Understanding the Science Behind the Candle Under Drinking Glass Experiment

[Phrak]

To amaze and entertain my 8 year old I did the candle under the drinking glass experiment.

I took a bowl of water, put a lit candle in the middle and placed a glass over the top. Sure enough, the water rose up the candle went out. For him it was an unexpected and wonderful result What made the water go up?! I was very happy to see the amazement in his face. Wonderful!

However, I told him knowingly--and knowing I had no idea what I was talking about, "The candle burned the oxygen in the air, so there was less air, and so the water had to rise up because now there's less air under the glass." Its what I was told when I was about his age :grumpy: --but now I question this patent answer.

What's the real story? I'm burning wax, which is fairly close to (CH₂)_n, and combining it with oxygen in the air.

2CH₂ + 3O₂ → 2CO₂ + 2H₂O

For every three molecules of oxygen, we get 4 molecules of gas. But the water level rose. Now I know, that air has about 21% oxygen and the water rose to reduce the initial volume by about 10 to 15%. To be sure, I'd have to try it again for better numbers. It's a bit warmer inside the glass then when the experiment started so the vapor is a bit less dense So what really happened?

 

[Dadface]

To amaze and entertain my 8 year old I did the candle under the drinking glass experiment.

I took a bowl of water, put a lit candle in the middle and placed a glass over the top. Sure enough, the water rose up the candle went out. For him it was an unexpected and wonderful result What made the water go up?! I was very happy to see the amazement in his face. Wonderful!

However, I told him knowingly--and knowing I had no idea what I was talking about, "The candle burned the oxygen in the air, so there was less air, and so the water had to rise up because now there's less air under the glass." Its what I was told when I was about his age :grumpy: --but now I question it.

What's the real story? I'm burning wax, which is fairly close to (CH₂)_n, and combining it with oxygen in the air.

2CH₂ + 3O₂ → 2CO₂ + 2H₂O

For every three molecules of oxygen I get 4 molecules of gas. But the water level rose. Now I know, that air has abou 21% oxygen and the water rose to reduce the initial volume by about 10 to 15%. To be sure I'd have to try it again. It's a bit warmer inside the glass then when the the experiment started, so what actually happened?

The volume of the candle decreased slightly and the steam condensed to water.As the air cooled the pressure dropped.

 

[Borek]

Change in the candle volume is neglectable if you ask me. Other than that I mostly agree with Dadface. There are two products of the combustion - water and carbon dioxide. Water condenses, CO₂ is well soluble in water. But my bet is that change in water level is mostly because when you cover the candle you trap hot (expanded) air, which gets colder after candle stops to burn, sucking water up.

 

[Phrak]

Well, when the glass is first placed over the candle, there's not a great deal of hot air inside. It should be a contributing factor though. It could be tested though. Hovering the glass over the candle for a while could cause the water to rise higher in the glass.

 

[LudusRex]

The candle under the drinking glass experiment is a classic demonstration of the relationship between air pressure and temperature. When you placed the glass over the lit candle, the flame used up the oxygen inside the glass and created carbon dioxide and water vapor. The water vapor takes up more space than the oxygen it replaced, causing the air inside the glass to heat up and expand. This expansion creates a decrease in air pressure inside the glass, creating a vacuum that pulls the water up into the glass.

This phenomenon is known as Charles's Law, which states that as the temperature of a gas increases, its volume also increases. This is because the molecules in the gas have more energy and move faster, taking up more space. In this case, the water vapor molecules have more energy and push against the air molecules, causing them to spread out and decrease the air pressure inside the glass.

Additionally, the water level rose because of the decrease in air pressure outside the glass. As the air inside the glass heated up and expanded, the air outside the glass cooled down and contracted. This created a difference in air pressure, with the higher pressure outside pushing the water up into the glass to balance the pressure inside.

So, in summary, the candle under the drinking glass experiment demonstrates the relationship between temperature and air pressure, and how changes in temperature can cause changes in air pressure that can lead to interesting phenomena like the water rising up into the glass. It's a simple yet fascinating experiment that can help us understand the science behind everyday occurrences.

 

[LudusRex]

The candle under the drinking glass experiment is a classic example of the relationship between gas and volume. When the candle is lit, it produces heat and light through a process called combustion. The candle wax, which is primarily made of carbon and hydrogen molecules, reacts with oxygen in the air to produce carbon dioxide and water vapor. This process releases energy in the form of heat and light.

As the candle burns, it uses up oxygen from the air, creating a low-pressure area inside the glass. This low pressure causes the water to rise up into the glass, filling the space where the oxygen was consumed. This process is known as the "candle sucking" effect.

The water level rises because the volume of gas inside the glass decreases as the oxygen is consumed. According to the ideal gas law, as the volume of a gas decreases, the pressure and temperature also decrease. As the air cools down, the water vapor inside the glass condenses into liquid water, creating a partial vacuum that pulls the water up into the glass.

So, to answer your question, the water rises because of the decrease in gas volume and the decrease in temperature inside the glass due to the burning candle. This experiment is a great way to demonstrate the relationship between gas and volume, as well as the concept of pressure and temperature in gases. I hope you and your child continue to explore and learn about science together!