In chemistry, volumes are not always additive. Can you think of an example for each of the following four situations?
(1) 19 cm3 of a compound is added to 89 cm3 of an identical compound, and the level of the liquid only rises to about 96 cm3.
(2) An appreciable amount of solid is dissolved in 100.0 ml of water, and the resulting volume of the solution is less than 100.0 ml.
(3) 50.0 cm3 of a gas is added to 50.0 cm3 of a gas. The resulting volume is 50.0 cm3 in some cases but 100.0 cm3 in others.
(4) 10.0 ml of one liquid is added to 5.0 ml of another liquid, the volume of the mixture is less than 15.0 ml.
(1) If ice is added to the water, part of the solid surfaces above the level of the liquid form of water. Only the submerged part displaces and increases the volume of water.
(2) The solid can be any soluble substance which changes the arrangement of water molecules as they attract and keep ions or molecules in solution. This arrangement is more orderly and slightly more compact than the free-flowing form of water.
(3) This can happen with any ideal gas mixture, where there are no interactions between particles and no chemical reactions. Of course you need a rigid container capable of withstanding the extra pressure ensuing from adding more molecules in the same volume. If there is a reaction, what happens? We know from Avogadro’s Law, that in a situation where you don’t have a rigid container ( molecules are free to expand) 50.0 cm3 of H2 will react with 50.0 cm3 of Cl2 to give close to 100.0 cm3 if the pressure and temperature are low enough to give near ideal behaviour. But if a single molecule is created fro joining two individual ones, the volume will not change.
(4) Alcohol and water also attract each other in solution, creating a mixture with a higher then expected density. If you could see alcohol molecules, a lot of empty space would appear. Because of H bonding between water and alcohol, water kind of fills the empty space as it mixes with alcohol. I prepared the following solutions to illustrate the point:
|Volume of 95% ethanol(ml)||V tap H2O added(ml)||Volume of mixture removed(ml)||total mass(g)||mass remaining(g)||mass of 10.00 mL sample(g)||actual density(g/ml)||V mixture(mL)||expected density (g/ml)|