1. Density Chapter 2

2. 2.1 What is density? What do you think will weigh more, a gram of salt or a gram of steel? Now, which one would have more volume, a gram of salt or a gram of steel? Density is the mass per unit volume. Salt has the same mass as iron in this example but has a greater volume than steel. Which one do you think has the biggest density?

3. The units for density contain the units that you use for mass and volume. Mass is generally measured in grams. The volume of solids is usually measured in cubic centimeters and volume of liquids is usually measured in milliliters. This means that the units used for density are going to be either grams per cubic centimeter or grams per milliliter.

4. An example of a density would be water. It has a density of 1.0 g/mL.

5. As you have already learned, density is a property of matter. Every type of matter out there has a certain density. The density of a pure substance is always going to be the same. The density of aluminum is always going to be 2.7 g/cm 3. Density will never depend on the size or shape of the object. No matter what the density will always be the same!!

6. Density will be used to identify unknown substances. Silver and aluminum look the same. If you have the same mass of both, you can find out the volume and that will help you to identify the substance because it will help you to find the density and the substance identity.

7. 2.1 How is density measured? To find the density, you need to know not only the mass of the object but also the volume of the object. You find the density by dividing mass by volume.

8. Finding the density if a liquid: 1. Find the mass of the graduated cylinder. Record the measurement. 2. Pour some of the liquid that you want to measure into the graduated cylinder. Write down the volume of the liquid. 3. Place the graduated cylinder with the liquid on the balance. Record the mass. 4. Find the mass of the liquid by subtracting the mass of the empty graduated cylinder from the mass of the graduated cylinder with the liquid.

9. Example: Find the density if a liquid that has a mass of 55g (liquid and water) and a volume of 35mL. The graduated cylinder by itself is 20g.

10. To find the density of a solid, you need to figure out volume of the solid first. You use the formula for volume: Volume = Length x Width x Height

11. Then you can find the mass and use the density formula. Example: You have a solid with the mass of 100g. The length of the object is 10cm x 10cm x 5cm. Find the density?

12. 2.1 What is Displacement? Displacement is something that was discovered about 2000 years ago by a scientist called Archimedes. He noticed that when he stepped into the bathtub, the water rose. When he sat down, the water spilled over the sides. This happens every time that an object is placed in water. The water level will always rise because the water is being pushed out of the way for the object. This is displacement: the replacement, or pushing aside, of a volume of water, or any fluid, by an object.

13. So when you place an object in water, the water that gets displaced is the volume of the object. You are going to be using displacement when you want to know the volume of an object that you cannot use the formula for volume.

14. You are going to pour water into a graduated cylinder to a volume that is known, lets say 20 mL. Then you are going to place the object in the water and read what the new volume is. Then you are going to subtract the old volume from the new volume and that is the volume of the object. Then you find the density of the object.

15. Example: You have an object with a mass of 20g. You have a graduated cylinder with water in it that has a volume of 20mL. You place the object in the water and the volume goes up to 35mL. What is the density of the object?