This is a strange question and probably has a pretty simple answer, but I can’t figure it out and it’s bugging me.
The hot water in our house takes a good 60 seconds or so to warm up completely when you turn on the hot water tap. Therefore, whenever I get ready to do the dishes or take a shower, I turn on the water and do something else while waiting for the water to get hot.
When it finally does get hot, the sound of the water rushing out of the faucet changes pitch and that’s how I know the water’s good and hot. It sounds slightly different than the colder water coming out before it. Why is that? Is the hot water traveling at a different speed than the cold? Does it have something to do with the pressure in the pipes from the hot water heater?
Actually, it is the heat, but it’s not the holes in the shower head. No, it’s the heat causing the washer to expand, which in turn causes the speed of the water flow to decrease.
The sound of water heating in a kettle also changes as the water gets hotter. Strangely enough, the sound is caused by the collapse of small bubbles of steam rising from the hottest part of the kettle. As the tiny steam bubbles rise through colder water, the steam loses heat, which is why the bubbles collapse.
Eventually, all the water is at boiling temperature, so the bubbles of steam get larger and don’t collapse before they reach the surface.
I’d guess that hot water coming out of a pressurized pipe will suddenly lose both dissolved air and steam, creating bubbles and sound.
No, even though the thermal expansion (of washers, pipes and valves as well as the water itself) is important, and the dissolved gasses have a role, the most important factor is the viscosity.
Boiling water has only a sixth of the viscosity of 0[sup]o[/sup]C water, and this gives rise to completely different characteristics as it passes faucets etc.
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Sure, viscocity is related to temperature - so is vapour pressure and many other properties. But, the pitch of the sound is related to density and elasticity (not viscosity).
Yes, but they only vary by a few percents in the spectrum of normal heating of water. Surely that cannot explain the wide variation in pitch that is perceived?!
I must admit that I haven’t investigated the matter fully, but I would have thought that the change in viscosity would change the turbulence quite a lot, which in turn would change the pitch of the sound.
But you are right, I probably haven’t understood the situation completely.
Regretfully I must confess that there have been no further results of the investigation.
Does anyone else have any ideas as to what might cause this effect? (I still strongly suspect the viscosity, but there is probably more to it than that.)
