Question

What is meant by -ve deviation from raoult's law?

Answer 1

Deviation simply means Change...Here when we add two liquids to form a solution, The vapour pressure of the solution generally lies under the range of Vapour pressure of the two liquids in their Pure state...

Such solution obeys or follows Raoult's Law...And solution which follows Raoults law are called Ideal Solutions...

In ideal solution, Del_Hmix = 0 which means when liquids are mixed together then heat or energy is neither absprbed or released...

According to Raoults law..

P(Solution) = p(liquidA) + p(liquidB) = S, say

But This is not always the case when we add two liquids together we get an ideal solution...
Some of the solution does not follows the Raoults law...Solution which do not obeys the Raoults law called Non-Ideal solution...

In such solution the value of P(solution) Either greater than S or less than S...

This is what we observe change in Solutions here...Remember this only happens if solution doesnt follows the Raoults law...If we apply Raoults law to calculate the Vapour pressure of such solution we get the value either less or greater than the value that we just get from Raoults law...

This change is called deviation here...

You can see the graph where you see the change or deviation...that deviation you will find on Y- Axis
clearly...Deviation above the P(Solution) is a positive Deviation and Deviation below the P(Solution) is a -ve Deviation..see graph..

for more details...Go through your book...Hope this help you too...