Kitchen Science

[Bob in a quantum-state-of-faith]

Today, I was making some new iced tea.

I wanted to make it in bulk, so I put about a gallon (US) of water in a pot, tore open 3 teabags, dumped the contents into the water, and bring to a boil.

Turn burner off, let the water cool to just not boiling.

While it's cooling down, prepare the container:  basically a 1 gallon plastic milk jug (washed out), with a homemade funnel (from a small bleach bottle, cut the top out-- it's smaller opening just snaps into the top of the milk carton).  Put a permanent coffee filter into the funnel (basically very fine stainless steel mesh with a plastic frame).

By this time, water's not actively boiling in the pot on the stove.  Carefully pour the tea through the funnel into the plastic jug.  (Careful! It's boiling hot...)

The hot water will act as a sterilizer for the inside of the milk jug, an added bonus.

Now, the science bit.

I wanted to quickly cool the tea, so I could put it into the fridge-- but boiling water is too hot for my old fridge, and would've raised the temperature of the other food 'way too much.

So, in an even bigger saucepot, I set the very hot plastic jug full of near-boiling tea.  Carefully! The plastic is a bit soft.

Fill the pot with cold water, until the plastic jug floats.

Question:  at what point does the jug full of hot water float in the pot of cold water? 

1) when the cold water is about 1/2 inch (1cm) below the top of the hot water in the jug?
2) when the cold water is even with the hot water in the jug?
3) when the cold water is about 1/2 inch (1cm) above the top of the hot water in the jug?

Why?

.....

If you answered 1), you're correct.   Roughly 1/2 inch or so below the top of the hot water, the jug floats freely.  Why? Because hot water is much less dense than cold water!

Totally cool.

Wait about 5 minutes, and the water in the pot gets pretty hot, and the jug has cooled a bit.  Dump the pot-water, and re-fill with cold water a second time.

How high does it get now?

Roughly 1/4" or less-- the jug water is not so hot, and is more dense than before.   

Wait about 15 minutes, and the jug of water is cool enough to put into the fridge, now.

More evidence that hot water is more dense than cold.

I put the cap tightly back onto the jug when the water was still near-boiling.  The jug's sides had actually expanded a bit, with the heat-- the jug looks as if it's swollen.

But, since the cap was on tight, the volume of the jug remains constant, yes?

No...as the jug cools down the sides of the jug contract.  The actual volume was less for the cold water than for the hot.

.............

_{Extremely picky scientists will ask what effect the tea dissolved in the water had on the whole thing.  Answer: except for altering the taste, the percentage of dissolved tea solids with respect to the water is negligible with regards to density and volume.}

[Aggie]

So, in an even bigger saucepot, I set the very hot plastic jug full of near-boiling tea.  Carefully! The plastic is a bit soft.

Next week's lab:  Temperature dependency of the aqueous solubility of pthalates.

[anthrobabe]

I have always made tea in a leftover milk jug--- would always fill the sink with cold water to pre-chill it before popping in fridge.

[Swatopluk]

Extremely picky scientists will ask what effect the tea dissolved in the water had on the whole thing.  Answer: except for altering the taste, the percentage of dissolved tea solids with respect to the water is negligible with regards to density and volume.

That's the method to distinguish between diet coke and "real" coke without having to taste it