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What's in Our Food?

In this worksheet, students will explore the chemical tests that can be used to identify different nutrients in our foods.

'What's in Our Food? ' worksheet

Key stage:  KS 3

Curriculum topic:  Biology: Structure and Function of Living Organisms

Curriculum subtopic:  Nutrition and Digestion

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

Food provides us with the energy to grow and move and helps keep all the parts of our body working properly.   

 

Runner

 

Marathon runners often have a meal high in carbohydrates the night before a race to provide them with the slow-releasing energy they need to run.

 

It's important for us to know, then, exactly what's in our foods so we can ensure we are getting the balanced diet that our bodies need. Below are a few tests that can be used to show if fats, proteins, starch or sugars are present in the foods we eat.

 

The Chemical Tests for Carbohydrates

 

There are two types of carbohydrate. One is starch and the other is glucose (a type of sugar).

 

The test for starch is using a chemical called IODINE SOLUTION. When added to something containing starch, iodine solution will turn from a red/brown colour to a blue/black colour.

The test for glucose is using a chemical called BENEDICT'S SOLUTION. When added to something containing glucose, Benedict’s solution will turn from blue to orange.  

 

The Chemical Test for Proteins

 

The test for protein is using a chemical called BIURET SOLUTION.  When added to something containing protein, Biuret solution will turn from a blue colour to a purple colour.  

 

The Chemical Test for Fats 

 

The test for fats is not quite as simple as the tests for carbohydrates and protein. It requires 2 steps:

 

Firstly, add the food to be tested to a small amount of ethanol (pure alcohol) and let it dissolve (the solution may need to be heated in order for it to dissolve fully. WE MUST NOT USE A BUNSEN BURNER OR A NAKED FLAME, because ethanol is highly flammable; therefore a water bath should be used.) 

 

Next, the dissolved food solution is added to a test tube of water.  If the solution remains colourless, there are no fats present. If a layer of cloudy white emulsion forms at the top, then there is fat in this type of food.

 

If necessary, read through those food tests again before you tackle this activity.

Complete the following sentence by writing the missing word in the box below:

 

There are two types of carbohydrates, starch and ____________ .

What chemical(s) would you use to test for carbohydrates? 

Iodine solution

Benedict's solution

Biuret solution

Ethanol

The presence of glucose will cause Benedict's solution will change from blue to what colour?

The night before a race, marathon runners normally have a meal high in which food group to provide them with the slow-releasing energy that they need to run?  

protein

fat

carbohydrates

vitamins

Match up the correct food type with the chemical used to test for it.   

Column A

Column B

starch
Iodine solution
protein
Benedict's solution
glucose
Biuret solution

Biuret solution is used to test for proteins.

 

Complete this sentence:

Column A

Column B

starch
Iodine solution
protein
Benedict's solution
glucose
Biuret solution

Ethanol is used in the 2 step process of testing for fats. The food substance being tested must be added to a small amount of ethanol and the ethanol must then be heated to allow the food to dissolve.  

 

Why must a Bunsen burner not be used to heat the ethanol?

The Bunsen burner will not heat the ethanol to a high enough temperature.

Ethanol is highly flammable.

Ethanol is a gas at room temperature.

Which of the following chemical solutions will change from red/brown to blue/black in the presence of starch.  

Iodine solution

Benedict's solution

Biuret solution

Ethanol

In the chemical test for fats, which of the following will indicate that presence of fat in the food being tested? 

a colour change from blue to orange

a colour change from blue to purple

a colour change from red/brown to blue/black

the formation of a milk like emulsion

A student wants to find out what food types are present in a sample of food. They test the sample with different chemicals and get the following results.  

 

Chemical Observation
Iodine no change
Benedict's solution Benedict's solution changed from blue to orange
Biuret solution no change
Ethanol A white (milk-like) emulsion formed 

 

From the above results, what food types are present in the food sample?

fats

proteins

starch

glucose

  • Question 1

Complete the following sentence by writing the missing word in the box below:

 

There are two types of carbohydrates, starch and ____________ .

