8th Grade Physical Science Curriculum Unit 1: Density and Buoyancy Lesson 2. Explore -- Liquids of Different Densities Objectives: Students will be able t construct a graph and interpret the relationship between the two variables, mass and volume. Students will understand that density is the term we use to describe the amount of mass a material has for a given volume. Key Questions: What is density? How can the density of a liquid be determined? How can we graph two related quantities? Outline This hands-on exploration allows students to actually measure the differences between the liquids we have been working with. It introduces them to methods for measuring volume and mass of liquids, using graduated cylinders and balances. The concept of density is introduced to explain the physical property that differentiates the liquids. Time: 40 to 50 minutes one day, 20 to 30 minutes the next Materials For each group of 4 students: Several plastic weights from a FOSS measurement kit (these are 20, 10, 5 and 1 gram weights, designed to be the density of water, but actually slightly denser.) Approximately 200 ml each of isopropyl alcohol, saturated salt water, and fresh water, each tinted a different color with food coloring. Note: To prepare salt water, the day before this lab, mix one part salt to four parts water. Stir and allow to sit over night. One 50 or 100 ml graduated cylinder. Assessment: Student graphs and explanations of the significance of the difference between the slopes of the three lines. OUSD/California Science Content Standards: 1e. construct appropriate graphs from data and develop qualitative statements about the relationships between variables. (Calif. # 8.9e) 1f. apply simple mathematical relationships to determine one quantity given the other two (including speed = distance/time, density = mass/volume, force = pressure x area, volume = area x height). (Calif. # 8.9f) 5a. density is mass per unit volume. (Calif. # 8.8a) 5b. how to calculate the density of substances (regular and irregular solids, and liquids) from measurements of mass and volume. (Calif. # 8.8b) How are the Liquids Different? Ask the students to explain their observations. Why do the weights float in salt water and sink in fresh water and alcohol? How are the liquids different? After they have done this in writing, ask them to share their responses. When someone says something about weight, seize upon it and ask how we could measure this? When they suggest weighing the liquids, ask them to propose a procedure. The desired procedure is as follows: 1. We must weigh the same amount of each liquid, so we have a fair comparison. After all, it wouldn't tell us much if a gallon of alcohol weighed more than a pint of salt water. 2. We must weigh the liquid in a cup (or the graduated cylinder itself), so we must weigh the empty cup first, then subtract the weight of the cup from our final weight. Suggest that a whole range of volumes be weighed. Each group should weigh 50 ml of each liquid, but then one group can weigh 10 ml, another 20 ml, and so on up to 80 ml. 3. If students have not measured liquids in a graduated cylinder before, explain how to read the volume, using the bottom of the miniscus (the curved surface of the liquid). Calculating Density Have students prepare a table like this, recording all the data from the class. Be critical of their reports. If they give you values you know are way off, be skeptical and ask for confirmation -- even do the measurement yourself -- or if time is too short, leave the bad data off.We want a graph at the end that is accurate. What you should get is three separate masses for each volume measured. For fresh water, the mass should be very close to the volume. For salt water it should be about 5% higher, and for alcohol, at least 10% lower. (Values are approximate) Have them calculate the density by using the formula: Mass/volume = density  Liquid  Volume  Mass  Density  Alcohol  20 ml 50 ml 80 ml (17 g) (43g) (70g)  (.87 g/ml)  Water  20 ml 50 ml 80 ml  (20 g) (49g) (81g)  (1 g/ml)  Salt Water  20 ml 50 ml 80 ml  (21.5 g) (53 g) (85 g)  (1.06 g/ml) Procedure Tell students what each color liquid is. The real mystery is not what the liquids are, but what it is that makes objects behave so oddly in them. Distribute the weights and only the cup of water and graduated cylinder. Ask them to record their observations of what happens when they put them in the water. (They will mainly sink, unless air bubbles are attached, or the weights are placed on the surface like a boat) Next, distribute the cups of The mass of each liquid is determined by students measuring. The densities are found through dividing the mass by the volume. Note that no matter what the volume is, the density will stay the same. Point out that the density of water is 1, and that alcohol is less than 1. Remind them of how alcohol and water behaved when placed together. Which floated? The less dense one. Graphing Density Using the data collected, we will now graph the densities of the three liquids. Distribute graph paper. Explain that we are going to graph their results. Put mass (grams) on the vertical axis and volume (ml) on the horizontal axis. If you connect these data points, you will have three lines upward. Water will be almost exactly 90 degrees, salt water will be steeper above, and alcohol less steep below. If you have wall-sized graph paper, you can create a class record that will be a useful reference point later. (Standards 1e, 1f, 5a and 5b) Unit 1 Home