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A Cup of hot chocolate - MyScience [archived]


ActivitiesTry This at Home - Dec 15, 2014

A Cup of hot chocolate

Although playing outdoors in the cold can be fun, a cup of hot chocolate crowns any chilling winter day. Here’s a neat little experiment for studying the solubility of cocoa powder in water.

Photo: Arnold Gatilao, Creative Commons

Some hot chocolate is perfect for a day like this!

Have you ever noticed that some cocoa powder remains in the cup after drinking it. Why does cocoa powder behave like this?

The cocoa powder used to make hot chocolate originates from the fruit of the cocoa tree. The harvested cocoa pods are sliced open, then they undergo fermentation which is a process where the pods and beans are stored in a warm and moist environment for a few days.

When stored like this the cocoa pods fall apart and the cocoa beans in them become exposed to air. During this process yeast, lactic acid bacteria and other microbes cause chemical reactions in the cocoa beans giving them more flavors. After the fermentation the cocoa beans are collected and dried. When they contain about 7% of moisture microbes and yeasts will no longer be able to act in the beans.

The dried beans are transported to chocolate factories around the world.

In the factories the beans are roasted and peeled. The shells are removed and the nibs are ground into a chocolate liquor.

Over 50% of the nutrients in cocoa beans are fats. Finely ground solid parts are easily mixed into a smooth liquid called cocoa liquor. The liquor can be used as an ingredient in chocolate or processed further by pressing the chocolate liquor through a fine fabric.

Cocoa butter is also an ingredient in chocolate production whereas the solid separated from the butter is dried and ground to make cocoa powder, the very same powder we use to make a cup of hot chocolate.

Most people have tried making hot chocolate by mixing cocoa powder with water or milk. But does the powder dissolve in the liquid or does it form a heterogeneous mixture that will slowly separate when the mixing stops?

Solubility might be difficult to determine in the kitchen but a simple test to roughly estimate whether or not cocoa powder is soluble in water can be done.

Samples of cocoa powder. Photo: F_A, Commons.

Experiment! Test the solubility of cocoa powder

You will need: a small pot, spoons, a glass or a plate, filter paper (e.g. coffee filter), a funnel or the filter basket from a coffee machine, water, cocoa powder

1. Boil some water and cocoa powder in the pot for about five minutes. You can use about 1 teaspoon of cocoa powder in 1dl of water.

2. While the water is boiling, place the filter paper in your funnel. Filter the hot water-cocoa powder mixture and gather the filtrated liquid in a glass or on a plate. You have used more cocoa powder than the water can dissolve and will have some cocoa powder left in the filter paper. Anything dissolved in water will pass through the filter paper with the water. Be careful not to burn your fingers! The filter paper and funnel can be as hot as the water you recently filtered through them.

3. The solution in your glass or on the plate has dissolved as much as possible from the cocoa powder and is therefore called a saturated solution. Solubility of different substances is temperature-dependent and so the amount of soluble cocoa powder will change when the liquid cools down.

Do you notice any change when the liquid has cooled to room temperature?

4. Leave the glass or the plate in an open place, e.g. on the table, until the water has evaporated. This might take a day or two. When the water has evaporated all you have left are the remains of the cocoa powder that were dissolved in water. Describe your findings!

Pure cocoa powder mainly consists of proteins (24%) and fat (21%). Also some starch (12%), fibres (11%) and small amounts of vitamins and other trace elements are present.

You can try to search for the solubility of these substances and try to figure out which parts of the cocoa powder most likely dissolve in water. Also check the information on the package of your cocoa powder to find out whether the powder you used contained some other substances that might be soluble in water.

In a laboratory it is also possible to determine more exactly how large quantities of cocoa powder are dissolved in water in comparison to another compound like table salt.

There are many different ways of making hot chocolate. Here you will have the instructions to a chili- version that will have a nice burning aftertaste.

Hot Chocolate with chili

For two small cups of hot chocolate:

  • 3 dl water
  • ¼ fresh chili
  • 4 teaspoons cocoa powder
  • ½ teaspoon cinnamon powder
  • a few drops of vanilla extract
  • ½-1 teaspoon of honey
  • 70g dark chocolate

Cut a piece of chilli, approximately ¼ of it will give a nice aftertaste. Bring to a boil the water and chili in a small pot. Add cocoa powder, cinnamon and honey. Stir till smooth. Let brew for a few minutes stirring every now and then. Remove the piece of chili. If you would like a milder taste of chilli you can remove the chilli earlier while brewing. The longer the chilli brews the hotter the taste will be. Remove from the stove and add the chocolate. Stir till smooth and pour into cups. Let the hot chocolate cool a bit before serving.

Read more aout the chemistry of chocolate:
The World Atlas of Chocolate.

Katariina Tammi is a soon-to-be teacher in chemistry and mathematics. Besides studying she is also as an instructor in the Gadolin laboratory and actively participates in different scouting and guiding activities.