CORRECT ANSWER
sugars
sugar
EDDIE SAYS
Sugars come in so many forms, like sucrose (table sugar), glucose, fruit sugar (fructose) and so on. Indeed, long starch molecules are made of long chains of sugar molecules. It is enzymes, like amylase, that break these chains up into their constituent sugar molecules.
  • Question 2

What chemical(s) would you use to test for carbohydrates? 

CORRECT ANSWER
Iodine solution
Benedict's solution
EDDIE SAYS
Iodine solution is used to test for starch. It will turn from red/brown to blue/black.
Benedict's solution is used to test for sugars. It will turn from blue to orange. Ethanol tests for fats/oils while the biuret test is for proteins.
  • Question 3

The presence of glucose will cause Benedict's solution will change from blue to what colour?

CORRECT ANSWER
orange
EDDIE SAYS
Benedict's solution will change from blue to orange in the presence of glucose. You will learn that glucose is an example of a 'reducing sugar' and the Benedict's test is for reducing sugars. Sucrose (table sugar) is not a reducing sugar and so the Benedict's solution stays blue with sucrose.
  • Question 4

The night before a race, marathon runners normally have a meal high in which food group to provide them with the slow-releasing energy that they need to run?  

CORRECT ANSWER
carbohydrates
EDDIE SAYS
Carbohydrates will provide long distance runners with slow-releasing energy throughout the race. For an energy burst, they can take a dextrose tablet (a pure form of sugar) or drink a high-energy drink, but starchy foods provide an energy store that their bodies can draw on over the race.
  • Question 5

Match up the correct food type with the chemical used to test for it.   

CORRECT ANSWER

Column A

Column B

starch
Iodine solution
protein
Biuret solution
glucose
Benedict's solution
EDDIE SAYS
In the same way that you nail the tests for gases like oxygen and hydrogen, you just need to nail these. Starch is easy: iodine solution turns blue/black. Start from there.
  • Question 6

Biuret solution is used to test for proteins.

 

Complete this sentence:

CORRECT ANSWER
EDDIE SAYS
Biuret solution will change from blue to purple in the presence of proteins.
  • Question 7

Ethanol is used in the 2 step process of testing for fats. The food substance being tested must be added to a small amount of ethanol and the ethanol must then be heated to allow the food to dissolve.  

 

Why must a Bunsen burner not be used to heat the ethanol?

CORRECT ANSWER
Ethanol is highly flammable.
EDDIE SAYS
Ethanol (which is pure alcohol) is highly flammable so must never be used near a naked flame. A hot water bath is used to raise its temperature instead.
  • Question 8

Which of the following chemical solutions will change from red/brown to blue/black in the presence of starch.  

CORRECT ANSWER
Iodine solution
EDDIE SAYS
Remember this test for starch: place iodine on a starchy food like potato, bread or pasta and the iodine will turn a definite blue/black colour.
  • Question 9

In the chemical test for fats, which of the following will indicate that presence of fat in the food being tested? 

CORRECT ANSWER
the formation of a milk like emulsion
EDDIE SAYS
If fat or oil is present in the food, it reacts with the ethanol so that when the mixture is added to water, something called an EMULSION forms - it's a new chemical that will not dissolve in the water, but remains suspended in it.
  • Question 10

A student wants to find out what food types are present in a sample of food. They test the sample with different chemicals and get the following results.  

 

Chemical Observation
Iodine no change
Benedict's solution Benedict's solution changed from blue to orange
Biuret solution no change
Ethanol A white (milk-like) emulsion formed 

 

From the above results, what food types are present in the food sample?

CORRECT ANSWER
fats
glucose
EDDIE SAYS
So what have we got? No change with the iodine or the biuret tests, so that means that neither starch nor protein are present in the food being tested. However, the ethanol test gave a milky white emulsion (a positive result for fats) and the Benedict's test gave an orange colour (a positive result for glucose). Looks like some sort of sweet food, like nougat.
---- OR ----

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