This is a fantastic way to illustrate a chemical change by combining two common household ingredients used in cooking, together with some food colouring and detergent or shampoo.

The reaction between the ingredients causes carbon dioxide gas to be produced and coloured foam bubbles are formed very quickly and a frothy mass rises up and overflows like lava from a volcano.

Advance warning!
Vinegar is a weak acid and will sting if it splashes into your eyes.
In step 8 below you may get a surprise when you are adding the vinegar but you must take care not to drop the cup containing the vinegar or to pull your hand away suddenly since this could cause you to spill some of it!

Instructions

1. About one quarter fill the large cup with tepid water

2. Add one heaped teaspoon of bicarbonate of soda

3. Stir thoroughly for 5 minutes so as to dissolve as much of the bicarbonate as possible.

4. Add 3 or 4 drops of food colouring and stir.

5. Add a few drops of washing up liquid and stir well so that a slight froth is produced.

6. Place the cup in a plastic tray which will prevent any mess from going onto the table.

7. About half fill the small plastic cup with vinegar.

8. The final step! Be prepared! Pour the vinegar from the small cup into the large cup in one go and be prepared for a sudden reaction in which the coloured foam quickly rises up the cup and overflows into the tray.

Explanation

First of all we must remember that the food colouring and detergent have nothing to do with the ‘chemical change’ which is taking place. They simply add to the effect and make things easier to see!

What is bicarbonate of soda?

The proper chemical name is Sodium Hydrogen Carbonate

The chemical formula describing exactly what it contains is NaHCO₃

_{The formula tells us that each individual particle (molecule or ‘formula unit’) of bicarbonate of soda contains}

_{1 atom of Sodium Na
1 atom of Hydrogen H
1 atom of Carbon C
3 atoms of Oxygen O}

_{All of these atoms are stuck together or ‘bonded’ and there are a number of ways we can make them split apart and change their appearance or change them for good into something different.}

_{1. Add water}

_{When water is added and the mixture is stirred the bicarbonate of soda starts to split apart and divide itself evenly in the water. We call this ‘dissolving’. The amount of solid bicarbonate of soda gets less and less until eventually we can’t see it any more. The bicarbonate of soda is still there but no longer as solid particles. It is completely and evenly spread out in the water. If the water was allowed to dry up or ‘evaporate’ we would get back all of the white solid bicarbonate of soda that we started with. Dissolving something doesn’t change it into something new although it does look like it may have. Dissolving can be reversed by taking away the water so it is not a permanent change. It is a reversible change.}

2. Heat

If we heat solid bicarbonate of soda strongly enough it starts to produce carbon dioxide gas and water. These are lost into the air so the bicarbonate starts to loses mass. We could check this using some weighing scales. This is a permanent change and it can’t be reversed. The carbon dioxide and water have resulted from the breaking up or decomposition of the bicarbonate of soda molecules. Something new has been formed from the original material. The white solid left behind looks the same as the original sodium Hydrogen carbonate but in actual fact it is a new substance called sodium carbonate.

_{3. Adding an acid such as vinegar}

_{When vinegar is added to bicarbonate of soda a vigorous ‘reaction’ takes place. The two substances together combine then rearrange to make 3 new substances which can’t be converted back into their original form. As with heating, the newly formed substances include carbon dioxide gas and water. In addition to this a new substance which is part ‘soda’ and part vinegar is made and it is called sodium acetate. The ‘acetate’ part of the name comes from the vinegar which has a proper chemical name of ‘acetic acid’.}

_{When the acid and bicarbonate combine an irreversible change takes place. The final product is no longer an acid and the carbon dioxide which is produced as bubbles of gas is lost to the air. Again there is a weight loss as with heating and this is s sure sign of a permanent or irreversible change.}

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