It says ‘Never add water to caustic soda, always add caustic soda to water’ - since it stank my kitchen out yesterday (blocked sink) I’m not going to try the former without asking here first.
So, what would happen if I took a cup of caustic soda (say, 200 grams) and added an eggcup full of water?
You always add chemicals to water and not the other way around.
There’s always the potential for splash back when you pour water onto a dry chemical.
Do you mean SOLID caustic soda? DON’T DO IT!!! (for the unititiated, that’s sodium hydroxide, AKA lye, or sometimes a blend of sodium hydroxide and sodium carbonate).
You’ll get a large release of heat, possibly an explosion (of sorts), and a fused mass of soda. Just buy commercial drain cleaner.
OK, if you MUST use lye, dissolve it in an equal mass of ICE-COLD water. Add the lye slowly, allowing the water to cool between additions. Now you have a strong solution capable of clearing drains.
But I still recommend Drano.
A whole bottle of ‘Mr Muscle sink and drain unblocker’ didn’t work, so I had to get something stronger. It still goes down fairly slowly though 
So, not recommended then. I think the work explosion has put me off sufficiantly.
Given 200g + eggcup of water, what kind of heat / reaction could I expect?
The water would quite rapidly come to a boil. If you happened to use a narrow container for this experiment, the boiling solution would expand rapidly and shoot up like a geyser; an unpleasant surprise if you’re not expecting it.
“If you’re doing like you oughter, add the acid to the water. If you want it in your face add the water to the base.”
I don’t remember where I first learned that. The point is, though, that when you dissolve an acid or a base in water, a lot of heat is released. If the acid or base is slowly added to a large body of water, the water won’t get too hot, but a small amount of water in a larger quantity of acid or base will boil very rapidly, likely resulting in spattering. You don’t want that in your eyes.
Damnit! Ate my post! Hamster! Feck!
OK, again. Use at least 200 ml of water – I have no idea how much “an eggcup” is. If you insist on making it this strong, keep the whole works in a shallow bath of crushed ice and a little water. Don’t spill.
WEAR GLOVES and EYE PROTECTION. 50% sodium hydroxide is extremely strong. Given time, it will chew through aluminum, it will etch glass, it will soften plastic, and it will cause third-degree burns. It takes no time at all to cause “milder” burns. Wear clothes you don’t care about, and stir with something plastic you never want to see again.
Sprinkle some sodium hydroxide flakes on just about anything with calories and add a few drops of water and you’ll know why. For some reason many chemists don’t even seem to know that you’ll get fire but my childhood bomb-making successes prove otherwise.
start with cold water, add the caustic slowly and stir with a wooden stick as you do it. Feel the outside of the container - you will feel it getting hotter. Give some time to cool down before adding more. ‘take it slow’ is the key here. Best do it outside.