SPM Chemistry Form 5 Notes



~Soaps and detergents are typically considered by many to be the same, due to both being surfactants which are also washing compounds. However, they are very different as soaps are made of natural material while detergents are more synthetic. The effects of soap as a washing compound is negatively affected when they react with minerals in water (especially hard water), while detergents are less affected by minerals in water. Nonetheless, soaps are said to be more health and environmental-friendly as compared to detergent.


~Soap = sodium salts (RCOO-Na+) / potassium salts (RCOO-K+) of long-chain fatty acids
~Alkalis react with fatty acids –> soap
~Fatty acids = animal fats or vegetable oils
~Soap preparation: hydrolysing fats or oils under alkaline condition
~Saponification – alkaline hydrolysis
~Saponification process – boiling fats / oils with concentrated sodium hydroxide solution or concentrated potassium hydroxide solution to produce glycerol + salts of fatty acids (soap)
~Saponification steps: 1. Hydrolysis of fats / oils –> glycerol + fatty acids. 2. Neutralisation of fatty acids + concentrated alkali –> soap + water
~Overall equation: Fats / oils + alkali –> glycerol + soap
~Sodium chloride – reduces the solubility of soap in water

Liquid soapSolid soap
Sodium hydroxide + saturated oils (coconut oil)Potassium hydroxide + unsaturated oil (sunflower oil / olive oil)
Uses: bathing purposesUses: shaving cream


~Detergent = sodium salt of sulphonic acid
~Alkalis react with sulphonic acid –> detergent
~Common detergent: sodium alkyl sulphate & sodium alkylbenzene sulphonate.
~Detergent preparation: 2 processes – 1. Sulphonation. 2. Neutralisation.
~Sulphonation: Petroleum fraction (long chain hydrocarbon compounds) converted into sulphonic acid with the present of concentration sulphuric acid. Water molecule is removed.
~Neutralisation: sulphonic acid neutralised with sodium hydroxide to form detergent (water molecule is formed).
~Other type of detergents: 1. Anionic detergent (dishwashing liquid detergent). 2. Cationic detergent (hair conditioner). 3. Non-ionic detergent (car washing detergent).

Cleansing action of soap and detergent

~Soap and detergent are surfactants. Surfactants are organic compounds that lower the surface tension of a liquid.
~Surfactants act as emulsifiers and foaming agents.
~Anion soap consists of carboxylate ion (hydrophilic = soluble in water) and long hydrocarbon tail (hydrophobic = soluble in grease /oils).
~Anion detergent consists of sulphate ion / sulphonate ion (hydrophilic = soluable in water) and long hydrocarbon tail (hydrophobic = soluble in grease / oils).

The action process

1.Soap or detergent is added to water.
2.This has lower the surface tension of water and increases the wetting ability of water.
3.Hydrophilic part of the anion dissolves in water.
4.Hydrophobic part dissolves in grease.
5.Scrubbing helps to pull the grease free and break the grease into small droplets.
6.These droplets do not coagulate and redeposit on the surface of the cloth. It is because therepulsion between negative charges on the surface.
7.The droplets are suspended in water forming an emulsion.
8.Foam produces to float the emulsion.
9.Rinsing helps to remove these droplets.

The effectiveness of the cleansing action of soap and detergent

Hard water (contain calcium & magnesium ion)Form an insoluble precipitate  = soap scum (formation of soap scum causes wastage of soap)Detergent is effective cleaner. Form soluble substances = calcium or magnesium salts (do not form scum)
Soft water (do not contain calcium & magnesium ion)Soap is effective cleaner.Detergent is effective cleaner.
Acidic water (contain hydrogen ion)Form insoluble long-chain fatty acids (reduces the amount of soap used for cleaning)Detergent is effective cleaner. Form soluble substances (Do not form scum)
EnvironmentSoap is biodegradableSoap is non-biodegradable

Additives in detergent

Biological enzymeRemove protein stainsAmylase, lipase & protease
Foam control agentControl foaming in detergentSilicones
BuilderEnhance the surfactant efficiencySodium phosphate
FillerMake the solid detergent dry and enable the liquid detergent to be poured easilySodium silicate
FragranceAdd fragrance-
Optical brightenerAdd brightness by convert UV radiation to blue light and whitenessFluorescent dye
Stabilising agentLower the production of foamSilicones
Suspension agentPrevent the dirt particles removed from redepositing onto cleaned fabricsSodium carboxymethylcellulose
Whitening agentBleach stainsSodium hypochlorite

Food Additives

~Food additive = a natural / synthetic substance that is added to food to
prevent spoilage
~extend food storage life
~improve food appearance
~improve food taste
~improve food texture

A. Preservatives

slow down / prevent the growth of microorganisms / bacteria / fungi
extend food storage life

PreservativesFunctionsExampleSide effect
SaltDraws the water out of the cells of microorganisms. Retards the growth of microorganisms.Salted vegetablesIncrease the risk of cardiovascular diseases
SugarDraws the water out of the cells of microorganisms. Retards the growth of microorganisms.Fruit jamTooth decay, diabetes and obesity
VinegarInhibits the growth of microorganisms.Pickled cucumber
Sodium nitrite / sodium nitrateSlows down the growth of microorganisms. Stabilise red colour in meats.SausageCarcinogenic
Benzoic acid / sodium benzoateSlows down the growth of microorganisms.Tomato sauce
Sulphur dioxideSlows down the growth of microorganisms.Grape juiceAsthma and allergies
*Alternative ways: freezing and refrigeration

Food Additives

B. Antioxidant

~prevent oxidation (causes rancid fats and brown fruits)

AntioxidantFunctionsExampleSide effect
Vitamin EPrevent oils from turns rancidPalm oil & sunflower oilNo
Vitamin CPreserve the colour of fruit juice and the formation of nitrosaminesFruit juice & cured meatNo
Sodium citrateStop rancidity in fatsIce-cream-
BHA (Butylated hydroxyanisole) BHT (Butylated hydroxytoluene)Retard rancidity in fats, oils and oil-containing foodsMargarine & cerealCarcinogenic

Food Additives

C. Flavouring

~improve the taste
~improve the smell
~restore taste loss due to food process

FlavouringFunctionsExampleSide effect
MSG (monosodium glutamate)Bring out the flavorSalad dressing & canned soupHeadache, thirsty, nausea & chest pain.
Aspartame (non-sugar sweetener)Sweeten food (about 180 – 200 times than sugar)Soft drinks & diet drinksIncrease the risk of leukaemia, cancer and neurological problems
Synthetic essences (esters)Produce artificial flavours = natural flavour. Cheaper to use than the real fruitsMethyl butanoate (apple flavour) & octyl ethanoate (orange flavour)-

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