AP Chemistry question

Test Preparation AP Test Preparation
1

<p>What does a dip in the cooling curve signify?</p>

<p>I’ll try my best to reproduce it with slashes:</p>

<p>Temperature v. Time</p>

<p>
-
–______________
-------------------
----------------------__________
---------Here -----> /------------
------------------------------------&lt;/p>

<p>*Ignore the dashes - formatting reasons…</p>

<p>What happens at this point, and why does the liquid and solid phase coexist (or liquid freezes) on the second plateau AND the dip inclusive? </p>

<p>This question showed up on a practice exam and in my review book (Peterson’s) - PM me for more details.</p>

<p>Thanks, a lot.</p>

2

<p>B U M P</p>

<p>(10char)</p>

3

<p>This is called supercooling. I’ll use (pure) water as an example.</p>

<p>Crystals of ice take some time to form, so if water is cooled fast enough, the water can remain a liquid past its normal freezing point before ice crystals begin to form. During this time, the temperature continues to drop as energy continues to be released. Then when ice crystals finally begin forming, the temperature rises slightly and then plateaus since the phase change from liquid to solid requires energy to complete.</p>

4

<p>I got ya’ covered dude.</p>

<p>Usually a curve like that shows up with negative phase changes (gas -> liquid, liquid -> solid). This lowest dip area is the temperature (boiling pt. or freezing pt.) for the particular substance. The reason it goes up is that these downward phase changes are exothermic processes (hence why steam burns are worse than water scalds).</p>

<p>The reason both states coexist on that plateau as well is that, since the temperature is not dropping, apparently there are still molecules condensing or freezing (to negate the effects of the temperature drops, sort of like an equilibrium). Then, after all the molecules have changed states, the downward trend continues once more.</p>

<p>Hope that helps, and good luck on Tuesday!</p>

5

<p>so does liquid and solid coexist from the point of the “bump” to the end of the plateau?</p>

6

<p>Yes.</p>

<p>Thanks wxmann and ecthelion!</p>

7

<p>To an extent, yes, but in reality, they’d probably start their reckoning at the beginning of the plateau…</p>