The sink water is slow to go down and it starts to bubble up. What is the best chemical to use?

If the person(s) using the bathroom has long hair then likely that's the problem. You need to remove the stopper and use a "hair snake" that will reach down the drain into the trap and pull the hair mass out.

See following video.

https://youtu.be/X2QIz1jAJy8?si=mJPR4BIh_szF8n9y

Since you used caustic drain cleaners you need to take APPROPRIATE SAFETY PRECAUTIONS to avoid chemical burns to skin and eyes.

OR, call a professional.

Hi there it might be that you have used a harsh abrasive chemical to clean the sink and there's a residue remaining from it eating into the surface causing theses cracks to. Appear
Or the chrome enamel surface. Is breaking down try using a 're chroming agent like silvo

Try running ice cubes through it, it will make a lot of noise but that should clean it out. Please slowly run the cold water with the ice.
I do not like chemicals if they can be avoided. d

Hello --
well -- was the head gasket mechanically or chemically repaired? -- If cemically, you may need to go to the other side and remove/ replace the gasket..
you may need to run the vehilce to "burp" out any further gases remaiing in the coolant system, esp if it was not completely drained and had some air pocekts in there somewhere..
Best regards
Ck

The Sink is siphoning into the dishwasher. Where it connects to the sink pipe make sure the tube loops up after it leavs the sink so it cannot drain down into the dishwasher when the sink is in use.

You can try using rock salt and boiling hot water down your drains. That will help if you have roots getting into your line. Chemicals are not very good for your pipes. If you have PVC drains the chemicals can eat a hole in the pipe because most of them have acid in them. They are also bad for your health (breathing in fumes, getting on skin, etc.) If the line is broken down, has a break in it, it will need to be repaired.

Sounds like your sink drain trap or line is clogged. When the dishwasher drains, the sink drain can't keep up, the water backs up, then backs down the dishwasher drain line into the dishwasher. You may not notice the slow line when you run a little water into the sink, but with the amount of water coming from the dishwasher the problem is worse. You can try taking off the trap under the sink and making sure it's clear. If the problem is farther down the drain line, you can try snaking it yourself, or call a plumber. I don't recommend using Liquid Plumr or Drano because if those don't work, they leave the line full of caustic chemicals for the next person who works on it. That can be dangeerous.

I would try baking soda and vinegar mix. Just sprinkle the soda in the sink and add the vinegar a bit at a time. Let it sit for awhile,use a soft scrubber,rinse,and wipe with a soft,dry,cloth.
You could also try CLR. Just follow the directions on the package.

Is the water slow coming out of the water dispenser? Your fridge should have the same water pressure as any sink in your house. Most of the time low water supply is from thw water valve in your basement or under the sink or wherever your is hooked up. Most people use piercing valves and they always plug up

Ooo bad news... Hope you like bubbles.
Don't worry, all is not lost.
Some people would suggest defoamers or other chemicals to cut the bubbles down, but if you just cycle the machine between the drain and fill steps, you will (eventually) dilute the soap down enough as to eliminate the problem.
It may be necessary to run cold water down the same drain as the dishwasher as cooler temps cause bubbles to decrease.
Hope this helps,
Kermit