • Oils and Fats are an important way of storing chemical energy in living systems and are also a source of essential long–chain fatty acids.

    • Plant oils and animal fats have a high energy density (higher than carbohydrates) and be easily stored in living organisms until they are needed to supply extra energy to power the chemistry of life.

    • If we take in more calorific food that we need e.g. excess carbohydrate or 'fatty food', then that excess energy supply is converted to fat and stored for future use, but, if it isn't used up, then obviously you will put on weight.

  • Fats and oils are esters formed from long chain fatty acids and the 'triol' alcohol glycerol diols triols and cyclo-alcohols structure and naming (c) doc b , which has three C–O–H groups.
    • Glycerol is the alcohol plants and animals use to make oils and fats which are esters we use in food and soaps.
    • Animals and plants combine glycerol and long chain fatty acids to make triglyceride esters – fats from animals and oils from plants.
    • Most of them are esters of the tri–alcohol ('triol') glycerol (systematic name propane–1,2,3–triol, but that can wait until AS–A2 level).

    • The carboxylic acids which combine with the glycerol are described as 'long–chain fatty acids'.

    • The resulting ester is called a 'triester' or 'triglyceride' and they are the major components in animal fat, vegetable oils, and processed fats like margarine etc..

  • The 'long–chain fatty acids' can be saturated, with NO C=C double bonds, and so forming saturated oils or fats (1st diagram below of the triglyceride formed from palmitic acid).

    • Animal fats are mainly saturated fats with no carbon = carbon double bonds in the fatty acid chain and they are low melting solids at room temperature.

  • The 'long–chain fatty acids' can be unsaturated, with one or more C=C double bonds, and so forming unsaturated oils or fats (2nd diagram below of the triglyceride formed from oleic acid).

    • If there is just one C=C double bond in the fatty acid chain, it is known as a monounsaturated oil or fat (upper diagram).

    • If there are at least two C=C double bonds in the fatty acid chain it is called a polyunsaturated oil or fat exemplified by the polyunsaturated fatty acid chain, with three carbon = carbon double bonds of the unsaturation, in the lower diagram.

    • Plant oils are mainly unsaturated fats with one or more carbon = carbon double bonds in the fatty acid chain and they are usually thick (viscous) liquid oils at room temperature. That's why plant oils are hydrogenated, adding hydrogen to the double bonds, to make them less unsaturated and raise the melting point to produce spreads like margarine.

  • Advanced Chemistry Page Index and LinksSome sub–notes on Oil and Fat Structure: (health issues dealt with further down)

    • Most oils and fats have quite long fatty acid chain molecules in their molecular structure, which can be ...

      • Saturated, with no double carbon = carbon bonds, so no atoms can be added to the molecule, in this context they are referred to as saturated fat molecules.

      • Unsaturated, with one or more carbon = carbon double bonds in the three fatty acid parts of the molecule. In this context these molecules are referred to as monounsaturated or polyunsaturated vegetable oils.

        • Atoms like hydrogen can be added to these molecules via the reactive carbon = carbon double bond that can open up and accept two atoms to bond to.

    • Unsaturated oils will decolourise bromine water, a simple test for unsaturation.

      • This is because bromine atoms can add across the double bonds like any alkene molecule.

      • Ignoring the rest of the molecule, the reaction in the unsaturated part of the oil/fat molecule is ...

        • C=C + Br–Br ==> Br–C–C–Br

        • The oil is colourless or pale yellow and the much brighter brown/orange colour of the bromine disappears and the saturated product is virtually colourless.

    • Although much shorter than polymer molecules, oils and fats have the same ester linkages as perfume molecules and Terylene plastic, but with different units, food for thought!

    • They are not as big as polymer molecules, but a lot bigger than a single petrol or a simple sugar molecule.

    • There can be 1 to 3 different saturated or unsaturated fatty acid components, so lots of variation possible in structure of the oil or fat.

      • The two  diagrams just assume three molecules of the same 'fatty' acid.

    • Monounsaturated fats have one C=C double bond in the fatty acid chains, polyunsaturated fats usually have at least two C=C bonds in their molecular structure.

    • For the same molecular size in terms of carbon number, unsaturated fats have slightly lower intermolecular forces because the C=C double bond produces a kink in the carbon chain and they can't pack as closely together as the saturated molecules.

      • This gives unsaturated fats a lower melting point and so they tend to occur as e.g. runny vegetable oils rather than saturated low melting fatty solids from meat and dairy products.

    • However, this means these unsaturated oils are not as conveniently 'spreadable' as 'butter'.

      • To overcome this problem, 'margarine' was invented in which the runny vegetable oil is 'hardened' by hydrogenation to produce a higher melting spreadable solid.

