Uss module 5 chpt 7 Food Chemistry

Module 5:
Food Matters
Chapter 7 Food Chemistry
1© Copyright Star Publishing Pte Ltd

7.1 How are substances extracted and
separated from mixtures?
7.2 What is a chemical reaction and what are
its
applications in daily life?
7.3 What are acids and how can they be
identified?
7.4 What are the properties of food acids?
7.5 What is neutralisation and what are its
7.6 What are the effects of untreated
wastewater

Objectives
 Explain that substances can be extracted
and separated through the following
techniques:
(i) dissolving (ii) filtration (iii) evaporation
(iv) distillation and (v) paper
chromatography
 Describe the applications of the various
separation techniques in homes and
food industries
 Use a measuring cylinder to measure
volume of liquids / solids 3© Copyright Star Publishing Pte Ltd

Separating Mixtures
 A mixture is one that contains
two or more different
substances that are not
chemically combined
together.
 The food we eat usually consists
of mixtures of solids and
liquids.
 For example, orange juice is
made up of orange pulps, sugars,
water and many other substances
including Vitamin C.
 The components in it can be
separated using separation
techniques.
© Copyright Star Publishing Pte Ltd 4
curry gravy is a rich
mixture of spices
a home purification system
for clean drinking water

Separating Mixtures
 Some separation
techniques are:
(a) Dissolving
(b) Filtration
(c) Evaporation
(d) Distillation
(e) Paper
chromatography
sugar cane juice is extracted
from sugar cane stems

Dissolving
 A mixture of two solids can be separated by using a
solvent which dissolves only one of them.
 For example, a mixture of salt and sand can be
separated by adding water to the mixture. Salt will
dissolve in water but sand will not.
 We can obtain sand by filtration of the mixture and salt
from evaporation of the filtrate.
separating a mixture of salt and sand

Dissolving of solutes
 Dissolving of solutes is commonly
used at home.
 Some examples include:
(a) Cooking of soup
(b) Drinks sweetener
(c) Cleaning of the household

Cooking of soup
 Soup stock (flavourings) either in solid
or liquid forms, is dissolved in water to
flavour the soup.
soup stock is added to water to make soup

Drinks sweetener
 Sugar is dissolved in drinks (e.g.
coffee and tea) to make the taste
better.
sugar is added to water when making tea

Cleaning of the household
 Detergent is dissolved in water to remove
dirt from clothes, and clean oily dishes and
the floor.
detergent is dissolved in water

Dissolving in the food
industries
 (a) Water is added in the
production of soft drinks,
to dissolve the mixture
of sweeteners,
colourings and
flavourings.
 Carbon dioxide gas is
also dissolved in the
drink to create the “fizzy”
effect which improves
the taste of the drink.
soft drinks contain a
mixture of sweetened
flavourings and colourings

Dissolving in the food
industries(b) In the production of
perfumes, flower
extracts such as
lavender and rose
are dissolved in
alcohol to
manufacture
perfumes.
(c) Chlorine is
dissolved in our
drinking water and
swimming pools to
kill harmful bacteria.
perfumes are mixtures of
natural plant extracts
bacteria in swimming pools are destroyed
when chlorine is added

Filtration
 Filtration is used to separate a
mixture of an insoluble solid from
a liquid.
 In the diagram on the right, the
mixture is poured into a filter
funnel with a filter paper.
 The filter paper allows the liquid
to pass through it but not the
insoluble solid particles of the
mixture.
 In this case, sand left on the filter
paper is called the residue and the
water that passes through it and
collected in the beaker is called
the filtrate.
filtration experimental set-up

Filtration
 At home, filtration is
used in air-
conditioners to filter
air that may contain
dust and smoke
particles.
 When cooking pasta,
the fastest and easiest
way to separate the
pasta from the boiling
water is to pour the
contents of the pot into
a strainer. This
separates the pasta
from the water. © Copyright Star Publishing Pte Ltd 14
a strainer is
used to separate the
pasta from water

Filtration
 A cup of tea can be made by using a
strainer to prevent tea leaves from entering
the cup.
filtering tea from insoluble
tea leaves

A simple filtration system
Activity Book Link
Activity 7.2
Water Purification

Simple filtration system
 A simple filtration system can be set up to
purify water.
 Impurities are removed by passing the
water through a bed of sand, gravel and
pebbles.
 The filtered water can then undergo further
stages (e.g. chlorine and UV treatment) to
improve the water quality to be fit for
drinking.

