Part I: How Dense Is It? Fundamental Question: What is matter, and how do we identify it? Everything on Earth is made of matter. Matter is as simple as a single element or as complex as the entire planet. Matter is anything that has mass and takes takes up space. In science, an amount of space is called volume. Matter can be any substance in the gas, liquid, or solid state. When we describe matter, we usually think of the physical properties of the matter, for example: color, smell, taste, feel, volume and mass. Another important physical property, density, is used to describe matter as a ratio of the mass to the volume of a substance. The more matter squeezed into the space, the denser the substance will be. A small potato is denser than a handful of cotton balls because the potato has more mass contained within the occupied space (volume). A small potato would still be denser than a bag full of cotton balls because density describes how much mass is contained within a specific volume. Density is a physical property that relates the amount of matter in a certain amount of space. How do you think this physical property is measured? Density measures how much mass is in a certain space, so density is measured in mass and volume. The scientific definition of density is mass per unit volume. As you think and learn about density, consider the following questions: Does a part or slice of a substance have a different density than the whole piece? How can the density of irregularly shaped objects be calculated? Record your ideas about these questions in your Student Journal. You will know the answer to these questions by the end of this Explore activity. 1
Part II: Know Your Units! Fundamental Question: How is density measured? To calculate density, divide the mass of the matter by the volume that it takes up. Here are the equations: Density For solid matter: D = g cm 3 mass volume D = In order to calculate the density of matter, you must use units of both mass and volume. For solid materials you use grams (g) for mass, and cubic centimeters (cm 3 ) for volume. For liquid materials you use grams (g) for mass, and milliliter (ml) for volume. The letter D represents density. or For liquid matter: D = g ml m V Calculating the Density of Solids with a Regular Shape The volume of a regularly shaped object is determined by measuring the l (length) x w (width) x h (height) of the substance, resulting in a measurement in cubic centimeters (cm 3 ). Procedure to Calculate Density of a Solid with a Regular Shape: 1. Carefully slice the end off a potato to create a flat side. Keep all unused portions of the potato for use later in the lab. Remove only enough potato for each lab member to cut out a regularly shaped potato cube with the following measurements: 1 cm x 1 cm x 8 cm. Warning: Be as precise with your measurements as possible because even small errors in measurement will affect your density calculation. 2
Part II: Know Your Units, continued Procedure (continued) 2. Use a marker to identify each potato piece with the initials of the lab partner. 3. Even though you tried to cut your potato cube to exactly 1 cm x 1 cm x 8 cm, there may be some variations. Carefully measure L, W, and H of the cube again and record the accurate measurements to the nearest whole centimeter in your Student Journal. Then use the volume formula (L x W x H = V) to calculate the volume in cm 3 of the potato piece. 4. Use a triple beam balance (or electronic scale) to determine the mass of the potato cube and record in the Journal to the nearest tenth (or hundredth if possible) of a gram. And then use the density formula to determine the density of the potato cube. 5. Compare the measurements and density results of your cube with two other students by completing the chart in your Student Journal and answering the question below the chart. You have determined the density of a regularly shaped object by making a potato cube and using the volume formula of V = l x w x h and then measuring the mass of the cube in grams. You probably found that your original potato density calculation varied somewhat from that calculated by other students. It is difficult to precisely cut a potato into a perfect cube. It s very likely that your potato cubes are slightly irregular shapes, which makes it challenging to determine volume using a ruler. When calculating density, it is very important to be as accurate as possible. You will be able to obtain a more accurate volume by using a different technique, the displacement method. The volume of a solid can be obtained using the the displacement method by slowly immersing the object into a graduated cylinder partially filled with water. The object will always displace an amount of water equal to its volume. Displacement a way to measure the volume of a solid by placing it in a known volume of liquid. 3
Part II: Know Your Units, continued Fundamental Question: How is density measured? Procedure to Calculate Density of a Solid using the Displacement Method: 1. Pour water into a graduated cylinder until it is about half full. Set the cylinder on the lab table and read the exact number of ml (remember to read the graduate scale at eye level at the base of the meniscus.) Record this reading in your Journal in the blank labeled initial measurement. 2. Slightly tip the graduated cylinder and slowly allow the potato cube to glide down the side so that it gently slides into the water. Place the cylinder on the lab table and read the new measurement on the graduate scale (again at eye level and at the base of the meniscus). Record this second measurement in your Journal in the blank labeled final measurement. 3. To obtain the volume in the graduated cylinder displaced by the potato cube, subtract the initial measurement from the final measurement. 4. Re-calculate the density of your potato cube using the displacement method to determine volume. Record your results in your Student Journal and then answer the questions. NOTE: 1 cm 3 = 1 ml 1 ml 1cm 3 = 4
Part III: Comparing the Density of Substances Fundamental Question: Can the density of an unknown substance be calculated? You have now used two procedures to calculate the density of your potato cube. First you measured the length, width and height of the cube with a ruler to determine the volume. This method is frequently used for regularly shaped objects. Then you used the displacement method to determine the volume. This method is frequently used for irregularly shaped objects that are not easily measured accurately with a ruler. Continue with your examination of potato density by taking some of the potato scraps and carving an irregularly shaped potato sculpture. Use the triple beam balance to measure the mass of the potato sculpture to the nearest gram. Use the displacement method to determine the volume of your irregularly shaped potato sculpture to the nearest ml. Then calculate the density of your sculpture and compare it to your potato cube density calculations. Your teacher will provide several other objects to test for density. In each case, determine the best procedure to use to calculate the density of the substance. Record your calculations in your Student Journal and answer the questions. 5
Part IV: Calculating the Density of a Liquid Fundamental Question: Can the density of an unknown substance be calculated? In this section you will learn a procedure to calculate the density of a liquid. Although the procedure is a little different from the one you used with solids, the calculation is the same. There are three beakers full of liquid at your lab station. Follow these instructions to calculate the density of a liquid: 1. Place a dry 100 ml graduated cylinder on the triple beam balance and determine its mass to the nearest gram. Record the mass on Part IV in your student journal. 2. Remove the graduated cylinder and add 40 ml of liquid from beaker #1. Record the volume of Liquid 1 in your data table. 3. Return the graduated cylinder with the liquid to the triple beam balance and measure the mass to the nearest gram. Record this measurement in your Student Journal. 4. Subtract the mass of the empty graduated cylinder from the mass of the graduated cylinder containing the liquid. This is the mass of the liquid. Record the mass in the data table. 5. Calculate the density of Liquid 1 using the density formula, Density = mass volume Record the density in the data table. 6. Repeat the process for the remaining two liquids and then identify the liquids. Be certain to start with a dry graduated cylinder each time. 7. Compare your density calculations to the Substance Density table provided by your teacher. 8. Complete the questions for Part IV in your Student Journal. Complete the Reflection and Conclusion questions on the last page of your Student Journal. 6