19 Jan 2018

Kitchen science: fire

From Summer Times, 10:45 am on 19 January 2018

Kids tend to be fascinated by fire and simple kitchen science is a fun, supervised way for them to learn about it, says science communicator Michelle Dickinson AKA Nanogirl.

"Teaching them about the dangers is really important so they don't feel like they want to play with it when you're not watching."

She tells us how to make a candle from a banana and get a boiled egg into a bottle.

Banana birthday candles

Traditional birthday candles are made from paraffin wax which is usually derived from petroleum – a fossil fuel.

"[Petroleum] takes a lot to get out of the ground and refine. A lot of energy goes into every candle."

A candle made from a banana and a small slice of almond is a good earth-friendly (and edible) alternative.

"It's fun and it's not using the planet's resources in the same way that petroleum is."


Peel the banana, then cut a 3 or 4 cm chunk with a flat base and flat top – "a cylinder of banana that stands up on its own".

Decorate it with chocolate and/or nuts. You can create patterns and if you like, match its decoration to your cake.

Then put either a slivered almond or an almond you've peeled and cut into a long thin piece into the top of the banana where the wick would be if it were a wax candle.

(If you're cutting your own slice of almond go a bit bigger than a slivered almond 'cause it will burn longer)

Stick it on top of a cake, light it and blow out as usual – then you can eat it with the cake!

"The almond will be burning just like a wick."


So what's happening here?

Wax candles burn when the cotton wick heats and liquefies the wax so it becomes a fuel.

In this case, the almond provides the fuel.

"We eat nuts 'cause they're full of energy – and the natural fats in them burn just as well as candles."

A banana candle will go out when the almond wick burns down to the moistness of the banana – usually within a couple of minutes, Michelle says.

She likes using bananas for candles cause their stickiness makes them easy to decorate, but you could also use a cylindrical slice of potato.

If you want a really long-burning candle, core an apple and fill its centre with nuts, then light these.

"It won't be like a candle but it will give you a soft flame in the middle of an apple."


Egg in a bottle

Air pressure from the atmosphere is on us and around us at all times, but we don't often feel it. With an egg and a bottle we can learn about its power, Michelle says.



Boil an egg for 10 or 12 minutes then cool it down and peel it.

Then take a glass jar or bottle that has a neck slightly narrower than the egg. Test it by putting the egg on top – if it falls in the neck is too big.

If you're using an edible candle just put it straight in.

If using a wax candle, drop a 1cm slice of banana into the bottom of the jar or bottle then place two wax candles into it and light them.

"They're burning because they've got oxygen and they've got a fuel (from the candle)."

Put the egg on top of the bottle and watch what happens when the flame runs out of oxygen.

"You'll see the candle goes out very quickly."

Then watch as the egg squeezes its way into the jar and eventually pops down inside.

"It's like a ship in a bottle. Nobody can really figure out how it got in there."


So what's happening here?

"Because we heated up the air inside this jar the air pressure has expanded. So you've changed the air pressure. Now the air pressure from the outside wants to get inside to equalise it.

"It's about air pressure and how heating up air can make it expand. And as it contracts it's gonna create this pressure on the egg. And the egg is soft and squishy enough that it can squeeze through.

"The hot air that was heated inside the jar is now cooling. So it had expanded and now as it's cooling it's got a lower pressure on the inside. As it cools there's less air 'cause it's getting smaller and smaller and it's taking up less space in there. So the air from the outside wants to fill that space now the temperature is becoming cooler. The air from the outside is pushing the egg so it can get inside so it pushes the egg out of the way."

To take the egg out of the bottle, turn it upside down and heat the bottle up again or blow on its neck – "you can create a positive pressure and the egg will just pop back right out."

You can find The Kitchen Science Cookbook here.




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