Friday Sprog Blogging: combustion

i-9abf49b9f425052a7293b8067c7ca7ea-Sprog-badge.jpgBoth Free-Ride offspring are charter members of the Order of the Science Scouts Special Children's Auxiliary. They have not, as yet, built their own fire, either in a fire pit or a laboratory. However, a discussion this week about the strange vapor seen emanating from a car's tailpipe one morning moves them further in the direction of being O.O.T.S.S.O.E.R.A.A.A.P. fire-certified.

Elder offspring: Remember that car we saw when we were walking to school, with the vapor coming out of its tailpipe?

Younger offspring: I made vapor come out of my mouth, too. It was cold.

Dr. Free-Ride's better half: What do you suppose that vapor was that came out of the tailpipe?

Elder offspring: Was it steam?

Dr. Free-Ride: Sounds good to me.

Younger offspring: Why was there steam coming out of its tailpipe?

Dr. Free-Ride's better half: What is steam?

Elder offspring: Gaseous water.

Dr. Free-Ride: So why would a car have gaseous water coming out of its tailpipe?

Elder offspring: Maybe there was water in the fuel tank?

Dr. Free-Ride's better half: Can you think of any other reason?

Dr. Free-Ride: What's the fuel people usually put in the fuel tanks of their cars?

Younger offspring: Gasoline!

Dr. Free-Ride's better half: That's right.

Elder offspring: But how does gasoline end up making steam come out the tailpipe?

Dr. Free-Ride: Do you know what the car has to do with the gasoline to get the energy out of it?

Younger offspring: No.

Dr. Free-Ride: It has to burn the gasoline.

Younger offspring: Like a fire?

Dr. Free-Ride's better half: Yep. And do you remember what a fire needs to burn?

Elder offspring: Oxygen!

Dr. Free-Ride: That's right.

Dr. Free-Ride's better half: So, do you remember what happens when we light candles and then let them burn all the way down?

Younger offspring: They melt!

Dr. Free-Ride: But if they just melted, all the wax that started out in the candles would end up dripping onto the table. We get a few drips, but not whole candles' worth of drips.

Elder offspring: What happens to the wax?

Younger offspring: Yeah, where does it go?

Dr. Free-Ride's better half: Let's see if we can figure that out. (Grabs a tealight candle, a 4 ounce canning jar, and a lighter.) OK, I'm lighting the candle. What will happen if I lower the jar over the candle?

Elder offspring: The flame will go out!

Younger offspring: (As the flame does go out) It ran out of oxygen!

Dr. Free-Ride: That's right. So that must mean that the oxygen gets used up when something is burning.

Dr. Free-Ride's better half: (Relighting the candle) What if I lower the jar more slowly so the oxygen doesn't run out so quickly? Can you see something forming on the inside of the jar?

Younger offspring: Is that wax?

Dr. Free-Ride: Soot?

Elder offspring: Steam! It's steam!

Dr. Free-Ride's better half: That's right. So, burning uses up oxygen ...

Elder offspring: And makes water!

Dr. Free-Ride: Do you know what else is produced when you burn something?

Elder offspring: Carbon dioxide.

Younger offspring: How do you know that?

Elder offspring: I don't remember. I must've heard it somewhere.

Dr. Free-Ride's better half: So, if burning the wax uses up oxygen and makes carbon dioxide and water, what can you say about what the wax is made of?

Dr. Free-Ride: (After some blank looks) What is carbon dioxide made of?

Younger offspring: Carbon and dioxide.

Elder offspring: Carbon and oxygen.

Dr. Free-Ride: And what's water made of?

Elder offspring: Hydrogen and oxygen.

Dr. Free-Ride: And you know that oxygen is getting used up when you burn the candle -- the oxygen that goes to make the water and carbon dioxide.

Elder offspring: So the carbon and the hydrogen come from the wax?

Dr. Free-Ride's better half: Yep. Wax has carbon and hydrogen, and so does gasoline.

Dr. Free-Ride: Hydrocarbon fuels. And the foods your body burns for fuel have carbon and hydrogen in them.

Elder offspring: Like carbohydrates?

Dr. Free-Ride's better half: And fats, and proteins.

Younger offspring: We burn our food?

Elder offspring: And sometimes have tailpipe emissions.

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That was utterly priceless. Your children are going to have lots of happy memories of their childhood.

you have smart sprogs. just a couple of hours ago my bigger one asked if the stuff coming out of a tailpipe was steam. we didn't get to the detail you did, but he did learn that some steam comes out of tailpipes. of course, what he was looking at was bluish, so he also got a very brief lesson in four stroke engine design and car maintenance :).

I've sometimes wondered why, on a cold day, one car will put out tailpipe vapors vigorously, and one will do little or none. Wondered if it was an effect of water in the gas line, or engine temp, or somesuch...