The Chemical History of a Candle · Platinum, part 3 · Part 7: The extra Lecture on Platinum (optional)
21. Fierce heat, a furnace of lime, and Faraday's farewell optional
Goal: Your child can explain what a blowpipe does, why lime makes a good furnace, and that even metals like gold and silver can be turned to vapor.
You'll need
- A metal spoon and a wooden spoon
- A mug of hot (not boiling) tap water
- Science notebook
What's in the book
The blowpipe of oxygen and coal-gas that melts platinum like wax; the lime-light again; a candle that melts a very fine platinum wire; iron burning in sparks; Deville's furnace of lime; mercury, lead, gold and silver driven off as vapor; and Faraday's closing words.
The lesson, step by step
Hook question
SayA candle flame is very hot. Why can't it melt a piece of metal?
Read together
Read pages 209 to 222, to the end of the lecture.
(Having made use of your friend, you get rid of him as quickly as you can.)
He means the lead: it was useful for cleaning the platinum, and then it had to go.
Heat enough, but not enough supply
Faraday melted a platinum wire thinner than a hair in an ordinary candle, but he could not melt a lump of antimony there.
You see we have, therefore, heat enough in the candle, as in the voltaic battery, or in the highly-exalted combustion of the blowpipe, but we do not supply a continuous source of heat.
SayA thin wire heats up fast. A big lump loses heat as fast as the flame gives it. A blowpipe drives a steady blast of hot gas against the metal, so the heat keeps coming.
A furnace of lime
Deville built his furnace from two blocks of lime. Lime does not melt in the fiercest flame, and it holds the heat in:
While the front part of the lime which you saw here was so highly ignited, I could at any moment touch the back of it without feeling any annoyance from the heat.
Try it gently: put a metal spoon and a wooden spoon in the mug of hot tap water for a minute. Touch the handles. Metal carries heat well; wood, like lime, does not.
Metals that boil away
Watch only: don't try this at home.
Faraday boiled mercury with the battery, made lead give off fumes, left a purple patch of gold vapor on limestone, and boiled silver in a green glow that Dr. Tyndall threw onto a screen. Mercury and lead fumes are poisonous. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
Faraday's farewell
Faraday ends by saying his memory is failing, and that he has wanted to retire:
I have desired to retire, as I think every man ought to do before his faculties become impaired; but I must confess that the affection I have for this place, and for those who frequent this place, is such, that I hardly know when the proper time has arrived.
Ask: How do you think he felt? What does it tell you about him?
Say it back and check
Narration prompts: What is a blowpipe for? Why did Deville use lime? What happened to the gold under the blowpipe?
Check questions:
1. Why could a candle melt the fine wire but not the antimony?
2. Why was lime a good material for the furnace?
3. Which two metals did Deville keep in the platinum on purpose?
Afterwards
Notice it this week: things that carry heat well (metal pans, radiators) and things that don't (wooden handles, oven mitts, mugs).
Tip
This is the very end of the book. If your child has kept a science notebook all the way through, let them look back through it and choose a favorite page.
Answers to the book's problems
1. The fine wire heats up faster than it can lose heat; the lump of antimony loses heat as fast as the flame supplies it, so it never gets hot enough. 2. It does not melt or change even in the fiercest heat, and it conducts heat so badly that it keeps the heat inside. 3. Iridium and rhodium, because they make platinum resist acids better.
Worked out for this site from the scan. If a number in your copy differs, trust the book.