Why Does Shaking Cause Expansion?

The shaking in and of itself does not cause expansion. The shaking accelerates the transfer of heat from the water to the air inside the jar. The warmed air expands and you get the "pop".

Try the opposite some time: Take an empty pop can (the aluminum kind) and place a few teaspoons of water in it (just enough to cover the bottom). Place on stove top and heat to boiling. Just before all the water boils away, use a pair of tongs to quickly invert the can into a bowl of ice-water. (Try not to jump to hard...)

Giorgia,

This is a result of the Air & Steam from the Hot Water expanding in the Jar.

Once you put the Lid On, you have created a fixed volume. The Hot water is doing two (2) things: -

1) It is heating the air Trapped inside the Jar with heat source (Hot Stuff) which when heated expands.

2) If the Hot water is hot enough to Steam, the water itself is expanding (as steam) which is also adding to the pressure.

When you then open the top, the pressure is released as a "Hiss-Pop" sound as the excess pressure is vented.

The reverse of this happens if you fill the Jar with hot liquids & gasses (or Jam, Chutney pickles etc). You fill it hot and the Jar will "Vacuum Seal" itself as the heated liquids & gasses "shrink" as they cool.

Hope this Helps,

Thought you might have been "Off Topic-ed" away.

Regards,
Sapper

Hi Giorgia,

I'm not so sure (I'm not an engineer or scientist as well).

My take on this is that the steam and hot water cools when you shake the jar (through contact with the jar's walls) causing the steam to condense and create a vacuum in the jar. When you open it, it pops because it is vacuum-sealed not because it is pressurized.

I'm not sure if it can be noticed or measured with any house-hold item, but if the lid is bulged, the jar is pressurized. If it's pulled in, the jar is under vacuum. I think that would be a way to find out what's really happening.

If your lid is too stiff, you might not be able to measure it. Try using a jar with a thinner lid.

regards,

Vulcan

Vulcan,

Your are partially correct.

The shaking does cause heat transfer, but because you are increasing the surface area of the liquid, the heat goes to the air in the Jar.

Try this using Warm Water (Not hot enough to Scald)

Get a container with a with a Press Seal lid (Tupperware(TM)) for example.
Fill about 30% with Warm Water and Shake..(Prepare to be Wet)....The top will usually Pop off.

When I was younger and Naughtier I used to do this with PET soft drink bottles.

They Go Hard (rigid) when you shake them and you can No longer squeaze the sides in...After about 15-20 minutes as the bottle cools... the bottle will crumple under vacuum....Lots of fun...

Regards,
Sapper

Simple - you have an implosion rather than an explosion. The shaking has nothing to do with it. When the contents cool the air inside contracts and forms a partial vacuum when you release the lid the pressure equalises with a "pop"

The answers that said the air is heated and expanded by the hot water were correct. But those that mentioned steam were very wrong. At normal pressures, steam is water at over 100 degrees C. You cannot create steam in an open pan of water (apart from the transient bubbles rising from the bottom) and the moment you pour the water into a cold container the temperature will drop way below one where steam can exist.

While we are correcting misconceptions, it is also worth mentioning that steam is a colourless gas, like oxygen or nitrogen. The stuff you can see above a kettle spout or factory chimney is not steam - it is a cloud of water droplets, condensing from the moist air.

To correct misconceptions.

Water boils at Normal (sea level) pressure only at 100° C Temperature and Will boil (turn into vapour) at lower temperatures. Increase the pressure and the boiling point increases.

The vapour pressure above the liquid will rise as the temperature increases and boiling point is reached when they are the same.

In this case, on cooling, any vapour in the jar will condense and the air will contract at the same time both of will cause a drop in pressure and a partial vacuum.

Dude, my head spins. I hope you're not a teacher.

Partial pressure of the water vapour (a mixture of air and water particles; not steam) becomes very high. By shaking you have created a very large surface for the water to enter into vapour by evaporation. Heat is taken from the hot water to pass from liquid to vapour - effectively the latent heat of evaporation. Your bottle has actually cooled down somewhat. If you were to do this even with cool water, you would still get a small "pop" and you would probably also be able to notice that the bottle becomes a little cooler (at these "room" temperatures we are more able to distinguish changes in temperature).

If you shake a can of drink then it also gets colder though much higher partial pressure will make the whole thing open with a helluva pop.

"I'm no engineer or scientist of any sort"
But I suspect a good observer - a truer scientist than most who have the title

Assuming accurate deduction (increased pressure), the cause is the combination of heating the air (that causes it to expand), and the addition of water vapour to the air. The shaking spreads the surface of the warm water, which speeds up both the transfer of heat to the air and the evaporation of the water.

The alternative explanation that you have created a vacuum is also possible - it all depends on quantities and timing. If all the hot water goes into the bottom of the jar and you put the lid on slowly, evaporation and heating of the air could already have occurred before you have sealed the jar. In that case the shaking would place more of the jar in contact with the water, so the water cools down quicker. Shaking the cooled water will also cool the air, so the vapour condenses and the air shrinks.

If you are uncertain which it is, you can check by quickly damping round the edge of the lid before you open it - bubbles pushed out would indicate pressure...

But I recommend you take the time to make batches of your own pasta sauce and freeze it - all the additives in the commercial product can't be doing any good.

all the additives in the commercial product can't be doing any good

Exactly right! Especially if you are a smoker, deal with stress every day, as sedentary as a pumpkin and drink like no tomorrow. The additives in the pasta will harm you the worst...

Nice one - but as sedentary as a pumpkin ?

Lol. Dont be ridiculous. If the additivies in the pasta were so dangerous in comparison to a combination of sedentary lifestyle, smoking and alcoholism, you would see a huge number of consumers (particularly since pasta is so prevalent in most peoples diets) be dead in a matter of a few of years. And, since we start consuming large quantities of pasta at a very early age as children, well... so what is the teenage mortality rate amongst say ethnic groups that consume particularly large quantities of pasta in their diet versus those who do not (but have the same availability of medical resources).