Evaporation
 Evaporation is used to
separate a dissolved
solid from a liquid. The
dissolved solid must not
decompose when heated.
 For example, salt can be
obtained from salt solution
by evaporation.
 Solid salt will be left
behind in the evaporating
dish.
water in a salt solution
evaporates, leaving
the salt crystals behind

Decomposition of sugar
 Sugar cannot be separated from sugar
solution by evaporation because it
decomposes and turns black upon
heating.
sugar blackens upon heating

Obtaining salt by evaporation
 Evaporation is used in salt mines.
When temperatures are high, the salt
water in the salt pans are evaporated
leaving the salt behind. The salt is then
piled up to dry.
piles of salt in salt pans
Activity Book Link
Activity 7.3
Making Table Salt

Condensation
 When the water boils in the kettle, it changes
into steam. When the steam touches a cold
surface, e.g. metal pot or spoon, it cools down
and changes into water.
 The change from a gas to a liquid is called
condensation.
steam from boiling water condenses on
the lid of the container

Distillation
 Distillation is used to separate a liquid from a
solution. It can also be used to separate a liquid from
a mixture of liquids having different boiling points.
distillation experimental setup

Distillation
 Pure water can be obtained from
seawater using distillation.
 Distillation can also be used to
separate a mixture of ethanol and
water.
 Ethanol will be distilled first from the
mixture because of its lower boiling
point 78°C) as compared to water
(100°C).

Process of distillation
Activity Book Link
Activity 7.4
Simple Distillation

Chromatography
 Chromatography is a method of using a solvent to
separate a mixture into its components.
 Food colourings are often used to improve the
appearance of our food and drinks. Food colourings
are usually a mixture of coloured dyes.
 Chromatography can be used to test if the dyes in food
colourings are safe for consumption. Chromatography
can also be used to detect banned drugs in urine
samples

Paper chromatography
(a) A small drop of ink is placed on one end of the
chromatography paper.
(b) When this end of the chromatography paper
is soaked in a solvent, the solvent travels up
the paper, carrying with it the dyes in the ink.
separating ink using chromatography
Activity Book Link
Activity 7.5
Investigating the
Components of
Food Dyes

Paper chromatography
(c) There are two coloured
spots formed on the
chromatogram. This shows
that the ink sample is a
mixture and it contains two
different coloured dyes.
(d) The dyes that are more
soluble will move faster
and further up the
chromatogram whereas the
dyes that are less soluble
will move slower and form
spots at lower
positions.
different coloured dyes
appear on the
chromatogram after
the ink has dissolved in
the solvent

7.2 What is a chemical reaction and what
are its
identified?
wastewater
on the environment?

Objectives
 Describe a chemical reaction as a
process that leads to the formation of
new products
 Give examples of everyday changes that
involve chemical reactions : (i) decaying
of food (ii) burning (iii) rusting (iv)
cooking of food
 Give examples of everyday situations in
slowing down / preventing chemical
reactions (e.g. Preserving food, reducing
amount of heat, fuel and air supply for
burning, tin plating against rusting)

What is a chemical reaction?
 Chemical reactions happen around us
and even inside us.
 Digestion of food is an example of a
chemical reaction and is an important
process before we can derive energy
from food.
 All chemical reactions lead to new
products being formed.

Chemical reactions in everyday life

Ways to slow down chemical reactions

its
identified?
wastewater
on the environment?

Objectives
 Describe the properties of acids by their
taste and effects on litmus paper and
universal indicator
 Describe acidity, neutrality and alkalinity
in terms of the pH scale (whole numbers
only)
 Measure the pH of various common
household substances and food using
homemade pH indicator, litmus paper,
universal indicator and a pH meter

Properties of Acids
 Acids are present in
everyday items. Acids can
be identified by the
following properties:
(a) Sour taste
Lemons, oranges and
grapefruits contain
an acid called citric acid
which gives them their sour
taste. Vinegar tastes sour
because it contains
ethanoic acid.
foods containing acids

Properties of Acids
(b) Turns blue litmus paper red
When both red and blue litmus papers are placed in an
acidic solution (e.g. lemon juice), the blue litmus
paper turns red, while the red litmus paper remains
red.
blue litmus turns red in acid while red
litmus paper remains unchanged

Properties of Acids
(c) Acids also produce colour change in universal
indicator. The table below shows the colour
change of the universal indicator when tested with
some sample substances which are acidic.

