2. An experiment with a wrong conclusion! RAVI DIVAKARAN Department of Chemistry, St. Albert’s College, Ernakulam 682018, India. Presentation by:

3. Background… For a very long time, an experiment is being described in school chemistry text books, an experiment which is supposed to demonstrate the percentage of oxygen in air…

4. Background… “Fix a small candle on the base of a trough and pour some water into it. Light the candle and cover it by inverting a drinking glass over it… …which goes like this:

5. Background… …carbon in the candle burns consuming oxygen and the flame slowly goes out. Water rises up to fill the space vacated by the oxygen. This is about 20% of the volume of the glass and therefore air contains 20% by volume of oxygen.”

6. Hmm… But what does the equation say? C + O 2  CO 2  That is, for every molecule of oxygen consumed, another molecule of carbon dioxide takes its place! Then why does the water rise?

7. Oh! Let’s try it ourselves… Okay. When we use one candle, water enters by 15-20%.

8. Oh! Let’s try it ourselves… Okay. When we use one candle, water enters by 15-20%. But what if we light two candles at the same time?

9. Let’s see… Wow! Water has risen to fill 30-40% of the volume! Now let’s see the effect of three candles.

10. There… Ooops! Almost 60% ?! So, percentage of oxygen in air is proportional to number of candles used! What’s happening here?

11. So, the conclusion… This experiment does not demonstrate the amount of oxygen at all! It only shows the amount of air expelled from the glass due to expansion by the heat from the candles! Suggested Reading: (1) J R Partington. A Text-Book of Inorganic Chemistry, 6 th ed., ELBS, 1963. pages 620-22. (2) Ravi Divakaran. Classroom. Resonance, May 1996, pages 72- 74.