Question

if we had red-hot water and put it in a tank of water with a lid, what would happen in the water

Answer 1

A few different things can happen — which ones dominate depends on how hot the “red‑hot” water really is, how much of it you add relative to the water already in the tank, and how tightly the lid is sealed. A clear point first: water does not “glow” red like metal; if you mean water at a very high temperature, here are the main effects and safety concerns.

If the hot water is below boiling at the tank pressure
- The hot water is less dense than the cooler liquid, so it will rise and set up buoyant plumes and convection currents. That drives turbulent mixing and eventually the tank approaches a uniform temperature.
- Heat is transferred by convection (fast) and conduction (slower) until equilibrium.
- Dissolved gases are driven out of solution so you’ll see small bubbles form and rise.

If the hot water is at or above the boiling point at the tank pressure (e.g., near 100 °C at 1 atm)
- You will generate steam. In an open tank the steam escapes; in a closed tank the steam will build pressure.
- Steam formation near the introduction point can cause vigorous boiling and mixing; if steam condenses on the cooler water or lid you’ll also get condensation and dripping.
- If the lid is sealed and cannot vent, pressure will rise. Higher pressure raises the boiling point, but continued heating/steam generation can cause dangerous pressures and possibly a sudden failure (lid blow off or tank rupture) — think “pressure cooker” behaviour.

If the water is superheated or you have rapid injection
- Sudden injection of very hot liquid into cooler liquid in a closed space can cause flash boiling and possibly explosive release of steam (steam‑explosion hazard), especially if a lot of heat is released in a short time.

Other effects
- Thermal stress: rapid heating can stress or deform the tank or lid material.
- Condensation on the lid and walls, then hot drips back into the water.
- If impurities are present, chemistry (scaling, salt concentration changes when water evaporates) can occur.

Safety takeaways
- Do not place very hot or boiling water into a tightly sealed tank. Vent the tank or open the lid to avoid pressure buildup.
- Add hot water slowly to allow mixing and heat dissipation.
- Wear protection — steam and splashes can cause severe burns.

If you want a more specific answer, tell me: approximate temperatures, volumes, and whether the tank is sealed or vented. I can then describe the expected pressure rise, steam production, or mixing timescales.