        • The melting point is still low, but not to low to remain liquid at room temperature ie a margarine is a soft solid at room temperature and doesn't go too hard in the refrigerator and spreads easily.

        • So, large quantities of vegetable oils are hydrogenated for the food industry to convert them from runny oils into low melting soft solids that spread on bread etc. easily.

      • The vegetable oils are reacted with hydrogen gas at 60oC using a nickel catalyst (Ni).

      • These are called hydrogenated fats and have higher melting point than unsaturated vegetable oils, so they are a low melting solid at room temperature rather than the sticky–syrupy vegetable oil you might use is cooking and salad dressings.

      • This reaction adds hydrogen atoms to the double bonds making a more saturated and more 'spreadable' higher melting soft solid fat that we call 'margarine'.

      • Saturated means no double bond and unsaturated means double bond in this context.

      • The reaction for any carbon = carbon double bond,

        • >CH=CH< + H2 == Ni ==> –CH2–CH2

        • which is converting an unsaturated part of the molecule to a saturated structure.

        • This type of reaction is called hydrogenation – quite literally – addition of hydrogen.

        • The >CH=CH< represents the unsaturated part of the hydrocarbon chain parts in the oil molecule.

        • While the margarine is still liquid, the expensive nickel catalyst can be recovered from the margarine by filtration and reused.

        • On cooling down the solid margarine is formed, but still soft enough to spread easily.

        • Hence polyunsaturated vegetable oils are hydrogenated to make margarine.

        • The diagram above illustrates in a simplified way the hydrogenation of a monounsaturated fat (one double bond per fatty acid chain) to a fully saturated flat. Note that one hydrogen molecule is added to each double bond giving the balanced equation for hydrogenating vegetable oils to margarine.

        • In the case of margarine, made from polyunsaturated vegetable oil, the oil is only partially hydrogenated, thus reducing the number of C=C double bonds in the molecule, BUT not making a saturated fat and still raising the melting point above room temperature. If it was completely saturated it would be too hard to spread.

      • BUT it does mean that it is more like animal fat now but various blendes have been developed to suit your dietary needs or desires!

      • The hydrogenated oils are used as spreads and general baking like cakes, bread and pastries.

      • Technically, margarine is only partially hydrogenated because fully saturated fats would be too hard and difficult to spread, but if a high % of the double bonds are hydrogenated, the texture of the margarine is a bit like butter and the 'buttery effect' appeals to many consumers.

      • Instead of butter, margarine and other partially hydrogenated vegetable oils are used in processed foods because they are cheaper and gives food products a longer shelf–life.

      • Margarine and other 'spreadable' fats based on vegetable oils are quite a mixture of molecules known as an emulsion. A typical mixture might be

        • 14–21% saturated fats (triglycerides with almost no double bonds in the hydrocarbon chains)

        • 15–30% monounsaturates in which there is about one double bond per molecule.

        • 14–22% polyunsaturates which have more than one double bond per molecule.

        • In terms of melting points, the order is saturates > monounsaturates > polyunsaturates.

      • Sodium chloride and water ('salt' solution'), small amounts of protein and carbohydrate and whey or buttermilk are added to the fat/oil mixture together with an emulsifier.

      • To stop the salt solution separating out from the 'oily' fats an emulsifier is added, which keeps the aqueous salt solution dispersed in the fats or they would separate into two layers and affect the look and taste.

        • Incidentally the emulsifiers may be mono– or di–glycerides of fatty acids, that is molecules like the vegetable oils but only 1 or 2 fatty acids attached to the glycerol rather than 3, which leaves 2 or 1 –OH hydroxy groups on the glyceride molecule.

        • These emulsifying molecules have the bifunctional structure (see diagrams D and E1 below) because through the action of intermolecular forces they bind with both fats (via hydrocarbon chain, 'water hating' hydrophobic end of molecule) and bind with water too (via hydroxy group OH, the 'water loving' hydrophilic end of molecule). This double interaction with the oil/fat holds the emulsion or dispersion together and stopping the formation of two layers (aqueous and oil/fat).

        • In margarine or butter there will be far more of the oil/fat than water, but the diagram is just meant to give an idea of how an emulsion is stabilised. The diagram below is better representation of margarine with its emulsifying agent which is often monoglyceride or diglyceride esters of fatty acids. The hydrocarbon tails sticking out from the minute water globules, make the water compatible with the hydrogenated vegetable oils.

        • For a more general and wider description of emulsions see Aqueous solution chemistry

Examples of food labelling on 'spreads'

Spread 1 is made from vegetable fats and olive oil. The oil/fat analysis shows it is a mixture of saturates, monounsaturates and polyunsaturates. The total oil/fat is 59% by mass, adding all the rest up means there is about 38% water in this oil in water emulsion?


