A First Course in Physics · Ch. XV, §§ 387–393 · Chapter 15: Effects of Electrical Currents
44. Chemical effects: electrolysis, plating and storage batteries
Goal: Your student can explain how a current carries ions through a liquid, describe the electrolysis of water, electroplating and the storage battery, and use Faraday's rule for the weight of metal deposited.
You'll need
- The book, open to these pages
- Red cabbage leaves, hot tap water, vinegar and baking soda (a cabbage-juice indicator)
- Two AA batteries in a holder, two wires with clips, two pencils sharpened at both ends (graphite electrodes)
- A glass of water with a spoonful of baking soda dissolved in it
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Purple cabbage juice shows acid and alkali forming at the two electrodes; ions moving through a liquid; electrolytes and electrolysis; splitting water into hydrogen and oxygen; electroplating, electrotyping and refining metals; Faraday's law (one ampere deposits 4.025 g of silver an hour); the lead storage battery. Eight problems follow.
The lesson, step by step
Hook question
SayGold-plated jewelry has only a thin skin of gold. How does the gold get there so evenly?
Read together
Read §§ 387–393. The book's idea of conduction in liquids:
it is thought to consist in the migration through the liquid of a swarm of positively charged ions in one direction, and of a corresponding swarm of negatively charged ions in the other direction.
Try it: cabbage indicator
Tear red cabbage into hot tap water and let it steep 10 minutes; strain it. Add vinegar to a little: red (acid). Add baking soda to another: green-blue (alkali). This is the same test the book uses in § 387.
Try it: split water
Stand the two pencil leads (sharpened at both ends) in the baking-soda water, not touching. Clip them to the two ends of the battery pack. Tiny bubbles stream from both leads, more from the one on the − side: that is hydrogen, and the other is oxygen, twice as much hydrogen as oxygen (§ 388). Add cabbage juice to the water and watch colors form at the two electrodes.
Use baking soda, not table salt (salt water gives off chlorine). Work near an open window, don't collect or light the gases, and pour the water down the sink afterward.
Safety: use only 1.5-volt AA or AAA batteries for every circuit in this course. Never connect anything to a wall outlet, and don't hold a bare wire straight across a battery for more than a few seconds (it gets hot and drains the battery).
Watch only: acids, cyanide and lead
Watch only: don't try this at home. The book's experiments use sulphuric acid, mercury, nitric acid, copper sulphate, silver cyanide (a deadly poison) and lead plates in acid. Describe what happens: copper coats the negative plate in § 389, and in § 393 two lead plates in acid become a rechargeable cell that rings a bell. Look up a video of electroplating copper in a school lab.
Explain the idea
Positive metal or hydrogen ions are pulled to the negative plate and deposited there; negative ions go to the positive plate (§ 387). Faraday found that a given current for a given time always deposits the same weight of a given element (§ 392). A storage battery stores chemical energy, not electricity itself (§ 393). The car battery is still a lead storage battery; phones use lithium-ion cells.
Say it back
Your student explains how electroplating puts silver on a spoon (Fig. 300).
Problems
Work problems 1–8 on book page 298. Use the book's figures: one ampere deposits 4.025 g of silver, 1.181 g of copper or 1.203 g of zinc per hour.
Afterwards
Look for plated things around the house (chrome faucets, silver-plated forks, zinc-coated nails) and for the storage battery in a car.
Answers to the book's problems
1. The plate that loses weight is where the current enters (the anode); the one that gains copper is where it leaves. 2. Hydrogen comes off twice as fast, at the negative electrode. 3. About 14.9 minutes (1 ÷ 4.025 hour). 4. About 0.30 g. 5. About 0.15 ampere. 6. Its plates are large and close together, so its internal resistance is very small. 7. 0.05 ohm (2 volts ÷ 20 amperes = 0.1 ohm in all, less the ammeter's 0.05). 8. No: cells in parallel give only about 1.5 volts, less than the 2 volts the storage cell pushes back with.
Worked out for this site from the scan. If a number in your copy differs, trust the book.