Our study of chemistry begins with the question: What is chemistry? Simply stated, chemistry is the study of matter and how it can be changed. In previous years, you learned that matter is anything that has mass and volume (takes up space). Students often ask, “what is the difference between mass and weight?” Mass is directly related to the number of atoms in a substance. For example, you are made of atoms. Your mass is the same whether you are on Earth or Mars. However, weight is dependent on gravity. Earth is more massive than Mars, so the force of gravity is greater. According to an article on Space.com, the gravity of Mars is 38% that of Earth. To figure out your weight on Mars, multiply your weight in pounds by 0.38 and that’s how much you would weigh. For example, if you weigh 100 pounds on Earth, you would only weigh 38 pounds (100 pounds x 0.38) on Mars!
In chemistry, we will often use mass in our analysis of matter. Certainly the chemistry of matter on Earth is very important, but so is the chemistry of matter on Mars, the Sun, and everywhere else in the Universe. As students of science, we want our learning to apply as widely as possible! Mass is measured in base units of grams (g). The amount of space something takes up, called volume, is measured in base units of liters (L). Distance is measured in base units of meters (m).
The meter is one of 7 fundamental units, called SI Units (read more about them here). The SI unit for mass is the kilogram (kg). The prefix kilo- means 1000, so adding kilo- to the base unit of gram means 1000 grams. Similarly, one kilometer (km) is equal to 1000 meters, and one kiloliter (kL) is equal to 1000 liters. Other commonly seen prefixes include milli- (1/1000) and centi- (1/100). For example, there are 1000 millimeters (mm) in 1 meter, and 100 centimeters (cm) in 1 meter.
- Question 1: How many mm are in 1 cm?
Brain break! Check out this inspired piece of musical art by the famed middle school science and math teacher Pete Hendley (aka KILA META):
…and we’re back. Meters, liters, and grams are all considered extensive properties of matter. An extensive property is specific to the amount of matter and therefore changes if the quantity of a substance changes. Imagine you have an empty two-liter (2 L) soda bottle. You measure out 500 mL of distilled water in a beaker and pour it carefully into the bottle, using a funnel to be sure not to spill.
- Question 2: How many liters is 500 mL?
- Question 3: After adding the 500 mL of water, what fraction of the 2 L bottle is filled with water?
Next, you measure another 250 mL of water in your beaker and carefully pour it into the bottle, increasing the volume of the water to 750 mL. This demonstrates that when we change the amount of liquid in the bottle (the volume of liquid), the number representing the volume also changes. Therefore, volume is an extensive property. Great job! Mentally pour out the water and then continue reading.
Earlier, we discussed the difference between mass and weight. If you have a bathroom scale at home, you can measure your weight in pounds. In science, we measure mass using a balance. Many students first learn to measure mass using a triple-beam balance. In high school chemistry, we work with small amounts of mass and often choose to use an electronic balance which allows us to precisely measure down to the nearest tenth or hundredth of a gram. On Earth, you can approximate your mass by taking dividing your weight in pounds by 2.2. For example, a person weighing 154 pounds would have a mass of about 70 kilograms. Similarly, you can calculate your approximate weight on Earth by multiplying your mass by 2.2.
- Question 4: Calculate the approximate weight (in pounds) of a German Shepherd dog with a mass of 35 kg living on Earth.
- Question 5: Imagine the 35 kg dog from Question 4 takes a rocket to Mars. Calculate the approximate weight of the dog on Mars.
- Question 6: For the German Shepherd from questions 4 and 5, what is the mass of the dog on Mars?
Back to our experiment! Imagine you measure out 500 g of liquid water using your electronic balance. As before, you carefully add the water to the empty 2 L bottle using a funnel. Next, you measure out an additional 250 g of water using the electronic balance and add that to the bottle.
- Question 7: What is the final mass of water in the 2 L bottle?
- Question 8: Is mass an intensive or extensive property?
While some properties of matter change based on the amount of matter present (extrinsic properties), others do not. Intensive properties do not depend on the amount of matter present and therefore can be used to identify matter. Intensive properties include density, boiling point, and the color of an object. Liquid water has a density of 1 g/mL (1 gram of water occupies a volume of 1 milliliter). Liquid water boils at 100 degrees Celsius (212 degree Fahrenheit). Liquid water is colorless (clear). If you were given the 2 L bottle from the thought experiment above, but did not know the identity of the liquid inside, you could quickly determine the color and then measure the density and the boiling point. That information taken together would help narrow down the identity of the liquid to likely be water. However, if you measured the mass or volume of the unknown liquid, that data would not help you determine the identity of the liquid, as you can imagine an infinite number of substances with a mass of 750 g or a volume of 750 mL.
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