Sec school seperation techniques chemistry help!

SgStudentStressed SG

Question 1:

Substance X: Melting point -50 degree celcius boiling point 40 degree celcius

Substance Z: Melting point 125 degree celcius boiling point 397 degree celcius

Explain why is it not possible to get a distillate of substance X when using simple distillation?

The Ans key's answer: Because X is volatile

i thought that it doesnt matter if its volatile? i mean in this case the substance will just reach a temperature of 40 degree celcius n X would be completely vapourised before substance Y gets vapourised? i think the answer key is wrong in this case. Am i right? thanks

Question 2: Why is cold water pumped into the bottom of a condenser and not the top?

Pls answer my questions in this format:

Question 1 ans: and Question 2 ans:

Thanks so much

UltimaOnline SG

Question 1:

Substance X: Melting point -50 degree celcius boiling point 40 degree celcius

Substance Z: Melting point 125 degree celcius boiling point 397 degree celcius

Explain why is it not possible to get a distillate of substance X when using simple distillation?

The Ans key's answer: Because X is volatile

Question 2: Why is cold water pumped into the bottom of a condenser and not the top?

Pls answer my questions in this format:

Question 1 ans: and Question 2 ans:

Thanks so much

Q1. Because the substance is very volatile, you won't be able to obtain much of it in a normal / standard distillation setup, as most of it would vapourize readily. You need to modify your setup, eg. using crushed ice to surround the beaker containing the distillate. So while you're correct in your thinking, the answer is also correct in its own way, but both of you failed to address the point of how you can obtain (without too much loss from vapourization) the volatile substance, ie. using crushed ice to keep the volatile substance from vapourizing.

Q2. Which do you think is more effective in condensing a greater percentage of the vapours? Pumping cold water from top to bottom, or bottom to top? Visualize the process and post your thoughts (on which option is more effective, and why) here. (I don't like to just spoonfeed answers, so you'll have to give your own ideas, and I'll comment on them.)

SgStudentStressed SG

question 1:ohhh yea i forgot about that. either that or i can put a stopper on the beaker so that it would not evaporate?

for question 2: isit like if i put it at the top, it would flow down very quickly thus it would not be as efficient if i put it at the bottom as the cool water has to collect itself for a while before it is disposed off in the top. will this be a good explaination?

UltimaOnline SG

question 1:ohhh yea i forgot about that. either that or i can put a stopper on the beaker so that it would not evaporate?

Q1. No, a stopper may cause the apparatus to crack due to the thermal expansion of gases.

Q2. No, it's not about gravity or speed. It's about the gradual warming of the coolant water as it travels (either up to down, or down to up). Which arrangement ensures a greater percentage of liquefaction of the volatile substance as its vapours travel downwards?

SgCoach SG

Q1. With or without the stopper, the volatile liquid will still evaporate (i.e. stopper does not stop evaporation). So in a simple distillation set up, you apply heat, which will cause even more gas to evaporate. Thus increasing the pressure and cracking the apparatus.

Q2. Usually in simple distillation, water is fed from bottom up. Both reasons are, in my opinion, valid. If you feed the water from top down, 2 things can happen: 1) water flows out the tubing without fully filling the condenser jacket. 2) when warm air interacts with the coolent at the top, the coolent becomes warm, and thus filling the jacket with warm coolent.

Thus, water is fed from bottom up. Firstly, the coolent jacket is filled, and secondly, the coolent interacts with the cooler air, and thus able to remain cool throughout the condenser to cool the air.

Cheers!

SgStudentStressed SG

but i tot the distillate will just condense and fall back into the reciever?

btw r u louis

SgCoach SG

I still won't recommend putting a stopper on the receiver. You might end up having the liquid condensing on the stopper and not being fully inside the receiver. I.e. UltimaOnline was trying to say that you have to make sure the receiver is cold so that the liquid does not evaporate from the receiver.

Cheers,

-Louis

SgStudentStressed SG

oh n wad does it mean by It's about the gradual warming of the coolant water as it travels (either up to down, or down to up)? i cant understand this, or nvm tmr tuition :)