The Chemical History of a Candle · Platinum, part 1 · Part 7: The extra Lecture on Platinum (optional)
19. Platinum and its family of metals optional
Goal: Your child can describe platinum and the five metals always found with it, and explain how platinum was made into solid bars by welding.
You'll need
- A periodic table (a free one online, or printed)
- A few small objects of the same size but different materials (a marble, a metal nut or bolt, a wooden bead)
- Science notebook
What's in the book
The extra lecture, given to the Royal Institution on Friday, February 22, 1861. Faraday explains that his friend Deville could not come to melt platinum, describes platinum and the metals found with it, the old way of purifying it with acids, and welds two platinum wires with a blowpipe and a hammer.
The lesson, step by step
About this lecture
This was not one of the Christmas talks for children. It was an evening lecture for grown-ups, printed at the end of the book. It is harder, so these guides are optional.
Faraday had promised that his friend Deville of Paris would melt thirty or forty pounds of platinum in front of the audience. Deville couldn't come, so Faraday explained the process himself.
Read together
Read pages 191 to 200, stopping at "The treatment which Deville proposes to carry out."
The metal platinum, of which you see some very fine specimens on the table, has been known to us about a hundred years.
A family of six metals
Platinum is found as grains of metal in river soils in Brazil, Mexico, California and the Ural Mountains of Russia, always with five other metals: palladium, rhodium, iridium, osmium and ruthenium.
Osmium has a specific gravity of 21.4, and is the heaviest body in nature
Find all six on the periodic table. They sit close together, which is part of why they behave alike.
Heavy for its size
Specific gravity means how heavy something is compared with the same amount of water. Platinum is about 21 times as heavy as water. Pick up your same-size objects one at a time and put them in order from lightest to heaviest. Now imagine a marble of platinum: about twenty-one times the weight of a marble of water.
Making a bar without melting
Platinum was too hard to melt in ordinary furnaces. So it was dissolved in acids, turned into a fine "sponge," then pressed, heated and hammered until the grains stuck together: welding, like a blacksmith joining iron.
He does not know the philosophy he calls into play when he sprinkles a little sand over the oxide of iron, but he has a fine philosophy there
Faraday heated two crossed platinum wires with a blowpipe, tapped them with a hammer, and they became one piece. When he pulled, the wire broke somewhere else, not at the weld. Watch only: don't try this at home. Search for a video of a blacksmith "forge welding" to see the iron version.
Say it back and check
Narration prompts: Why did Faraday give this lecture himself? Where does platinum come from? How was it made into bars?
Check questions:
1. Name the five metals found with platinum.
2. What did Faraday say was the heaviest substance in nature?
3. How did Faraday prove his weld was strong?
What we know now
Osmium and iridium are so close in density that careful measurements were needed to show osmium is very slightly the denser. Platinum is used today in car exhaust cleaners (catalytic converters), jewelry and laboratory equipment.
Afterwards
Notice it this week: look for the word "platinum" on jewelry, spark plugs or labels.
Tip
Faraday was 69 when he gave this lecture. If your child finds it heavy going, read the stories (the missing friend, the welded wire) and skim the lists.
Answers to the book's problems
1. Palladium, rhodium, iridium, osmium and ruthenium. 2. Osmium, with a specific gravity of 21.4. 3. He pulled the welded wire apart: it broke where the pincers gripped it, not at the weld.
Worked out for this site from the scan. If a number in your copy differs, trust the book.