The labelling on this fat spread made from vegetable oil is packed with nutritional information. Apart from the oil/fat composition in spread 2 (assume similar in spread1) there added vitamins, salt, water, emulsifiers, flavourings etc. etc. In spread 2 there is, by mass, 14% saturated fats, 15.9% monounsaturated fats and 25.5% polyunsaturated fats.


  • Since fats and oils are important to our diet, there is the ever present danger of over–consumption (speaking as someone who loves chips and spicy crisps!).

    • So there are health and social, as well as 'molecular' issues to address!

    • Vegetable oils are an important source of energy and even vitamins like vitamin E in seed oils.

      • Vegetable oils are high calorie high energy food.

    • Vegetable oils contain essential fatty acids which are bodies need for certain metabolic processes.

    • So we need both oils and fats as sources of important essential fatty acids and energy.

    • We need both saturated and unsaturated fats or oils.

      • Animal fats tend to be saturated molecules and vegetable oils tend to be unsaturated fatty molecules.

      • The main sources of saturated fats are from meat and dairy products e.g. 'dripping', butter, lard from pork fat, blubber from whale fat, cod liver oil from fish, ghee butter oil.

      • The main sources of unsaturated fats are plant oils e.g. olive oil, walnut oil.

      • Animal fats are usually solids at room temperature, though with low melting points, but vegetable oils/fats tend to be liquids.

    • It is recommended that we do not overdo the fat intake but we do need both saturated and unsaturated fats.

      • Whatever fat or oil you use in cooking – food preparation, you are significantly increasing your calorie intake from these energy rich molecules and it doesn't matter the oil/fat is polyunsaturated, partially hydrogenated or fully saturated.

      • In general unsaturated fats are more healthy to consume than saturated fats and reduce the level of cholesterol in your bloodstream.

      • However, too much saturated fat raises cholesterol levels and is not too good for the heart – increased blood pressure and poor blood circulation from blocked arteries and heart disease can result from a diet high in saturated animal fats – but you do need some and eating saturated fats in moderation shouldn't be a problem.

    • Natural highly unsaturated vegetable oils like walnut oil, olive oil, sunflower oil etc. do tend to reduce cholesterol levels.

    • The consumption of trans fats, and animal fats in general, increases the risk of coronary heart disease by raising levels of LDL cholesterol and lowering levels of 'good' HDL cholesterol.

    • However even partially hydrogenated vegetable oils contain 'trans–fats' which are not supposed to be good for you, because they also tend to increase 'bad' cholesterol levels and decrease 'good' cholesterol levels in your blood stream, and therefore the risk of heart disease, so, eating lots of food containing margarine etc. is not good for you!

    • See also Aspects of diet, food additives and cooking chemistry

  • SOAP what is it? How is it made? This is another use of ESTERS

  • 'Traditional' soap is a product of the hydrolysis of fats from animals and vegetable oils from plants

    • 'Soapy' soaps (not modern detergents) are the sodium salts of long chain fatty acids formed by heating fatty oils with concentrated alkalis like sodium hydroxide or potassium hydroxide to hydrolyse them.

      • This is known as a saponification reaction and a typical equation is illustrated above and the general word equation quoted below.

      • vegetable oil/animal fat + sodium hydroxide ==> soap molecule + glycerol

    • Advanced Chemistry Page Index and LinksThis reaction breaks the fat molecule down into one glycerol molecule (a triol alcohol) and three sodium salts of the long chain carboxylic fatty acids that formed part of the original oil/fat ester.

