Investigation One:

Introducing Density

Matter is considered to be any substance that has mass and volume. Matter exists as one of four different phases, gas, liquid, solid, and plasma, and can transition between the four phases depending on both the behavior of the particles that constitute the matter and on the temperature of the matter.

Solids are identified by their ability to hold a definite shape. As a result, they have a definite volume. Liquids have a definite volume as well, but cannot hold a shape without a container. This explains why salt crystals will remain in a pile on a table unless disturbed, but an equivalent volume of water will spread into a thin film on the same table. As with solids and liquids, gases have mass. However, their volume is defined solely by the volume of their container, as gas molecules will expand to fill the available space. Solid, gas, and liquid states of matter are commonly encountered by students in their daily life. Plasma occurs only at very high temperatures and is found only in places such as the Sun or in the Earth’s core. Plasma is not a state of matter that students will need to know in the course of this CELL.

The transition from one phase to another does not alter the chemical nature of the matter. Water serves as a useful example of one type of matter that easily transitions between three phases since water can exist as gaseous water vapor, liquid water, and solid ice.

Atoms, Elements, and Chemical Bonds

Matter consists of atoms of different elements. The atoms in matter associate with one another through strong interactions termed chemical bonds. Two different types of bonds hold matter together. Covalent bonds arise through the sharing of a pair of electrons by two atoms. The covalent bonds in table sugar attract and hold the carbon, oxygen, and hydrogen atoms together. When atoms associate through covalent bonds, the resulting combination of atoms is termed a molecule.

Students will be asked to describe density in Investigations One through Four using the term molecule. As students conduct and analyze experiments in Investigation Five, they will encounter the ionic compound sodium chloride (table salt).

Typically, in a gas, the interactions between particles are so weak that the gas particles remain distant from one another and separated by a significant amount of space. This accounts for the impression, especially of students, that gases do not possess mass or volume. Gases simply expand due to the absence of any significant interaction between particles.

In liquids, slightly stronger interactions between particles result in less space between the particles. The attraction between particles is strong enough that liquids possess obvious mass and volume. However, the attraction is too weak, and the distance between particles is so great that liquids cannot independently assume a stable and specific shape. The movement of the particles is great enough that liquids flow and must assume the shape of their container.

The attraction between the particles of solids is strong enough that there is virtually no space between particles. Due to the strong attraction, solids have obvious mass, volume, and are able to independently assume a stable and specific shape. The illustration below captures much of what we have discussed about the relationship between density and physical state.

Notice that the size of the three boxes, A, B, and C, are identical. That is, the volume of the blue, purple, and red boxes are the same. Yet, each box contains a different amount of molecules. Box A contains the most, while Box C contains the least. Simply looking at the three boxes allows us to draw two conclusions. First, since all matter, every molecule has mass, Box A clearly has more mass than boxes B and C. One might say that the molecules in Box A are more densely packed than those in Box B and Box B is more densely packed with molecules than Box C. The more densely packed molecules are in an object (in this case, the identical size and volume boxes) the tighter/stronger the attraction between the molecules. Box A has the most molecules, mass, and the strongest interactions between its molecules.

The high degree of interactions between molecules in Box A permits the sample to maintain its shape. When placed in a container, Box A maintains its previous shape because the interactions between its molecules are so strong. That is why, in the illustration, Box A still looks like a perfect square or cube when transferred to the beaker at the bottom of the illustration. We say that Sample A is a solid.

The less-densely packed molecules in Box B do not have such a high degree of interactions with each other compared to Box A, so when this sample is transferred to the flask, the molecules “spread out” and take on the shape of the container (the flask). However, the interactions between the molecules in sample B are strong enough to maintain a weak connection to each other and all of the molecules remain in the sample, which we call a liquid.

Box C has the weakest interaction between its molecules. When placed in the flask, the molecules actually repel each other so that the molecules “spread out” as far as they can and fill the entire flask. We call sample C a gas. If the black rubber stopper were removed, the gas molecules would continue to expand into the space of the room until they are evenly distributed within the entire room, no matter how large the room is. This is why you may smell the odor of cookies baking in the kitchen even though you are in your bedroom – the gas molecules from the baking cookies filled the kitchen and then continued filling space until they reach you in your bedroom.

Phase and Temperature

The phase that matter assumes also depends on the temperature of the matter. Increasing the temperature of matter will increase the kinetic energy of the particles by increasing their energy content. Increasing the temperature causes the particles in a sample to move faster, weaken their interaction with each other, increase the distance between them, causing matter to transition from solid to liquid to gas. Reversing the process, decreasing the temperature, decreases the kinetic energy of the particles by decreasing their energy content. This causes the particles to move more slowly, decreasing the distance between them, causing matter to transition from gas to liquid to solid. So we see in the illustration below that the density of a substance is opposite to its temperature. That is, the higher the temperature of a sample, the lower the density of the sample, while the lower the temperature, the higher the density of the sample (compare the top and bottom triangles in the illustration below). We say that this is an inverse relationship, or that temperature and density are “inversely related”.

Water, for example, will transition from solid ice to liquid water to water vapor as the temperature is increased and the energy content is increased. Conversely, water vapor will transition from liquid water to solid ice as the temperature is decreased and the energy content is decreased. As with all matter, the transitioning from one phase of water to another does not change the chemical nature of the water molecule. Finally, the illustration below contains all of the information and relationships we have just discussed.

Density

Density is a physical property that depends upon the spacing between particles of matter as the illustrations above depict. As in a gas, when the spacing between particles increases, fewer particles, and therefore, less mass can be packed into the same volume. As in a solid, when the spacing between particles decreases, more particles and therefore more mass can be packed into the same volume. Density is defined as the amount of matter found in a certain volume. A small mass of matter found in a certain volume corresponds to a low density and a large mass of matter found in a certain volume corresponds to a high density. Gases, with large amounts of space between particles, generally possess low densities since low amounts of a gas’s mass are found in a certain volume. Solids, with small amounts of space between particles, generally possess high densities since high amounts of a solid’s mass are found in a certain volume. Liquids generally possess intermediate densities since the space between particles is intermediate between gases and solids.

A common misconception among students is the confusion between density and mass. Students often believe that a highly dense material possesses a greater mass than a less dense material that has the same mass. A useful example is that one kilogram of lead possesses a mass equal to one kilogram of feathers. The fact that the lead is found in a smaller volume than the feathers misleads students into believing that the lead possesses the greater mass. Investigation One will not only serve to introduce students to density but also serve to dispel this misconception among students.

In Investigation One, students will investigate the relative differences in density of different solids and liquids. Students will observe that differences in density determine whether a solid floats or sinks in a liquid and whether a liquid floats or sinks when added to a second liquid. Students will use their observations of to determine the relative densities of the different solids and liquids.