Density and Pure Substances

So what is the volume of 18 g of water when you measure it directly? If you measure out 18 g of water in a graduated cylinder, its volume will be 18 mL. Try measuring the mass of 0 mL, 20 mL, 40 ml, 60 mL, 80 mL, and 100 mL of water.

water
volume (mL) mass (g)
0.0 0.0
20.0 20.0
40.0 40.0
60.0 60.0
80.0 80.0
100.0 100.0

What do you notice about the relationship between the volume and the mass of water? They are directly proportional to each other. Which only makes sense… if you double the volume of water, then you would expect the mass to double as well. In the case of water, each milliliter of water adds one gram of mass to the total. Therefore, we say that the “density” of water is 1 g/mL (or 1 g/cm3). The density of a material is defined as its mass per unit volume, and the density can be calculated by dividing a material’s mass by its volume.

material density* (g/cm3)
helium (gas) ≈00.0002
air (gas) ≈00.0012
gasoline (liquid) 0≈0.7400
oak (wood) 0≈0.7500
ethanol (liquid) ≈00.7900
polypropylene (plastic) 0≈0.9000
water (liquid) ≈01.0000
honey (liquid) 0≈1.3600
table salt (solid) ≈02.1600
aluminum (solid) ≈02.7000
granite (solid) 0≈2.7000
diamond (solid) ≈03.5000
iron (solid) ≈07.8700
gold (solid) 19.3000
* at 20 °C and standard atmospheric pressure

Drilling down to the molecular scale once again, the density of a material is determined by the mass of its molecules and how closely packed together those molecules are. The mass of a water molecule is 18 u (2.99 × 10-23 g). At 20 °C and standard atmospheric pressure, the average distance between water molecules in the liquid state is 3.85 × 10-8 cm and there are 3.34 × 1022 water molecules packed into 1 cm3 of space. Meanwhile, the mass of an ethanol molecule is 46 u (7.65 × 10-23 g). At 20 °C and standard atmospheric pressure, the average distance between ethanol molecules in the liquid state is 5.7 × 10-8 cm and there are 1.0 × 1022 ethanol molecules packed into 1 cm3 of space.

The average distance between molecules and the average amount of space a molecule occupies depends on the size and shape of the molecule, the attraction between molecules, and the temperature of and pressure on the molecule. The stronger the attraction between molecules and the higher the pressure, the closer together the molecules will become. The higher the temperature, the faster the molecules will be moving, the more space the molecules will occupy. So we would expect the density of a material to increase if the attraction between molecules becomes stronger and when the pressure increases. And we would expect the density of a material to decrease when the temperature increases.

Density of Liquid Water at 4 °C
Effect of Temperature on the Density of Water
temperature (°C) state density* (g/cm3)
500 gas 0.0003
100 gas 0.0006
100 liquid 0.9584
80 liquid 0.9718
60 liquid 0.9832
40 liquid 0.9922
20 liquid 0.9982
0 liquid 0.9998
0 solid 0.9167
-180- solid 0.9340
* at standard atmospheric pressure
Density of Ice and Liquid Water at 0 °C
Intermolecular Bonds and Chemical Bonds

Density is a characteristic property of a pure substance. At 100 °C and standard atmospheric pressure, water vapor will always have a density of 0.0006 g/cm3. At 20 °C and standard atmospheric pressure, liquid water will always have a density of 0.9982 g/cm3. And at 0 °C and standard atmospheric pressure, solid ice will always have a density of 0.9167 g/cm3. The density of a pure substance only depends on the mass, size, and shape of the molecules, the attraction between molecules, and the temperature of and pressure on the molecules. So if you are dealing with water and water molecules, except for the temperature and pressure, those properties are never going to change.

We say that density is a “characteristic” property of a pure substance because it can be used to identify the pure substance. If you had a glass of water (H2O) and a glass hydrogen peroxide (H2O2), you could identify which was water by measuring the densities of the two liquids. At 20 °C and standard atmospheric pressure, liquid water has a density of 0.9982 g/cm3 while liquid hydrogen peroxide has a density of 1.450 g/cm3. (You cannot say a liquid is water just because it has a density of 0.9982 g/cm3. Many liquids may have the same density. But if a liquid does not have a density of 0.9982 g/cm3, you can definitely say that a liquid is not water.)

Pure substances and mixtures

When I referred to density as a characteristic property, I used the term “pure substance.” A pure substance is a substance that is made up of only one type of molecule. If you could examine every molecule in a glass of pure water, you would find that they are all water molecules (H2O… two hydrogen atoms chemically bonded to one oxygen molecule). On the other hand, if you could examine every molecule in a bottle of air, you would find many different types of molecules. You would find mostly nitrogen (N2) and oxygen (O2) molecules; some argon (Ar) atoms, carbon dioxide (CO2) molecules and water (H2O) molecules; and a handful of other molecules. There is no such thing as an “air” molecule… air is a mixture, not a pure substance.

Now if you went to your kitchen sink and drew a glass of water from the tap, you would not have a pure substance. Tap water is a mixture of water molecules and other dissolved minerals and gasses. When you make ice cubes from tap water, the ice cubes are often cloudy because of these impurities. If you were to make ice cubes from boiled, distilled water (boiling will drive out many of the dissolved gasses that even distilled water can contain), you would have crystal clear ice cubes. This, again, is an issue between scientific language and everyday language.

For example, your local pharmacy will carry products that are called, in everyday language, isopropyl alcohol, hydrogen peroxide, and ammonia. None of those products are pure substances; they are all mixtures. Isopropyl alcohol, hydrogen peroxide, and ammonia all exist as pure substances. There is an isopropyl alcohol molecule (C3H8O), a hydrogen peroxide molecule (H2O2), and an ammonia molecule (NH3). At your local pharmacy, all three of those substances are sold in a diluted (mixed with water) form. Rubbing alcohol is often 70% or 91% isopropyl alcohol, with the remaining 30% or 9% being water. Hydrogen peroxide is usually sold in concentrations of 3-6% and ammonia in concentrations of 5-10%. So when we talk about “water,” we need to be clear whether we are using everyday language or scientific language.

Density is not a characteristic property of a mixture for obvious reasons. Consider salt water: a mixture of salt (NaCl) and water (H2O). Because you have a mixture, salt water can have many different concentrations of salt. As the concentration of salt increases, the density of the salt water also increases.

Density of Salt Water by Concentration
salt concentration
(by mass)
density* (g/cm3)
0% 0.9982
4% 1.0268
8% 1.0559
12% 1.0857
16% 1.1162
20% 1.1478
* at 20 °C and standard atmospheric pressure

If you look at the table of densities for different materials at the top of this page, you will see that some of the densities are given as approximations (≈). Gasoline, oak, polypropylene, honey, and granite are all mixtures that can have different compositions that can affect their densities. Air is also a mixture, but when we say “air,” we are referring to the specific composition that makes up the Earth’s atmosphere. The composition of air has changed over the millennia, but dry air currently contains roughly 78% nitrogen (N2), 21% oxygen (O2), argon (Ar), carbon dioxide (CO2), and small amounts of other gases. We would not say that a mixture that is 60% nitrogen and 40% oxygen is air. You will learn much more about pure substances and mixtures later in this unit.