    • Examples of long chain 'carboxylic acids, known as 'fatty acids', used to make soaps and detergents are shown below ... where they typically have 16 to 20 carbon atoms in the chain ...
      • ... with the diagrams of the organic molecules or ions involved
      • Diagram S1: The stearic acid molecule C17H35COOH or CH3(CH2)16COOH is a typical long chain fatty acid obtained from naturally occurring plant oils and used to make traditional soaps.
      • Diagram S2: The salt sodium stearate C17H35COONa+, formed when stearic acid is neutralised with sodium hydroxide is a typical soap molecule.
      • These salts are naturally formed on hydrolysing the triglyceride triesters i.e. when the oil or fat is boiled with sodium hydroxide solution (see equation above).
  • How do soaps and detergents work?
    • The diagram above represents the effect of mixing a soap/detergent with some clothes being washed.
      • This diagram illustrates the mechanism by which soaps wash oily/greasy clothes or surfaces.
      • The 'neutral' end of the soap molecule (hydrophobic, 'water hating') forms intermolecular bonds with the 'blob' of oil or grease, because they are compatible at the molecular level.
      • The other ionic negative end of the soap molecule (hydrophilic, 'water liking') forms intermolecular bonds with water (there would also be repulsion between the negative hydrophilic ends of the soap molecule.
      • The result is that the 'blob' of oil or grease is surrounded by a coating of soap/detergent and is dislodged from the fabric surface and dispersed into the washing water and hence can be washed away.
      • This mechanism applies to washing greasy dishes with 'washing-up-liquid' in the kitchen.
      • The washing process is further described and explained in more detail using the three diagrams below.
    • Most traditional soaps are actually ionic compounds which dissolve in water forming a long singly charged negative ion (anion), which is balanced by a singly charged metal ion e.g. a sodium ion, Na+.
    • So soap molecules have a negative ionic hydrophilic 'head' ('water liking'/'oil hating' end of molecule) and a hydrophobic 'tail' ('water hating'/'oil liking' end of molecule').
      • eg the stearate ion from the soap sodium stearate shown above.
      • When you shake soap with an oily/greasy material (washing clothes or scrubbing a surface), the oil/grease breaks up into tiny droplets or globules and removed from the surface to which they were attached. Why? ...
      • The hydrocarbon hydrophobic tail of the soap dissolves in the oil or grease globule and the negative head is on the surface of the globules/droplets but in contact with water.
        • The long 'hydrocarbon' hydrophobic tail can only interact at the molecular level with oil/grease ie is attracted to oil and grease (its due to intermolecular forces, a bit like dissolving).
        • The 'ionic' hydrophilic head can only interact with water and forms weak bonds with water, just like when ionic compounds readily dissolve in water.
        • Two negative hydrophilic heads cannot interact with each other and tend to repel each, but strongly interact with water.
        • In effect, the soap anions allow oil/grease and water to mix because the globules of oil/fat get a surface coating of the soap and the negative end sticks out into the water, and that end sort of dissolves in water.
        • Therefore the oil/grease blobs cannot remain attached to the fabric, or any other surface, and become dispersed in the washing water, then washed away.
        • This argument to any 'dirt' on any surface that soap can interact with.
        • A general name for these hydrophobic–hydrophilic structured molecules is surfactants and includes soaps, detergents and naturally occurring non–ionic molecules like lecithin found in egg yolk.
        • For more notes see Colloids, Emulsions, Paints and Pigments
  • Dry cleaning explained - no water involved!
    • Dry cleaning is a washing process that doesn't involve water, but uses other organic solvents.
    • These organic solvents are better than soaps/detergents in removing oil or grease stains from clothes or any surface e.g. cleaning a metal surface before treating it in some way.
    • These solvents will also remove stains that will not dissolve in water.
    • Unlike the complex action of soaps and detergents in 'water washing', these solvents will completely dissolve the oil, grease or stains.
    • So, why is it that these solvents have such useful and effective 'dissolving power'?
    • When the organic solvent interacts with oil/grease there are three lots intermolecular forces (weak intermolecular bonds) operating, which are e.g. for oil ...
      • solvent...solvent, solvent...oil and oil...oil
      • and these three forces are all comparable in strength,
      • therefore, with an excess of solvent molecules, the particles of the oil stain become surrounded by solvent molecules and bond to them,
      • so there is now a strong interaction between the oil molecules and solvent molecules because of the intermolecular forces,
      • the solvated oil particles are now little different that the solvent particles and are now very compatible with each other,
      • AND this is the same as a dissolving process and so all of the oil stain (and any other) is completely removed in a dissolving action.
    • -
  • Washing clothes at lower temperatures - biological detergents
    • Biological washing powders or biological detergents contain enzymes.
    • Enzymes are biological catalysts which help decompose, or break down, larger insoluble molecules (like food) into smaller soluble molecules which can easily be removed in the washing process.
    • Enzymes tend to work best at relatively low temperatures (30oC to 40oC), so using biological detergents to wash your clothes you can use a lower temperature for your washing cycle.
    • Using a lower temperatures gives you several advantages over using hotter water e.g.
      • (i) you save on your energy (electricity) bill,
      • (ii) you can wash more 'delicate' items of clothing and less chance of washing out fabric dyes.
    • It should be noted if you wanted to do your washing cycle at a higher temperature and still using biological detergents, well, its NOT a good idea.
      • These enzymes become denatured above 40oC and wouldn't be very effective!


GCSE/IGCSE/O Level Oil Products & Organic Chemistry INDEX PAGE

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Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier–foundation–level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder–higher–level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and (c) doc b 3 linked easy Oil Products gap–fill quiz worksheets

ALSO gap–fill ('word–fill') exercises originally written for ...

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b Worksheet gap–fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

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