1B: Volume and Chemistry

Transcription

1 Chapter 1 1B: Volume and Chemistry Key Question: How are quantities of liquids used in chemistry? In this investigation, students learn and practice how to measure volume, while making solutions of colored water. Volume measurements are taken using a graduated cylinder and a pipette (or dropper bottle). Students will learn how many drops there are in a milliliter (ml), so that they may convert drops to milliliters as needed during future experiments. Students will experience a chemical reaction by adding household bleach drop by drop to remove the color from their solutions. As a practical application students will determine the volume in milliliters of bleach required per milliliter of colored water. This will allow them to determine the percent concentration by volume of bleach needed to remove color. Here is an example of how to perform a measurement of the liquid level in a graduated cylinder. The reading is taken at the bottom of the meniscus. In this case the reading is 6.6 ml. Not 6.8 ml. Reading Summary Questions and Goals Students read section 1.1 What Chemistry Is About before the Investigation (pp.7,9,14-18) Volume is an important measurement in science. Chemists use the SI units of liters (L) and milliliters (ml) to represent volume measurements. Volume measures the amount of space taken up by an object. The volume of a cubic centimeter (cm 3 ) is equal to one ml. A graduated cylinder is the most common tool used to measure volume, and it is important to read the lowest part of the meniscus. Volume is needed to calculate the density of a substance. Density describes how much mass is in a given volume of material. Measurements of volume are never exact, because the tools we use to measure are not precise enough to determine exact values. For this reason, we estimate and round our values, and this involves human error. When reading a graduated cylinder it is important to consider the number of significant figures. For a graduated cylinder we can estimate to a tenth of a graduation. A conversion factor helps us to convert between units. In this investigation, this concept will be helpful in allowing us to find out how many drops are in one ml. Main Questions Learning Goals Key Vocabulary What are some techniques that chemists use to measure volume? Why is volume important in chemistry? How do careful volume measurements contribute to good results? By the end of the Investigation, students will be able to: Measure volume by reading the meniscus. Understand how important careful volume measurements are. Evaluate how much bleach is required to remove the color from a dye solution. Measurement, volume, liter, milliliter, graduated cylinder, precision, accuracy, conversion factor. 10 Chapter 1: The Science of Chemistry

2 Materials and Setup Details 1 B Students work individually or in pairs. Each pair should have: Food coloring, dropper bottle of bleach, 100-mL and 10-mL graduated cylinders, 4 test tubes, empty dropper bottle. Preparation Students will measure and make their own solutions. You will need enough materials for each pair of students. An empty dropper bottle is needed for them to calibrate the number of drops in a ml of solution. Hints: Show the students how to read the meniscus of a graduated cylinder. Sketch a diagram on the board and explain why the bottom of the meniscus is read for accuracy. Explain that the meniscus must be at eye level, and the graduated cylinder must be set down flat on the lab bench. Make the students aware of the fact that there is human error involved in reading the meniscus. It is quite possible that the readings obtained by different students will not agree. Suggestions: Have each person read the meniscus, so that they both learn how to do it. Students are often tempted when working together to have one person perform all the measurements. For this reason, it might be worth having students work individually for parts of this investigation. Measure the number of drops in 1 ml yourself using a dropper bottle and using a pipette, so that you know approximately what value your students will obtain. Test the concentration of the bleach solution by counting the number of drops required to turn the color to clear, using 3 ml of bleach just as the students will. Teaching Time Assignments Misconceptions One class period Outline of the Investigation Section 1.1 What Chemistry is About in the Student Text before the Investigation. Students think they can read the meniscus while holding the graduated cylinder in their hands. Explain to them that it must be set down on a flat table and read at eye level. Introducing the Investigation 1 Setting up 2 Calibrating a dropper 3 A chemical reaction 4 Things to think about 5 A chemical mystery 6 Going further Volume and Chemistry 11

