# Interesting phenomena of boiled water and my coffee.

#### BOAS

Hello,

whilst making my coffee this morning (more like this afternoon, but hey i'm a student), I noticed something that I thought was cool and i'm interested in hearing what you guys think may be happening and/or what you have to say about my ideas.

So, I have these 'coffee filters' that are halfway between proper coffee and instant coffee. They are essentially open plastic containers with a filter paper sachet filled with ground coffee as the base that fit in the top of a coffee cup. You pour boiling water into the top and the coffee drips down into the cup.

I poured the water into the top and walked away to put my bread in the toaster. I came back and noticed no coffee had dripped through, but there was fizzing in the hot water. Enough that little droplets were being thrown a few centimeters above the surface.

The coffee filter creates a reasonably tight seal around the cup so what is going on?

Is the hot water heating the air inside the glass raising it's pressure and forcing it out through the water, or perhaps the coffee trying to drip into the glass is raising the pressure by decreasing the volume the air can occupy. Or is there something stranger going on causing the water to reach boiling point again?

I know this is lacking in the physics side of things, but it would be cool to understand why.

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#### mfb

Mentor
Is the hot water heating the air inside the glass raising it's pressure and forcing it out through the water
That looks like a possible explanation. Bubbles of air can look like boiling water, even without actual boiling. And if you have no other way for air to escape...
or perhaps the coffee trying to drip into the glass is raising the pressure by decreasing the volume the air can occupy
I thought no coffee had dripped through?

#### BOAS

Thanks for the response.

That looks like a possible explanation. Bubbles of air can look like boiling water, even without actual boiling. And if you have no other way for air to escape...
I thought no coffee had dripped through?
Yeah, that does seem like the most likely explanation and yes, no water had dripped through so that idea is unlikely.

I think this further observation strengthens the idea of the air pressure being increased; If I break the seal by lifting the container out of the glass for a moment, equalising the pressure, coffee begins to drip frequently, whereas if I leave it to it's own devices it can take upwards of a minute for anything to happen!

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