Colour changes of the universal indicator in
different acid conditions

The pH Scale
 pH is a measure of the strength of an
acidic or alkaline solution.
 The pH scale is a range of numbers
from 0 to 14 that shows how acidic,
alkaline or neutral a substance is.
 Neutral solutions have a pH of 7.
 Acidic solutions have pH values of
less than 7.

The pH Scale
 A solution with a lower pH is more acidic
than one with a higher pH.
 Alkalis have pH values of more than 7.
 A solution with a higher pH is more alkaline
than one with a lower pH.

The pH scale
Activity Book Link
Activity 7.6
Testing for Acids

Indicators
 Indicators can be used to test for the
presence of acids and alkalis.
 An indicator will change colour
depending on whether the solution
tested is acidic or alkaline.
 There are many types of indicators
commonly used in the laboratory to
indicate the pH of solutions.

Litmus indicator
 The commonly used litmus
indicators are the red and blue
litmus.
 Litmus is a dye obtained from
plants and can be used as a
solution or in paper form.
 Blue litmus turn red in acidic
solutions, and red litmus turn
blue in alkaline solutions.
 In neutral solutions, both the red
and blue litmus will not change
colour.
litmus solution and
litmus paper

Natural indicators
 Some flowers, vegetables and berries
can be used as natural indicators.
 For example, the coloured juice
extracted from the red cabbage turns
pink when tested with acidic
substances and green when tested with
alkaline substances.
 Other examples include extracts from
grape peels and orchid flowers.

Natural indicators
red cabbage juice is a natural indicator
as it changes colour in different pH
berries, grapes and are natural indicators
orchid flowers are natural
indicators

Universal indicator
 The universal indicator is a mixture of
several different types of indicators. It can
show a variety of colours depending on how
acidic or alkaline a substance is.
 Each colour corresponds to a pH value. The
universal indicator is used in the form of a
solution or a paper.
 The pH of the acid of or alkali can be
determined by comparing the colour of the
universal indicator against the colour
chart.

Universal indicator solution
and universal indicator paper

pH meter
 A pH meter is an instrument used to
measure the pH of a substance accurately.
 A pH meter consists of a pH probe which is
connected to a datalogger.
 A pH probe is placed in a solution to
determine its pH value and the pH value of
the solution is recorded on the datalogger.
 Among the four indicators, the pH meter
gives the most accurate pH reading of a
solution.

pH meter can measure an
accurate pH reading of a solution

its
identified?
wastewater

Objectives
 Describe the characteristic properties
of food acids on the following items
that are commonly found at home
(i) metals ( e.g. cooking utensils )
(ii) bases ( e.g. antacids )
(iii) carbonates ( e.g. baking soda,
effervescent Vitamin C tablets)

Reaction of Food Acids with Metal
Utensils
 Some common examples of acids found in
food are lemon and orange juice (citric
acid), tea (tannic acid) and soft drinks
(carbonic acid).
lemons contain citric acid
soft drinks contain carbonic acid

Acid reaction with metals
 Acids react with some metals to produce a salt
and hydrogen gas.
 For example, when some magnesium strips are
added to a test tube of lemon juice, bubbles
can be observed to form on the magnesium
strips.
 The bubbles on the magnesium strips are
hydrogen gas.
 The presence of hydrogen gas can be tested
using a lighted splint. Hydrogen gas will
extinguish a lighted splint with a “pop” sound.

