The importance of structure: chocolate

This activity would be a good introduction to the topic of structure and bonding. It emphasises how important it is to know about the structure of a substance and demonstrates that structure can make a big difference to the properties of a material. The concept of polymorphism is introduced and can be built on later. The activity is fun to do, interesting and the taste tests are always popular.

Prior knowledge required

Students need to know about changes of state and be able to interpret graphs showing changes of state.

Equipment required

Chocolate to eat:

• At least two squares of milk chocolate per student.
• Half needs to be melted and rehardened first.
• Take a whole chocolate bar (the ones that are fully wrapped in one sealed wrapper are best).
• Put it somewhere warm (such as on a radiator) and allow it to melt.
• Once it has melted, put it in a refrigerator (not the one where the chemicals are stored) to harden quickly.
• Once it has set, remove it and allow it to return to room temperature prior to the lesson.
• The remaining chocolate should be of the same make and type but simply stored at room temperature.

Practical work – for each pair or group of students:

• 1 square of milk chocolate
• 1 square of the same type of chocolate which has been pre-melted and rehardened (see above) •
• boiling tubes
• Beaker, 250 cm³ or access to a hot water bath
• Kettle (for boiling water)
• Thermometer
• Timer.

Health, safety and technical notes

• Read our standard health and safety guidance
• Students should not eat in a laboratory.
• Check whether any students are diabetic or have other disorders that preclude them eating chocolate.
• Boiling water can cause scalding.
• Warn students to take care.

Suggested lesson plan

The introduction must be carried out away from the laboratory because students need to eat the chocolate samples.

Give each student two pieces of milk chocolate, one from an ordinary chocolate bar and one from a bar of the same kind which has previously been melted and quickly re-hardened.

Students should note the differences that they can see, try snapping the pieces and note what happens then eat the chocolate and notice any differences in taste and texture. This should not be a blind trial – they should know which piece is which.

Once they have eaten the chocolate they can carry out the rest of the activity in a laboratory.

They should be warned not to eat any more of the chocolate during the lesson.

Discuss the differences between the two pieces of chocolate.

Emphasise to students that the chemical composition of the chocolate that was melted is unchanged because it was heated and cooled within the wrapper.

Nothing has been added or removed but the chocolate has clearly changed.

The way the atoms are arranged and the structure of the components have changed.

Ask students how chemists find out what a substance is and what its structure is like.

Explain that they will see how scientists at CCLRC in Harwell, Oxfordshire found out about the structure of chocolate, then they will plan and carry out an experiment for themselves.

Students work through the information on the student sheet and attempt the first few questions.

Students plan and carry out the practical work. A set of clue cards is included in this resource to assist students who struggle with the planning aspect of the activity. Photocopy and cut out the clue cards before the lesson.

Students should take one at a time and think about how the information helps them to plan an experiment to find out whether chocolate samples contain Form V cocoa butter. A few blank cards have been provided should you wish to create your own clues.

They should aim to use as few clues as they can when they design their experiment.

Extension

The pre-melted chocolate has a lower melting point than the other sample. If the chocolate is liquid for longer while it is in the mouth then it is more likely that the volatile flavour compounds will be released and will reach the mouth and nose.

If more flavour compound molecules are released, a greater sensation of flavour is achieved.

You may wish to discuss why this form of chocolate is not used instead. The answer is that it does not rate as well in terms of texture, ‘mouth feel’ and ‘snap.’