If you mix sugar Crystal in a glass of water and mix it well, the level of water will not rise.The reason they say is that matter have spaces between them.If matter have spaces between them , How come that empty space is not visible to our eyes.If I look at a glass of water , I see all the molecules mixed up well.Not like there are some areas where there is no H20 molecule and some they’re are.

What does it exactly mean and how does it look like and what is happening there?

My thinking: Is it like there are intermolecular forces between H20 molecules but they are at a separation from each other and still have the bond.Why does level inc if I put my finger.

  • 1
    $\begingroup$ your eyes cannot perceive distance that is that small.. Your finger is much bigger than a grain of sugar which the crystals are made up of. $\endgroup$ – Safdar Nov 14 '20 at 5:36
  • $\begingroup$ Is my thinking correct or is it sth elese. $\endgroup$ – srijan Sri Nov 14 '20 at 5:37
  • $\begingroup$ To be clear, dissolving sugar in a cup of water does cause the water level to increase, even if there is no solid left over. $\endgroup$ – Nicolau Saker Neto Nov 14 '20 at 10:31
  • 1
    $\begingroup$ Sucrose ( bulk, not loose/powdery) density is 1.59 g/mL. Volume of separate 900g water and 100g sucrose is about 900+100/1.59=963 mL . Density of 10 w/w % sucrose solution is 1.038 g/mL, so the total volume changes from 963 mL to about 1000/1.038 = 963 mL. So the total volume practically does not change. $\endgroup$ – Poutnik Nov 15 '20 at 22:53

Note that there is the law of mass and energy conservation, but there is no law about volume conservation.

Molecules of matter are in eternal motion. Molecules of gases move freely by flying between collisions. Nitrogen or oxygen molecules of air have an average speed of a supersonic fighter, colliding at rate typically 10 billions collisions per second, with the mean free flight distance typically 70 nm. Such motion creates space between them. ( Try to keep a sworm of vivid children in tight packed formation. )

Molecules of liquids and solids are held together by attractive forces. Loosely for the former, so they continuously separate and rejoin. Tightly for the latter, so they just vibrate.

Another reason for space between molecules is electrostatic repulsion of their electrons, if they get too close. If you hit a wall, you did not really touch it. The wall started to repulse you by the mighty electrostatic force, when your and it's electrons got too close.

If you mix 1 L of ethanol and 1 L of water and let it cool down ( because it warms up ), the total volume will not be 2 L, but about 1.96 L. It is due the fact the average energy of bonds water-ethanol (via hydrogen bonds) is greater than the average energy of bonds water-water and ethanol-ethanol. This leads to shorter average distance between molecules ( fractions of nanometre ), as stronger bonds are shorter, and to the volume contraction.

Aside of this chemical reason, there is geometrical one as well, both adding to each other:

Fill a container e.g. by peas/beans, and fill an identical container by rice. Then mix them and try to fill both containers again. You will see some volume will remain free. Rice will partially use otherwise free space between peas/beans.

The similar phenomena probably happens for the system sucrose-water, as I have no such personal experience.

Generally, such volume changes, smaller or bigger, always happen when 2 components are not indifferent with respect to each other and properties of the mixture are not a linear function of its composition.


Not the answer you're looking for? Browse other questions tagged or ask your own question.