Maya's sugar. Maya drops one sugar cube into a cup of cold water, and one sugar cube into a cup of hot water. She uses a timer to see how long each one takes to disappear.
Chemistry · Week 3 · The man who weighed the air
A log burns down to a little pile of ash. So where did the rest of it go?
Tutor view: press T. Turn it off before you share your screen.
Three quick questions about the last lesson. Just say what you think. There's no score yet.
Maya's sugar. Maya drops one sugar cube into a cup of cold water, and one sugar cube into a cup of hot water. She uses a timer to see how long each one takes to disappear.
Let's learn these before the story, so nothing surprises us later.
Something in an experiment that can change.
Like a dial you can turn: the water's temperature, the amount of sugar, the time.
How much stuff is in something. We find it with a scale.
We measure it in grams (g) or kilograms (kg).
Closed up tight. Nothing can get in or out, not even air.
Like a jar of jam before you open it.
A guess you can check. IF is the thing you change on purpose. THEN is what you measure.
If the water is hotter, then the sugar will disappear faster.
You can do something and measure something to find out if it's true.
“Hot water is nicer” is not testable. “Hot water dissolves sugar faster” is.
About 250 years ago, almost everyone believed the same thing about fire. You can see why. A big log goes into the fire, and only a tiny pile of ash comes out.
So they said: “Burning destroys matter.”
That sounds right. But sounding right isn't the same as being tested.
Antoine Lavoisier, France, in the 1770s. He weighed everything.
Antoine Lavoisier was a French chemist who loved his scales. He didn't want to just look at fire. He wanted to weigh it.
1. He put a piece of shiny tin metal inside a glass jar.
2. He sealed the jar, so nothing could get in or out.
3. He weighed the whole sealed jar on his scale.
4. He heated it for a long time. The shiny tin slowly turned into a dull white powder, just like burning.
5. He weighed the whole sealed jar again, still closed.
Tin, sealed inside glass, being heated.
Now let's be scientists about it. Remember Maya's sugar from the warm-up: one thing was changed on purpose, and one thing was measured. Lavoisier's experiment has the same shape.
Here is what the people of 1770 believed, turned into a proper hypothesis. Watch how the IF and the THEN work.
Now you finish the other side of the argument.
Made-up numbers, to show the idea.
The same. Not one bit lighter.
The tin had changed completely, from shiny metal to white powder. But the sealed jar weighed exactly the same. So burning did not destroy any matter. The old idea was wrong.
Lavoisier wasn't finished. Next, he pulled the cork out. Before we see what happened, it's your turn to write the whole hypothesis yourself.
Whoosh, and the jar got heavier.
When he opened it, air rushed in. Now the jar weighed more. Why? While the tin was heating, it had pulled some of the air inside the jar into itself. That left room in the jar, and new air rushed in to fill it.
So the tin didn't lose anything. It gained something, and it gained it from the air. That's why he's called the man who weighed the air: he proved air is real stuff, with real mass.
Most of the log goes up into the air, as gases you can't see.
Here's a true story. About 100 years before Lavoisier, an English scientist named Robert Boyle did almost the same experiment. He got the wrong answer. Can you find out why?
Robert Boyle, England, in the 1670s.
What Boyle did:
1. He sealed some metal in a glass jar and heated it.
2. He opened the jar.
3. Then he weighed the metal. It was heavier!
His conclusion: “Fire must have weight. The fire went through the glass and stuck to the metal.”
A brand-new experiment. This time you write the whole hypothesis, with no help.
A fizzy tablet is dropped into water inside a bottle. A balloon is stretched over the top, so the bottle is sealed. The whole thing sits on a scale. The tablet fizzes, and the balloon starts to puff up.
Next time: a hand warmer gets hot, and an ice pack gets cold, but they look like the same trick. Where does the heat come from?
Six questions on the things that were hard last time: variables, what you change, what you measure, one change at a time, and writing your own IF/THEN. New examples, same skills.
Whatever we don't finish in class becomes homework.
Name: ______________________ Date: ____________
Chemistry homework: the hypothesis skills. Circle one letter, or write your answer on the lines.
Leo's ice. Leo puts salt on one ice cube, and no salt on another ice cube that's the same size. He times how long each one takes to melt.
Ava's candles. Ava wants to know if a candle burns longer inside a big jar than a small jar. She puts a thick candle in the big jar and a thin candle in the small jar.