If water boils at 100°C, and boiling is the process of turning liquid into gas, why are bathrooms full of steam when showering at only 40°C?
In: Chemistry
Boiling is the process of turning liquid in the center of the mass into gas. Exposed surfaces evaporate at nearly any temperature. Even solid ice slowly turns to water vapor on the surfaces (sublimation).
Because boiling is not just evaporation. Water evaporate all the time even at 5°C, but much slower. 100°C is the temperature at which water can turn into steam not only at the surface, but also inside of water. That’s what these bubbles are. It’s water that reach 100°C and can no longer remain in liquid state, even if it’s surrounded by water.
The “steam” in the 40 degree bathroom is actually condensed water vapor consisting of tiny droplets of water. It is not actual steam which is a gas.
Because the water doesn’t need to be at boiling point to evaporate! Warm water increases the rate of evaporation, and a hot shower becomes a steamy room because it’s full of tiny water droplets suspended in the air, aka humidity. Plus, showers are basically little steam machines since it’s a confined space with constant heat.
To slightly tweak some of the other answers given here:
Boiling occurs when a (currently-liquid) substance becomes so hot that the pressure of the atmosphere can no longer keep the substance in liquid form–*all* of it will evaporate so long as it remains at boiling temperature. Molecules bounce around and fly apart from each other unless high pressure (force squishing them together) or low temperature (lack of energy keeping them from flying apart) prevents them from doing so. This is why water instantly boils when you put it in vacuum; there *isn’t* any atmosphere to push against it, so it immediately starts flying away.
But the thing is…boiling isn’t the only way for things to evaporate. “Volatile” compounds are ones that can evaporate on their own, without needing to boil first. Water is a very volatile compound, and easily evaporates even when it is barely above freezing. Many other compounds also do this, and that’s how your sense of smell works–your nose responds to tiny concentrations of volatile compounds floating in the air. Alcohol, for example, is a well-known volatile compound. Menthol, the substance that gives peppermint its “minty” taste, is also volatile. (Some volatile compounds are very dangerous, such as gasoline or nitroglycerin–so don’t think only foods can be volatile.)
When you’re in the shower, there’s a large temperature difference between the hot water (40 C, as you say) and the air around you (probably 25 C or thereabouts). This means that the water has excess energy, and the air has a comparative dearth of energy. Energy can be exchanged between them by having the water evaporate–that cools off the hot water, and warms up the air. But since the water evaporating loses some of its energy in the process (heating up the air molecules), it becomes less volatile (volatility is in part a function of initial energy). As a result, some of the water vapor ceases to be an invisible gas, and condenses into a very fine mist of microscopic water droplets–bigger than single water molecules, but much smaller than a true “drop” of water. You’ll notice more of this process if your house is dusty, because dust provides nucleation sites, for the water molecules to “cling” to in order to form these microscopic droplets.
It’s actually not boiling, it’s evaporation. Hot water from the shower increases the temperature and humidity, allowing water to evaporate more easily and form steam (which is just tiny droplets of water) even though it’s not at boiling temp.
We think of water is a single thing, when that’s not really the case. Having a cup of water is more like having a cup of very tiny marble balls, where the heat/temperature does not evenly distribute (it’s warmer at the top than the bottom as a rule of thumb if not heated or stirred). When water becomes gas where it’s exposed to air (i.e marbles that are on the top) that is evaporation and you don’t need to boil the water, because there is always a little amount of water that can evaporate, but the warmer it gets the more water evaporates. Boiling is when you turn water to gas inside the water and not on the surface.
So in a bath, you can easily see tons of evaporation (cuz its much warmer water than usual), but not boiling.
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That’s water vapor, not steam. Steam is overheated water vapor. You get vapor all the time even under 100°C.
First of all there is the concept of partial pressures in a mixture of gases. If I have two gases mixed together, with a pressure of 1 bar, and I have 25% of gas A and 75% of gas B, then gas A will behave the same way pure gas A at 0.25 bar will behave, and gas B will behave the same way pure gas B behaves at 0.75 bar.
Water boils at 100°C at 1 bar pressure. At lower pressure, it boils at a lower temperature, and at higher pressure it boils at a higher temperature. At 40°C, water boils at about 0.073 bar. That means, if I have a mixture of water vapour and air at 40°C, I can only have 7.3% water vapour in that mixture. If there is more water vapour, it will start to condense and form droplets.
If I take a hot shower, some of the water will evaporate. It will continue to evaporate until the air is saturated. At 40°C, that happens when 7.3% of the air-water vapour mix is water vapour. When I turn off the shower, the hot water is no longer making the air hot, and because the rest of the house is colder, the air will start to cool down. As it cools down, the fraction of water vapour that can be contained in the air goes down too. That causes the excess water vapour to condense as clouds of water droplets (commonly mis-named “steam”), or to form as water drops on surfaces like walls, mirrors, windows etc.
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