Intensive Property-Concentration

[andyrk]

Main Question or Discussion Point

How is concentration an intensive property? I mean if you add more amount of a substance(mass), the number of moles increase and so the concentration increases. So why is concentration not an extensive property and is an intensive property?

 

[Borek]

Imagine having a lot of 2M solution of sulfuric acid.

You take 1 mL and weight it. It weights 1.1206 g.

You take 1000 mL and weight it. It weights 1206 g.

Conclusion - mass of the solution is an extensive property, it depends on the amount of the solution.

Now you take 1 mL and check the concentration of the acid. It is 2M.

Now you take 1000 mL and check the concentration of the acid. It is 2M.

So the concentration is an intensive property, it doesn't depend on the amount of the solution.

Note that concentration is a RATIO - in a way you divide "amount" (of substance) by "amount" (of solution) and what is left no longer depends on the "amount", as it canceled out.

 

[andyrk]

Ok, suppose we have a 2M solution of sulphuric acid and then we add some more suplhuric acid to it. As concentration is an intensive property, the concentration of sulphuric acid even after adding suplhuric acid should remain same but it is not like this. Or does it still remain the same? Because, I think that when we add more sulphuric acid though we are increasing the number of moles of sulphruic acid but we are also increasing the volume of the solution (as the new coming suplhuric acid makes the volume increase). So does that make the concentration remain the same?
The definition of an Intensive property is: Properties which do not depend upon the quantity of mater present are called intensive properties. So concentration should not depend upon the quantity of sulphuric acid (matter) present. So whether we add 4 gram or 4kg the concentration shouldn't change. But is it so? I don't think so. So that's why I am confused.

 

[Borek]

When you add PURE sulfuric acid to the solution, you do change its concentration, as you change the RATIO of sulfuric acid to solvent.

If you add 2M sulfuric acid to 2M sulfuric acid, concentration doesn't change.

So you need to be careful about what you mean by sulfuric acid in this context (2M or pure) and you have to remember your mixture has more than one component (the other being solvent - most likely water).

 

[andyrk]

So "Independent of amount of matter" , here matter means the whole solution of suplphuric acid and water and not pure sulphuric acid..right?

 

[Borek]

Yes.