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Every morning, I fill my electric kettle from the fridge at room temperature or below ($\approx 70 \ ^\circ \text{F}$, $\approx 20 \ ^\circ \text{C}$), set the water to boil, then leave the room to brush my teeth, then pour the kettle into my coffee. At that time, the water boiled, and then cooled to below the point where there was significant visible wet steam ($\approx 190 \ ^\circ \text{F}$, $\approx 90 \ ^\circ \text{C}$).

Today, and a few days ago, I neglected to press the button to start it boiling. Both times, the noise of my electric toothbrush, and being in another room prevented me from (consciously at least) being able to tell that the kettle had not boiled.

On pouring the water from the kettle into the cup, I immediately noticed that the water "looked too cold"; stopped only about a quarter of the way into filling the cup; and checked the temperature of the kettle to find that it was not warm; and was able to boil the kettle and pour the now-boiling water into the mug, allowing me to still have a nice warm mug of coffee without having to redo it from scratch.

Since I've now done it twice, it feels to me as if there is a definite and reliably noticeable difference in the way water pours at fridge temperature compared to boiled temperature. I've never seen this mentioned, so:

  • What is the difference? To me, it seems as if the cue I'm picking up on is that cold water looks too laminar/viscous, while hot water looks more turbulent/runny. As it's a mostly-unconscious cue at the boundary of my awareness, I'm not 100% sure. Consciously, it just looks and sounds "wrong". I don't think I'm picking up on less direct, unconscious cues like "I didn't press the button", "I didn't hear it boil" or "I don't see wet steam condensing", as those aren't the parts of the pour that look wrong to me.

  • Why? What causes this difference? <- This is the main question I'm asking, but I suspect that first understanding the "what" is necessary to provide the "why".

[Edit: a similar question on the acoustics of different water temperatures is here: I've edited to remove acoustic aspects from this question, to focus on the visual aspect not covered by that question. However, it may not be worth reopening as I suspect there'll be the same answer to both: $20 \ ^\circ \text{C}$ water has triple the viscosity of $90 \ ^\circ \text{C}$ water.]

Qmechanic
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Dewi Morgan
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2 Answers2

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Viscosity will be the main attribute that changes the flow of water. Heat, say ~20c to ~90c can change viscosity by 3 times.

There’s also surface tension which changes with the increase of temperature. These will make water prone to turbulent flow. This should be noticeable visually.

Density changes slightly with temperature, but this would be a very subtle effect.

References:

Water Viscosity Calculator

Always check Wikipedia Temperature dependence of viscosity

Is it plausible that the viscosity of water at different temperatures can be detected visually?

DrJay
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I have noticed this too!

As the temperature of water approaches its boiling point, its surface tension falls appreciably and its viscosity approaches some minimum value.

As water is cooled, its surface tension increases to a certain value and its viscosity increases (but BTW goes through a sudden increase very close to the freezing point).

So water dumped out of a boiling kettle tends to exit as a blubbling (my invented term), gushing, fast-moving blob but water dumped out of a cold kettle forms a distinct and mostly self-contained stream.

niels nielsen
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