"sucking back"

[asto5]

in a chemistry experiment, what does the term "sucking back" mean. its got something to do with a saftey precaution when the apparatus cools?

cheers in advance

[theprof]

The best way I can show you what this means is to give you an example.
A standard method of producing Ethene gas in a lab is to heat ethanol with concentrated Sulphuric Acid in a round bottomed flask with a small quantity of sand added to prevent "bumping". The gas produced is bubbled through 10% Sodium Hydroxide solution in a wash-bottle and then collected via a delivery tube into an inverted water-filled test-tube in a water bath.
After the Ethene has been collecting in the test-tube for a few minutes, the delivery tube must be disconnected from the round bottomed flask before the heat is turned off.
The reason for this is that as the gas cools it would contract, resulting in the wash-bottle solution sucking back into the round bottomed flask. The reaction between the Sodium Hydroxide and hot concentrated Sulphuric Acid would be massively explosive.

[Bert]

There's a chap where I work who always spells "safety" as "saftey". Is it something to do with the brain and the fingers (I often type "form" when I mean to type "from")?

[asto5]

cheers theprof, thats basically what i thought but needed confirming!

i dont know about that one bert, its probably in your case that your more used to writing from! in my case i was just in a hurry!

[theprof]

Glad to have been of help.