Boiling and melting point of impure substances

  • Thread starter sgstudent
  • Start date
  • #1
739
3
They say that an impure substance has an increased boiling point and reduced melting point (i know that they will melt/boil at a range of temperature). But does impurity mean that its melting and boiling point is higher than the substance itself or it doesn't matter?

I think that the impurity has to have a higher boiling. When we dissolve salt and water, the boiling point increases to about 117 degree celcius. when we say the boiling point increases, we take the water's 'perspective'. meaning we see it in water's point of view. however, if we look at salt's perspective, the boiling point has decreased. So i think that the impurity has to have a greater boiling point. Also, is this assumption correct: We take the perspective of the substance with the lower boiling point when an impurity is present. And the boiling point of that substance will increase. So using this assumption, when we pour ethanol and water into a beaker and heat it up, the boiling point will be around 85 degrees? As, i take the perspective of ethanol being the lower boiling point substance.

I also think that the impurity has to have a higher melting point. When salt is dissolved in water and frozen then melted, the melting point of the substance decreases to around -5 degrees. Again, we take the perspective of the substance with the lower melting point. Water's melting point is 0 degrees while salt has a large boiling point. so when they say impurities decrease melting point, they are taking the perspective of water. So is this assumption right: We take the perspective of the substance with the lower melting point when an impurity is present. And the melting point will further decrease from this. So, using this assumption when alloys are made the impurity will be the metal/non-metal with a higher melting point and thus the other metal will have a lower melting point. So, i look at the lower melting point metal and decrease it further. This seems true since alloys have a lower melting point than the metals in it.

thanks for the help!
 
Last edited:

Answers and Replies

  • #2
Borek
Mentor
28,703
3,192
It can be even more complicated than that. Statement "impure substance has a higher/lower boiling/melting point" is only an approximation, and it works exactly only when the impurity has much, much lower vapor pressure than the main component of the mixture. Hence it works OK for a salt in water, but in the case of water/ethanol mixture it becomes much more problematic.
 
  • #3
739
3
oh ok. i also read somewhere that if a volatile substance is placed into water (for example) then the boiling point will drop again taking into perspective or the water. this makes sense as in all my impurities case, i was taking the impurity to be non volatile with a high boiling point.

so are impurities in these case considered to be non volatile with high boiling and melting point?

thanks for the help!
 

Related Threads on Boiling and melting point of impure substances

Replies
9
Views
131K
Replies
10
Views
21K
  • Last Post
Replies
1
Views
845
  • Last Post
Replies
13
Views
19K
  • Last Post
Replies
2
Views
6K
Replies
1
Views
3K
Replies
17
Views
6K
Replies
6
Views
4K
Replies
1
Views
2K
  • Last Post
Replies
1
Views
1K
Top