[user84985]

The question's in the title, and I've heard every answer: I used to think that the water level would rise slightly, then heard from some people that it doesn't change at all, and I then even heard from others that it even falls a little. Which one is true, and why?

Thanks!

[Pradeep]

My self I dont have a clear idea. But I like to make a guess.

When Salts are dissolved in water, water mollecules make a solvation( hydration) sphear arround those ions. So randomly orders water mollecules become more order manner. According to me this can decrease the volume of water.

Open For coments.

[Borek]

What salt?

Ordering effect is possible, but remember after the dissolution you have to deal with volume of solvent AND salt.

Density of 10% w/w Na₂S is 1.1150 g/mL, density of 10% w/w NaCl is 1.0857 g/mL. Has the volume increased or decreased?

Is there one answer to the question?

[user84985]

Thanks for the replies. 
Regarding the kind of salt, I was referring to ordinary table salt, NaCl. And I'm not sure why the volume change is ambiguous -- I meant that you measure (for example) 200 mL of water, then add a tablespoon of salt. Is the new volume greater than, less than, or equal to 200 mL, and why?

Again, thanks for the replies, but I'm still not sure about the answer.

[Borek]

Try to use density information I have given to calculate volume of the solution. Assume you have prepared 10% w/w solution starting with 90 mL of water. How much salt do you have to add? What volume do you end with in each case?

[user84985]

I'm not sure if I'm doing this correctly (Chemistry is neither my weakest nor my strongest subject), but from what I understand, if you start with 90 mL water, then you're starting with 90 g water, and if you add 10% NaCl weight for weight, then you get 99 grams, correct? Then using the density, we get (99 grams) / (1.0857 g/mL) = 91.19 mL, which indicates that the volume would increase by 1.3%.
In contrast, it seems that if you use Na₂S instead, then the volume would become 88.79 mL instead, which would mean it would decrease.

Is this correct? And so is the answer that "it depends on the solute"?

[Borek]

Minor issue with your calculations - to get 10%w/w solution you need it contain 10g of solute in 100g of solution, so you should add 10 g and final mass would be 100 g. 9/99*100% = 9.09%.

Other than that your calculations are OK and lead you to the right conclusion

Note that in most cases you will observe volume increase, Na₂S(aq) is a rare case of solution decreasing its volume - but such salts do exist, so you can say that volume always increase.

[aeacfm]

i think the volume is not changed as :
if you get the density of Na2SO4 =  2.664 g/cm3 (anhydrous) but for NaCl = 2.165 g/cm3 so the density of the solution is specific for the material but the volume i think it is constant !!!!!
i dont know may be iam wrong

[Borek]

No, volume is not constant.