GED Science: Mass, Volume, and Density

Chemistry is the study of the composition, structure, properties and change of matter. Matter is sometimes defined generally as the substance that makes up all observable physical objects.  In scientific terms, matter is anything that has mass and volume, meaning that it takes up space. Humans, rocks, air, atoms, and neutrons are all examples of matter. We will start our study of chemistry first by describing matter; then, in the next lessons, we will learn about the structure and interactions of matter.

One way in which to describe matter is by describing the mass, volume, and density of that matter.Volume is a measure of how much space something takes up.  For example, we can say that the water in a tub has a volume of 40 liters.  Mass is a measure of the amount of matter something contains.  For example, we can say that a person has a mass of 55 kilograms. Mass and weight are often considered to be interchangeable terms in everyday language, but they are distinct measurements. Weight is the measurement of the pull of gravity on an object. An object’s mass doesn’t change when an object’s location changes, since the amount of matter in the object remains the same. Weight, however, does change with location. For example, a person who weighs 120 lbs. on earth will only weigh 20 lbs. on the moon, because the moon’s gravitational force is only 1/6 as strong as the earth.

Thus, one can describe an object or a substance by describing its mass and its volume.  However, alone these measurements are not particularly helpful in chemistry.  You can have 40 liters of water, 5000 liters of water, or 2 liters of water.  Knowing the volume of the water does not tell you much about the properties of water.  Mass and volume are considered extrinsic, or non-characteristic, properties of matter.  Extrinsic properties are ones that are not specific to the material, or in other words, ones that cannot be used to identify the material.  For example, if someone said “I have a material here with a  volume of 2 liters….what is the material?,” you would be unable to answer the question.

On the other hand, intrinsic, or characteristic, properties of matter are ones that are specific to the material and can be used to identify the material. One example of an intrinsic property is density, which depends on mass and volume. Density is a measure of how tightly packed matter is within a certain volume. Density is measured in units such as g/mL (grams per milliliter) or g/cc (grams per cubic centimeter).  For example, Styrofoam has a very low density (~.90 g/mL), while iron has a very high density (7.9 g/mL).   Density is an intrinsic property, because it does not change with the amount of material.  A very small bit of iron, and a large block of iron will both have a density of 7.9 g/mL.  Water is a reference point for density, having a density of exactly one g/mL.

The following diagram depicts a range of materials with low density to those with high density.  The green circles represent particles of matter.  As you can see, as you move from left to right, the green particles become more tightly packed.  The square on the far left would represent a material with low density, while the square on the far right would represent a material with high density.

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People are inclined to confuse mass and density.  They will say “oh, Styrofoam is light and iron is heavy.”  While this is intuitive, it will cause confusion in your understanding of chemistry.  It is better to say something like “Styrofoam is light for its size” if you want to describe the density of Styrofoam.  An example that will help you to understand why this distinction is important is to consider the mass and density of a large ship.  The mass of a large ship would be very high.  However, its density is low, which explains its ability to stay afloat in water.  You would not say “the ship is light” (as it floats in water), yet this is the same way people describe the density of Styrofoam (it is light).

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