3 Investigation sections Part Ideas and Dialog Introducing the Investigation Review the SI units of volume with your students. Help the students understand how important measuring volume is to our daily lives. Setting Up Go over the instructions given on the Investigation sheet. Demonstrate how to use a graduated cylinder, so that students clearly understand. In your reading you learned about volume and how it is measured. Give an example of the units used to measure volume and describe what volume actually measures. The units of milliliter (ml) and liter (L) are most commonly used. 1 ml = 1 cm 3. Volume is used to measure the amount of space occupied by matter (an object). Measurements need to be accurate and precise, but they can never be perfect. Why not? Explain. Our measurements are limited by the instruments or tools that we use. This is because none of our equipment can measure exact amounts. All measuring tools have some limit to their precision, which requires us to estimate the last decimal place and round our number. The more precise the measurement tool, the more accurate our measurement will be. Today you will learn (or perhaps review) how to measure liquids in the laboratory. By using some simple guidelines you will be able to make a reliable volume measurement. This skill we be very helpful to you in future experiments. 1 Gather the materials listed on the Materials list at the beginning of the investigation. You may work with a partner, if your teacher approves. Have each student practice reading the meniscus of their graduated cylinders using just water. Encourage them to compare their measurement with another student. Ask the students for some real life applications where measuring volume is important. Also ask them to think of one example where the volume is large, such as the amount of water a bathtub can hold, and another where it might be small, such as the amount of a medication (like a teaspoon of cough syrup). Have a couple of students explain their examples. Read and follow the directions for your Investigation. Try your best to take careful volume measurements. You will submit a lab report that includes your measurements (tables 1 and 2) and observations. Discuss why you think the new techniques that you learned would be helpful in future experiments. Calibrating a dropper Explain to the students that sometimes we measure by the drop and that such small amounts can be important. 2 Counting drops can be tedious. Challenge the students to see if they get close to the same number each time they measure the number of drops in 1 ml. Taking the average is important when measuring such small amounts with a dropper. It is also important because students get distracted when counting a large number of drops, like 100 or 150. Your goal for this part of the investigation is to get a sense of how many drops are in one milliliter. Go slowly and read your meniscus carefully. It is fine to start again if you lose count. Remind the students to compare their results with each other, to see how their results compare. The average value should be close among students. If there is a large discrepancy, have the students repeat the experiment and see if they reproduce the same results. Some students may not have enough patience to count accurately. If that is the case just have them count the number of drops in 1 ml, i.e. not the volume of 150 drops. You will use your result of the number of drops per ml in the next part of your investigation. 12 Chapter 1: The Science of Chemistry

4 Investigation page Sample answers Teaching tips The importance of reading the meniscus can not be over-emphasized. Students need to have the graduated cylinder resting on their lab bench in order to read the meniscus properly. Students often make the mistake of holding the graduated cylinder in their hand to measure the meniscus at eye level. We do not want this close enough approach. It is helpful to count the number of drops by 10 and measure the volume every 50 drops. Since it takes only a couple of minutes to place 150 drops in the test tube, it is a good idea to perform the experiment a couple of times and take the average. Encourage students to record the appropriate number of significant figures. A review of significant figures is given in Appendix C at the back of the laboratory investigations book. Example Answers 2.1 It should only take a couple of minutes to place 150 drops in the test tube. 2.2 There are approximately 30 drops per ml. This is calculated by dividing the number of drops counted by the volume measured. The number of significant figures shown should be compatible with the accuracy of the measurement. For example, if they read 1.7 ml for 50 drops, the result should have 2 significant figures is rounded to 29. Similarly if they read 1.65 ml for 50 drops the result should be 30 drops per ml. 2.3 The average ml per drop is calculated by taking the inverse of drops per ml. Here again the number of significant figures is important. In the example data notice that the number of significant figures is two. 1 B: Volume and Chemistry 13

