Whenever an election comes round, you'll see that people who voted have an ink mark on their fingers. Why do they get it, and why does it not rub off?
Keeping track of who voted
In a democratic country, citizens get to vote in an election to choose their rulers. As all citizens are equal, each citizen is allowed to vote only once in an election. But how does one make sure someone doesn't cheat and votes more than once?
A simple way is to mark everyone who has voted. Then the mark will show and the person cannot vote again. But what kind of mark does one put?
A mark that rubs off quickly is useless. But it must go off in the time the next election comes, so that the voter can vote again. So what we need is a mark cannot be rubbed off for sometime, but will wear off by the time the next election comes. What chemical can make a mark like that?
Have an election in class to choose your class leader. Use a piggy bank as the ballot box, and make small squares of paper for the ballots. Everyone has to secretly write the name of the person they want to choose as class leader, fold the paper and put it in the ballot box.
Ask your chemistry teacher to help you with a little bit of silver nitrate solution from the lab. After each person has voted, dip a glass capillary into the silver nitrate and put a dot on their finger. Hold the finger till it dries. When everyone has voted, you can open the ballot box and count who got the most votes.
Why Silver Nitrate
Silver nitrate is just the chemical we need. It is soluble in water, so you can make an inky black solution. It comes in bottles with a brush in the lid (like nail polish bottles). When you go to vote the first time, you'll see the election officer draw a thin line over your index finger and its fingernail with the tiny brush. Then they'll hold your finger till the ink dries, giving you a violet mark.
When it is put on skin, it reacts with the salt present on it to form silver chloride. Silver chloride is not soluble in water, and clings to your skin. It cannot be washed off with soap and water. Not even hot water. Not even if you use alcohol, nail polish remover, or bleach. (But please don't try these things, they are dangerous.) But as new skin grows and the old skin sloughs off, the ink stain will disappear. The ink on the skin goes off in a week. The ink on the nail takes longer, as the nail grows out.
The ink only works if it dries. If you rub it off while still wet, it will go off.