Familiar question? This is found in an assessment book that anxious Singaporean parents eagerly snap up. The answers should also sound familiar. According to this book, the aim of this experiment is to show that air is needed for burning and the explanation is that as oxygen is used up by the burning candle, water rushes in to fill the space in the glass jar.
Yes, oxygen can be “used up” but it doesn’t just disappear from the glass jar. In fact, every atom of oxygen that was in the glass jar would still be present in the glass jar after the candle has extinguished. Combustion uses oxygen and produces carbon dioxide. It shouldn’t create a vacuum.
Next, if the depletion of oxygen really results in the creation of a vacuum in the glass jar, the water should start rising while the candle is burning. If you actually conduct this experiment, you’ll find that the water does not rise when the candle is burning. The water level only rises after the candle flame has extinguished.
I’m not sure if this question is ever asked in the exam, but I certainly hope they don’t give marks for wrong answers. The concept behind this experiment is somewhat beyond primary school kids. It demonstrates Charles’ Law. Put simply, Charles’ Law states that for a given volume of gas, the change in volume is directly proportional to the change in temperature.
Temperature. Have we been so focused on oxygen that we forgot all about the temperature? When the glass jar is placed over the flame, heat from the flame increases the pressure in the glass jar. Fewer and faster moving air molecules fill the glass jar. In some cases, bubbles may even be expelled from the glass jar when the jar is placed over the candle. Once the flame is extinguished due to the depletion of oxygen, the temperature of the air in the glass jar falls rapidly. Volume decreases following Charles’ Law and water moves in to fill the space.
It’s a mistake to think that making questions difficult and “sophisticated” will enhance and improve science education for our kids. I think it’s more important to teach and get the basics right. Such sophistication not only cause stress among parents and kids but may also result in teachers and examiners getting it wrong themselves.