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Skip to 0 minutes and 6 seconds Penicillins are sometimes used in combination with other medicines. For example, for people who have peptic ulcer disease and are known to be infected with a bacteria called Helicobacter pylori, at least two antibiotics are used. This is in addition to medicines, which are used to reduce the acid secretion in the stomach and a bismuth-containing compound used to protect the stomach lining from acid. This so-called triple therapy is complicated because it involves taking as many as 20 pills a day. To reduce the number of tablets a patient has to take, sometimes a pill can contain more than one active substance, such as antacid tablets.

Skip to 0 minutes and 46 seconds Alka-Seltzer is a medicine that works as both a pain reliever and an antacid - it relieves minor aches and pains, an upset stomach, headaches, and indigestion. The pain-relieving agent is aspirin while the antacid that helps neutralise stomach acidity, which can cause heartburn, is formed from sodium bicarbonate. Sodium bicarbonate is commonly called baking soda. The tablets also contain citric acid, a weak acid that adds flavour - interestingly, as we will see it also provides an important source of hydrogen ions. To take the Alka-Seltzer tablets, they first need to be dissolved in water.

Skip to 1 minute and 24 seconds In the solid tablet the sodium bicarbonate and citric acid do not react, but when placed in water they react in an acid-base neutralisation reaction, producing sodium citrate, water and lots of bubbles of carbon dioxide - hence the fizzing. It is the sodium citrate, which acts as the antacid, when the mixture of Alka-Seltzer and water is ingested. In this activity we will make use of this reaction and the release of carbon dioxide to form a colourful lava lamp. Adding a tablet of Alka-Seltzer to a mixture of vegetable or olive oil with a smaller amount of water, in a bottle, produces the desired effect.

Skip to 2 minutes and 3 seconds The bubbles of carbon dioxide in the water make buoyant blobs that float in the vegetable oil - when they reach the surface, the bubbles pop so the blobs of water become less buoyant and sink again. To stamp your personality on this experiment, try adding a food colouring to the mixture, and see what happens. Try adding small pieces of more than one tablet. What about repeating with warm water, or with cold water? Were there a lot more bubbles produced, initially, in the hot compared to the cold water? For an authentic lava lamp, place the bottle in a darkened room and backlight the ‘lamp’ with a torch. Don’t forget to take photos and to post your spectacular results.

Making a medicinal lava lamp

As well as the active ingredients contained within a single pill or capsule, there are inactive substances called fillers and flavourings, and also buffers to maintain the optimal pH at which the active ingredients will be effective.

The following ingredients will be required:

  • Alka-Seltzer tablets
  • A selection of food colourings
  • Cooking, olive or vegetable oil

The following items will be required:

  • A tall, clear glass

Ensure that you read through all of the instructions before beginning the experiment to ensure that you understand exactly what will happen at each step.

Step 1: Measuring

Fill approximately one-fifth of the glass with tap water.

Fill the remainder of the glass with cooking or vegetable oil. Ensure to leave a small gap between the oil level and rim of the glass as some “foam” may be produced.

water and oil volumes

If you wish to add any food colouring for an enhanced effect, then do so now. Different quantities of food colouring will produce different shades of the relevant colour.

Step 2: Adding the active tablet

Take your Alka-Seltzer tablet and drop it centrally into the glass. It should sink to the bottom of the glass and after a few moments bubbles should begin to form.

adding tablet

reaction 1

reaction 2

Repeat the experiment with different shades of food colouring, or alternatively different liquids in place of water. Do different temperatures of water produce different results? Will crushing the tablet into smaller pieces produce more bubbles? If cabbage juice is used in place of the food colouring, why is there a change of colour?


Be creative, add a backlight to create a true “lava lamp” effect or try dropping in small pieces of glitter to see what happens. Don’t forget to post your results on our open Padlet (we have included some examples from previous courses to help inspire you) and/or on Twitter or Instagram hashtag #FLchemistry so that everyone can see! Why not make a video with accompanying background music - we went for Ride of the Valkyries by Wagner.


  • Dispose of the completed lava lamp safely: We strongly advise to not consume the “lava lamp” at any stage of the experiment. Dispose of the lava lamp down the sink, washing equipment thoroughly with cold water, once complete. It may be necessary to use warm water to wash away the oil (even if we normally use cold water to wash up after an experiment).

Lava lamp competition

There will be a prize for the best photo or video posted on our Padlet site and/or on Twitter or Instagram by 9am on Monday 13 July (week 3). The prize will be given to the person whose lava lamp, in the opinion of Andy, is the most original, striking, imaginative and memorable. We will advertise the name of the winner in week 3 (in the Comments below) who will be sent a molecular model set and Chemistry@York fidget spinner. (This competition is being run by Andy / University of York and is not affiliated with FutureLearn and any personal details submitted by the learner will only be used for the purpose of sending the prize.)
Why not give it a go?

Extension activity – How much of the active ingredient is present in an antacid tablet?

To relieve ‘heartburn’ we have seen that an antacid tablet contains active ingredients that are bases, that neutralise the acid in our stomachs. Besides the active ingredient we know that the tablets also contain inactive ingredients, such as as a filler or binder. If you are interested in finding out how to determine the amount of the base present in an antacid tablet, take a look at the antacid analysis pdf in the downloads section below. This describes the use of titration, a common laboratory method of quantitative chemical analysis, and it provides another opportunity to do some calculations.

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This video is from the free online course:

Exploring Everyday Chemistry

University of York