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Saturday, September 4, 2010

A Recipe for a Rainy Day - Where did the Oxygen go?

Here is a little bit of kitchen science that will give your little ones a head start on science this year.  Schools usually just draw pictures and tell the kids about oxygen and how much there is of the stuff in the air, but you can give your kids an actual visual representation.  They can watch it happen and see the results.  I always think that doing is better than being told.  So if you have a rainy day and want to smarten up the kids, have a go at this recipe for success and let the kids help. (you will like it too, I promise)

Room air contains about 21% oxygen and an exhaled breath about 16%. This demonstration will provide a visual to support these values in a way that children can understand. Oxygen is a necessary component of any fire. This demonstration also stresses this in a way that small children can understand.


What you need:

1 shallow bowl

1 candle - This can be any type but small birthday candles work best. Attach the candle to the bottom of the bowl using hot wax.

A few drops of food color so we can see the water – blue is the preferred color but any will do.

A clear jar that is tall enough to invert over the candle and has straight sides; you will want make a scale on the side of the jar – I use duct tape. Mark of the jar in tenths


The first experiment will be with room air.


Put some water in the dish (about 1/3 of what the jar can hold) into the dish with the candle and add a few drops of food coloring to make the water more visible. Light the candle and quickly invert the jar over the candle and into the pool of water sealing the air and candle inside the jar. Now watch as the candle flickers and goes out. You will see the water rise inside the jar. It will rise to be near the 2nd mark on the scale or 2 tenths which is 20%. (If you do this over again, make sure to force new room air into the jar as the air from the last burning will be trapped inside and your results will be inconsistent.)


Questions to ask:

Why did the candle go out? . Because there wasn’t enough oxygen in the air in the jar to support a flame any more, and without oxygen you can’t have fire.


How much oxygen was in the air? You should have just about 20% of the jars capacity filled with colored water.


Now do this again with exhaled air.

Refill the dish and light the candle. Have one of the children blow hard into the jar until they run out of breath. Quickly invert the jar over the candle just like you did before. The candle will go out much faster and a water level in the jar will rise to just about 1/10th of the jars capacity or the first mark on your scale.


Questions to ask:

Why did the candle go out so fast? When you exhale air it doesn’t contain very much oxygen because you use it in your body.


How much oxygen was left in the air? About 16% or less.

Have fun with this.