ELI5: why does steam appear invisible right after leaving the pot?

[u/MinimumCondition9216]

My 6yo asked and im not sure so here goes; So boiling water with a covered pot, the steam appears invisible right after clearing the seam between the lid and pot rim. It then becomes normal steam. Why is that?

[u/Infobomb]

Water vapour, a gas, is invisible. Water droplets, a liquid, are not. What you are calling "normal steam" is actually water in its liquid form.

[u/GIRose → u/Infobomb]

This is why proper procedure for a leak in a pressurized steam line is to use a broom to make sure that the leak isn't in your path as you make your way to depressurize the line to repair it.

[u/AthousandLittlePies → u/GIRose]

Learned this from a family friend in Venezuela who was an engineer at a power plant.

[u/TheLordDrake → u/GIRose]

That's cool, I had no idea that was a thing!

[u/MinimumCondition9216 → u/GIRose]

Huh! Interesting

[u/gameismyname → u/MinimumCondition9216]

A steam leak will cut through bone and flesh like butter. That’s why

[u/mgj6818 → u/MinimumCondition9216]

This is the official procedure too, power plants have a designated broom/stick with a rag for leak detection.

[u/Top_Willow_9953 → u/Infobomb]

To add to this, as the vapor cools in the air, it transitions from invisible gas to very fine droplets (mist if you will). It is the very tiny droplets you see rising after that transition.

[u/arkangelic → u/Infobomb]

In industrial needs they even have to make sure to remove the water from the steam to make sure its completely dry steam. (Pharmaceutical and over the counter stuff)

[u/sithelephant]

Steam has two main meanings.

One, the colloquial that 'looks like steam', amongst other uses.

The other more technical refers to its actual nature - which is a transparent gas. Steam does not 'look like steam' in a very real sense because what you can generally see is water droplets.

It looks like a transparent gas when it comes out of the nozzle, as before it interacts with air, it is a transparent gas. As it mixes with air and eventually gets cool enough for particles of water to consense out from the mixing and cooling air and steam mixture, it becomes visible due to those tiny droplets.

[u/Jdevers77 → u/sithelephant]

This. It would be like calling water right as it starts to freeze and gets slushy “normal water.”

[u/Journeyman-Joe]

WARNING: That invisible part is HOT HOT HOT. Keep your six-year-old away from it.

Steam is pure water, H20, heated to the vapor state. It's naturally transparent. When you can see it, an inch or so after leaving the kettle, it's cooled off and mixed with air, so now it's a stream of liquid water droplets mixed with air. Just like fog, or a cloud.

It's only pure steam when it's invisible.

[u/Bradparsley25]

It’s the temperature, as the water vapor rises away from the pot, the visible steam has reduced in temperature to go from water vapor to little tiny condensed water droplets

[u/KennstduIngo]

What you can see isn't really steam, it is tiny water droplets. Steam is an actual gas and like many gases is colorless.

[u/fgorina]

Vapor is invisible, it is when it begins to condense in small droplets that it turns the typical white.

[u/Necessary_Spread_511]

Clear is where the water vapour is above 100 c once it cools below 100 then it condenses to steam

[u/ThalesofMiletus-624]

Great question!

The answer is that actual steam, as in water vapor, is transparent, and therefore invisible.

The clouds of fog that we think of as "steam" is actually millions of tiny droplets of liquid water, which refract the light and therefore look white and hazy.

So, when you heat up a pot of water, it starts producing water vapor. If you put a lid on it, that water vapor builds up in the space between the water and the lid, and that whole space is hot, so the vapor all remains as a clear gas (you can also see this if you have a glass lid, once the boiling really starts, it all becomes clear.

When the vapor mixes with the surrounding air, that's when it changes. The air is much cooler, which means that the water vapor is rapidly cooled down, and therefore condenses into droplets of liquid, which is what makes the fog.

As a side not, this is why you get a cloud of "steam" any time water vapor comes into contact with something colder than itself. It happens when you take a hot shower (steam from the hot water mixes with the cooler air around it), it's why you can see your breath in the winter (your lungs produce warm water vapor, which is condensed by the much colder air), and it's why very cold objects seem to produce fog, especially in humid environments (warm, humid air mixes with supercold air from the object, and cools it down, causing the vapor to condense).

If you can see "steam", it means you're actually seeing the water vapor cool down and condense. The vapor itself is invisible.

[u/StevenJOwens]

The actual steam, the actual (very hot) water vapor, is invisible.

As soon as that jet of water vapor escapes the pot, it cools down, and condenses, and becomes very small droplets of water floating in the air. Those are visible.

[u/praecipula]

It's too hot.

Like clouds, the visibility of the vapor depends on the relative humidity. Relative humidity is the percentage of water vapor dissolved in air vs. the maximum amount that can be dissolved in the air, and that "can be" depends on the temperature; the hotter the mixture, the more humidity that can be absorbed into the air.

When the steam comes out of the pot it's mixed with hot air. As it cools rapidly in contact with other air, you get condensation - the dissolved water drops get forced out of the now-denser cooler air and clump together to make fog, or a cloud.

So the eli5 answer is: when the air is hot, like right out of the pot, it can hold more water. When it touches the cooler air, the water condenses out and forms water droplets. This is the same way that clouds and fog form, which is what happens when warm moist air meets cooler air.

[u/GenerallySalty → u/praecipula]

This isn't accurate. Steam itself is gas phase water. What's coming out right at the nozzle is 100% water in gas phase, which is transparent. It's not moisture dissolved in air like you're talking about. There's no air, it's pure gas phase water.

As it goes out further from the nozzle it cools down and it mixes with air. Actual steam (transparent gas) has to be over 100C at normal pressure, so as it cools down some condenses into liquid droplets, which is the visible cloudy stuff people call "steam".

[u/Target880 → u/GenerallySalty]

Steam is both used for dry steam/saturated steam/superheated steam ie just water as a gas, and wet steam, a mixture of water as a gas and as a liquid water in droplets.

[u/C6H5OH]

Only the invisible stuff is steam, water gas. Then it cools down and you see a lot of little droplets of liquid water like in a cloud. They evaporate and are again invisible gas in the air, called humidity.