Question

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

Answer 1

First, a correction: water cannot be “red‑hot.” At atmospheric pressure liquid water boils at 100 °C, so anything much hotter than that is steam, not liquid. I’ll assume you mean “very hot (near boiling) water.”

What happens depends on whether the tank is open/vented or tightly sealed.

1) Open or vented tank
- Mixing and convection: hot water is less dense, so it tends to rise and form a warm layer at the top. Depending on how you pour it and how different the temperatures are, you can get stratification (hot layer on top), turbulent mixing, or both. Over time convection and diffusion cause the whole tank to approach a single temperature.
- Steam and splashing: if the hot water is at or near boiling, you’ll see steam (vapour) at the surface. Pouring boiling water into much colder water can also cause vigorous bubbling and splashing; that’s a burn hazard.
- Final temperature (no boiling): if no vaporization occurs, energy conservation gives the final temperature Tf ≈ (m1 T1 + m2 T2)/(m1 + m2) (same specific heat assumed for both). Example: 1 kg at 90 °C mixed into 9 kg at 20 °C → Tf ≈ 27 °C.

2) Sealed (rigid) tank with a lid that traps steam
- Pressure rise: any steam produced or thermal expansion of the liquid will raise the pressure inside the tank. Higher pressure raises the boiling point, so trapped water can stay liquid at temperatures above 100 °C — but the trapped steam/pressurized water will be dangerous.
- Possible outcomes: if the vessel is not pressure‑rated or there is no relief, pressure can build to the point of deforming the tank or violently blowing the lid off. If the lid is bolted on, catastrophic failure is possible. If the lid vents, steam will escape until pressure balances.
- Flashing: when very hot liquid contacts cooler liquid, some of it can instantly vaporize (“flash”) into steam, adding a rapid pressure impulse in a closed space.

Other effects
- Thermal expansion changes liquid level slightly.
- Heat loss to the tank walls and surroundings will reduce temperatures over time.
- If the hot water contains dissolved gases, heating will drive gases out (bubbling).

Safety
- Don’t pour boiling or near‑boiling water into a closed container.
- Use pressure‑rated vessels and a working pressure relief valve for sealed systems.
- Beware of scalding from steam and splashes.

If you tell me approximate masses/volumes and temperatures and whether the tank is vented or sealed, I can estimate the final temperature, how much steam might form, or how much pressure could build.