Question

if we had very redhot water and put it in a thank of water with a lid, what would happen in the water

Answer 1

I think you mean: if you pour very hot water into a tank (container) that already has water in it and the tank has a lid, what happens? Short answer: the hot water will transfer heat to the cooler water and the air above it, causing convection currents, steam and condensation, possible pressure increase if the tank is tightly sealed, and loss of dissolved gases. There are also safety risks.

What specifically happens (processes):
- Convection and mixing: hot water is less dense than cold water so it tends to rise. That creates buoyant flow and mixing until temperatures even out. If the temperature difference is large you’ll get vigorous convection and turbulence; if gently added you can get stratification (a warm layer on top) for a while.
- Heat transfer: heat moves from the hot water into the cooler water (and into the container walls and lid) until thermal equilibrium is reached.
- Steam and vapor: if some of the water is above its local boiling point (or vaporizes when splashed), you’ll see steam forming. In an open container the steam escapes; with a lid the steam will hit the lid, condense to droplets and fall back into the water.
- Pressure change in a closed tank: if the lid seals the tank so vapor and expanding air can’t escape, the heated air and vapor will increase the pressure. Higher pressure changes the boiling point and can be dangerous if the container can’t withstand the pressure (risk of popping the lid or rupturing the tank).
- Degassing: heating releases dissolved gases (oxygen, CO2). You’ll often see lots of small bubbles rising that are gas coming out of solution, not necessarily boiling.
- Thermal stress: if the tank or its components (glass, plastic, metal) are cold and you introduce very hot water quickly, the container may crack or warp from thermal stress.

Safety notes:
- Never heat a sealed container of water — pressure can build and cause an explosion or scalding.
- Be careful of splashes/steam — they can cause burns.
- If the container is glass, avoid pouring very hot water into a cold glass tank (risk of shattering).

If you want a more precise answer, tell me: is the tank sealed or open, what are the temperatures, what are the volumes, and what material is the tank made of?