Measuring Matter
An earlier version of this article appeared in Greg’s now-defunct chemistry website, Science Help Online for Chemistry.
Chemistry, the branch of science involving the study of matter, is a quantitative subject. This means that measurements and calculations involving matter are central to the subject at hand. Matter is sometimes defined as "anything that has mass and volume." Another definition of matter is "anything that has the property of inertia." This lesson deals with some of the most basic and important concepts of Chemistry, in that the concepts involved are necessary to understand the definitions of matter.
Each time that you do a laboratory activity this year you will be collecting data. Mass and volume are probably the most common types of quantitative measurements that you will take in chemistry, so it is important for you to have a clear understanding of what you are measuring. When you know the mass and volume of a substance, you can find its density. Weight is not something that you will often have to worry about in chemistry, but since many students confuse the concept with mass, I will also explain weight here. The unit called The Mole will also be introduced here.
Mass - The balances that you use in the laboratory measure mass, not weight. Mass is sometimes defined as the amount of matter in an object. 10.0 grams of gold would contain twice as many gold atoms as 5.0 grams of gold. Students sometimes confuse mass and volume because the term "massive" can mean "large" in English. In Chemistry, mass has nothing to do with size. The SI unit for mass is the kilogram (kg).
Conservation of Mass - One of the basic scientific laws of Chemistry is called the Law of Conservation of Mass. The law states that matter is neither created nor destroyed in an ordinary chemical reaction. Now, it turns out that mass and energy can be converted into one another, in a nuclear reaction, but (unless something goes horribly wrong) we will not be carrying out any nuclear reactions in the Chemistry lab. This means that no matter what we do in the Chemistry lab, the mass that we put into the reaction will be the same as the mass that we get out of the reaction. In other words, in each chemical reaction that we observe in the lab, the total mass of all of the products will be equal to the total mass of all of the reactants.
Inertia - The inertia of an object represents its ability to resist changes in its motion. This change in motion could be in terms of speed, or in terms of direction. If you are in a car that stops suddenly, your body continues to move forward, because of your inertia or your resistance to a change in your motion. When a car makes a sharp turn, you might feel yourself moving to one side, due to your inertia. The more mass an object has, the more inertia it has. Imagine the difference between hitting a golf ball with a golf club and hitting a bowling ball with a golf club. The bowling ball has more inertia, a greater resistance to a change in its motion, due to its greater mass.
Volume - Volume is the amount of space that an object takes up. When you buy a 2-liter bottle of soda, the soda takes up 2 liters of space. A 200 cm3 sample of gas is twice as large as a 100 cm3 sample of gas. The volume of liquids can be measured using graduated cylinders, beakers and flasks. You can determine the volume of regularly shaped objects solids with a meter stick. Solids that have an irregular shape are often measured through what is called "the water displacement method." This means that you can determine the volume of the object by finding out how much water it displaces. In Chemistry we often measure volume in milliliters (ml) or cubic centimeters (cm3). It is important for you to know that milliliters and cubic centimeters are equivalent units, so 1 ml = 1 cm3. There are 1000 milliliters in one liter.
Density - Density is the amount of matter in a given unit of volume. It can be measured in grams per cubic centimeter (g/cm3). It is a measure of how tightly packed the atoms of a substance are. When we say that ice is less dense than water, we mean that the water molecules are more tightly packed when they are in the liquid state. The formula for determining density is:
Mass m
Density = --------------- or D = ------
Volume v
Submarines, scuba divers and many types of fish can alter their depth in the water by adjusting their density.
Weight - Weight is a measure of the force of gravitational attraction between two objects, one of which is usually the earth. In the United States, we usually measure weight in pounds, but the SI unit for weight is Newtons (N). The weight of a certain object can change as it moves closer or further away from the earth, so you do actually weigh less while flying on an airplane than you do on the ground!. On the moon, objects weigh about 1/6th of what they weigh on earth, because the moon doesn’t attract things as strongly as the earth. Mass, on the other hand, does not change with location. To gain or lose mass an object must gain or lose atoms!
The Mole - As you might imagine, we will not be able to measure the mass or volume of individual atoms or molecules in the chemistry lab. Molecules are so small that a single drop of water contains billions and billions of them! Human beings often take small items and group them together in number. We often group eggs in “dozens” so we can sell 12 at a time. We often sell a “case” of soda, containing 24 cans. Just as eggs are grouped in dozens, and other items are grouped in grosses, atoms and molecules are grouped in moles. How many items make up a mole of items? 6.02 x 10^23 ! A mole of helium atoms would be 6.02 x 10^23 atoms. A mole of carbon dioxide molecules would be 6.02 x 10^23 molecules.
We may not be able to measure the mass of one water molecule in the lab, but we can measure the mass of one mole of water molecules. In fact, one mole of water has a mass of 18.0 grams. Just as a dozen eggs has a different weight than, say, a dozen cars, moles of different atoms have different masses and weights. What is the same is the number of items in each mole. Just as each dozen of items, regardless of the weight, contains 12 items, each mole of atoms, regardless of the element, contains 6.02 x 10^23 atoms.
Let’s go over a few questions to check your understanding. Try to answer the questions on your own, before reading my answers.
The following questions are based on comparing a typical bowling ball to a typical beach ball as shown in the image below.
Image courtesy of Paint 3D
Question 1. Which has more mass? The bowling ball or the beach ball?
Question 2. Which has more volume? The bowling ball or the beach ball?
Question 3. Which has more density? The bowling ball or the beach ball?
Question 4. Which has more inertia? The bowling ball or the beach ball?
Question 5. Which has more weight? The bowling ball or the beach ball?
Answer 1. - Remember, in chemistry, mass does not refer to size. Mass is a measure of how much matter is in an object. The bowling ball has more mass than the beach ball. How do I know? The bowling ball is heavier than the beach ball, when they are located at the same spot in the Earth’s gravitational field. Things with greater mass have a greater attraction to the earth when the distances are the same.
Answer 2. - In chemistry, volume is the amount of space that an object takes up. The beach ball clearly looks bigger, so it has greater volume.
Answer 3. - Density is a measure of mass per given unit of volume. In other words, it is a measure of how tightly-packed the particles of a material are. If the bowling ball has more matter packed into a tiny package, it also has a greater density than the beach ball.
Answer 4. - Inertia is a measure of an object’s resistance to a change in its motion. Inertia is also a measure of how massive something is. The bowling ball has more mass and, therefore, more inertia. Imagine kicking both of them. Which one would be harder to get moving?
Answer 5. - This is a bit of a trick question. Weight actually depends upon the location of the object. The weight of an object is actually a measure of the force of its attraction to another object. The other object is usually, but not always, the earth. This attractive force depends on the mass of the two objects in question and the distance between them. If both the bowling ball and beach ball are located the same distance from the center of the earth, the bowling ball will weigh more than the beach ball. The difference in mass between the two of them is actually so great that the bowling ball will weigh more as long as they are both on earth.
Teachers and students can click here to access a practice worksheet and answer key.
Students, do you have any handy tips for keeping the differences between mass, volume and weight straight in your heads? Teachers, do you have any tips for teaching these topics? Join our team by joining the discussion below.