Testing for hydrogen gas

Acid reaction with metals
 The general word equation to represent the reaction of
a metal with acid is:
acid + metal  salt + hydrogen gas
 Most cooking utensils are made of metal.
 Over time, these utensils may become rusty. The rust
can be removed by soaking the rusty utensils in
vinegar or rubbing them with lemon slices.
 The acid in the vinegar or lemon will react with the
rust to form a soluble substance that can be easily
washed off.
rusty utensil

Reaction of Food Acids with Carbonates
 Acids react with carbonates to give off
carbon dioxide gas.
 Some examples of carbonates found in or
daily life are baking powder, egg shells,
Vitamin C tablets and sea shells.
 For example, when some egg shells are
added to vinegar, bubbles of gas can be
observed to form on the egg shells.
 The bubbles formed are carbon dioxide
gas.

Testing for carbon dioxide
 The presence of carbon dioxide gas can be
tested by bubbling it into limewater.
 If carbon dioxide gas is present, the limewater
will turn chalky.

 Vitamin C tablets consist
of an acid and a
carbonate. When the
tablet is added to water, the
acid and carbonate
dissolve and react to
produce carbon dioxide
gas which results in the
fizzy bubbling effect
 Baking powder contains
sodium bicarbonate
(baking soda) and an acid.
During baking, the baking
soda and the acid present
in the baking powder react
to produce carbon dioxide
gas. This gas helps to
raise the cake and makes
it soft.

 The general word equation to represent the
reaction of a carbonate with acid is:
acid + carbonate  salt + carbon dioxide
+ water

Reaction of Food Acids with Bases
 Acids react with bases to produce a salt and
water.
 Bases dissolve in water to form alkalis. For
example, our stomach produces an acid which
helps us in the digestion of food.
 At times if we eat too much and too fast, the acid
may flow backwards into our oesophagus.
 We may feel a burning sensation. This is called
acid reflux or heartburn.

Backflow of stomach acid into
oesophagus causes heart burn

Antacids
 If we do not have regular
meals, the acids produced in
our empty stomach may
cause us to suffer from
gastric pains.
 Over time, this may cause
damage to the inner walls of
the stomach.
 Antacids, which contain a
base, can be used to
neutralise the excess acid.
This process is called
neutralisation.
antacids neutralise
stomach acid
Activity Book Link
Activity 7.7
Properties of Acids

its
identified?
wastewater

Objectives
 State that neutralisation takes place
when an acid reacts with a base and
the products are salt and water only
 State some applications of
neutralisation (e.g. action of tooth
paste, fabric softener and hair
conditioner, controlling of pH in soil,
neutralising industrial wastes)
 Relate pH within the mouth to its effect
on dental health

Neutralisation
 Neutralisation occurs when an acid reacts with a base
to produce a salt and water only.
 The general word equation to represent neutralisation
is:
acid + base → salt + water
 For example, mixing sodium hydroxide and
hydrochloric acid produces two new substances,
sodium chloride and water.
 The word equation is:

Neutralisation
 If sulfuric acid is used, the word equation
becomes:
 Sodium chloride and sodium sulfate are salts.
 Neutralisation reactions are important because
there are many useful daily applications for
them.

Useful applications of neutralisation reactions in
everyday life

Useful applications of neutralisation reactions
in everyday life

Useful applications of neutralisation reactions
in everyday life
Activity Book Link
Activity 7.8
Neutralising
Chemical Waste

7.2 What is a chemical reaction and what
are its applications in daily life?
identified?
wastewater on the environment?

Objectives
 State some of the effects of phosphate
detergents (e.g. increase growth of
algae and water weeds) and acidic
wastewater on the rivers and seas
 Understand the use of microbes in
sewage plants to treat the wastewater
before they are released to the rivers
and seas

Effects of Untreated Sewage on the Environment

What are microbes and their uses?
 Microbes such as bacteria are nature’s “recyclers”.
 They can break down bodies of dead animals and
plants into simpler substances.
 Bacteria are very useful in the treatment of waste
water because they produce enzymes to break down
the waste substances into smaller and simpler
substances.
 The bacteria population is also well regulated as they
grow quickly when there is a lot of “food” for them.
 When the “food” supplies decrease, the bacteria
population will decrease rapidly.

Microbes and their uses
magnified view of the
bacteria
scientists analysing bacterial colonies
multiplying on “waste materials” as
their source of food

Sewage treatment
 In sewage treatment plants, microbes such as aerobic bacteria are
used to decompose the waste sewage material.

Uss module 5 chpt 7 Food Chemistry

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