5 Investigation sections Part Ideas and Dialog A chemical reaction Determining the amount of bleach depends on mixing and careful observation. 3 Give the students time to carry out the procedure. Allow them to work as independently as possible. Remind them to mix after each drop and wait a little while after the addition of each drop. The active ingredient in bleach, hypochlorite (OCl - ), takes time to react. As the bleach is added the color becomes clearer over time. Record the number of drops of bleach required to make each color become colorless. Students should not find a large difference in the amount of bleach required to turn each dye clear. If they work in pairs be sure each partner takes a turn and makes their own observations. Repeat the experiment so that you have 2 trials for each color if time allows. If students do repeat their trials it is likely that their results will be quite different. This is due to their technique and how well they mix the solution, as well as how long they wait before adding more bleach. Things to think about Ask students to consider what makes color and how bleach works. Have them consider chemical vs. physical change. A chemical mystery Following student discussion, reveal that green dye is a mixture of blue and green. 4 Now we need to consider why you observed what you did, and what it means. Take your time and consider each of the questions. In this part students may need help in understanding that molecules in the food colorings make the colors. It is helpful to show them some dye molecules and explain why our eyes see colors. For example, we see the color green because it is reflected, and all of the other colors in white light are absorbed. Why do you think bleach removed the color? Have the groups discuss their ideas. Some students may suggest a change in the molecular structure of the dye. Many should think that there was a chemical change, because there was a color change. You can then have them consider more carefully what happens during a chemical change, namely that bonds are broken and reformed and atoms are rearranged. Students will more easily arrive at the fact that the dye molecules were changed. Did each color behave in the same way? Do you think this change was chemical or physical? Students should see that the colors behave similarly. Keep in mind that lighter colors such as yellow do not appear to change as dramatically, but the color slowly fades in a similar manner. Adding dye to water is a physical change. The concentrated dye molecules mix through the process of diffusion making the entire solution appear colored. The more dye the deeper the color, to a point. 5 Which color do you think is a mixture of two others colors? Support your answer with evidence. Most students will arrive at this answer quickly, because they have mixed paints and pigments before. For experimental evidence they will mix the two dye colors together to produce green. 14 Chapter 1: The Science of Chemistry

6 Investigation page Sample answers Example Answers 4a. Dye molecules make the color. 4b. Bleach changed the dye molecules. 4c. The dye molecules are chemically similar, reacting with bleach in a similar fashion. 4d. This is a chemical change. By definition a change in color is considered chemical because bonds are broken and reformed. 4e. Physical change. Dye molecules are distributed by diffusion in water. The molecules themselves are unchanged. 5. Green is a mixture of blue and yellow. 1 B: Volume and Chemistry 15

7 Investigation sections Part Ideas and Dialog Going farther Calculate the percent volume of bleach needed to remove each color. 6 Find out how many ml of bleach it took to remove the color, by converting the drops you counted to ml using your conversion factor (drops per ml) determined in part 2. Have students determine the volume in ml s required to remove dye from 3mL s of colored water. Students will find it helpful if you walk through a sample calculation with them. Specifically showing them how to calculate the percent by volume of bleach required for each color. Using the same concentration of color as in your experiment, determine the amount of bleach (ml) needed to remove a yellow food color stain. Assume your washing machine holds 15 gallons of water. Assuming 25 drops = 1 ml, and that 21 drops of bleach are required to remove the yellow color from our solution, students should find that roughly 28% bleach by volume is required. This answer will vary based on the drops per ml determined by the students. Encourage them to average this value so that class results can be compared between groups. See the next page for sample calculations. 16 Chapter 1: The Science of Chemistry

8 Investigation page Sample answers Example Answers 6a. For example if using 3 drops bleach then: 3 drops (1 ml / 25 drops) = ml bleach ml bleach / 3 ml colored water = or 3.2% by volume. Note: This is for our concentration of 1 drop of dye in 20 ml of water. (Depending on the type and the amount of dye used the answers to these questions can be over a wide range. So, do not be surprised if you find that the answer is 20%) 6b. 4% bleach x 56,775 ml water (in washing machine) = 2271 ml bleach x (1 cup / 237 ml) = 9.6 cups of bleach. This real life connection does not seem realistic. 9.6 cups of bleach in a load! The reason for this is that the type of dye in the food color is not the same as the dye in the clothes. The concentration of the dyes in the two cases are also not the same. Teaching tips Typically, the amount of bleach will be high in this experiment because they are adding it to concentrated dye. Normal laundry stains are much less concentrated than this dye. The reaction of bleach with colored water is time dependent, so it is important for the students to wait a few minutes for the reaction to take place. If they add the bleach too quickly it may appear as though they need a larger amount. Students should find that their amounts compare favorably with the class data. The reproducibility of this lab is quite high. If students have poor technique initially they will likely improve with experience. It is important that they feel confident in their measuring skills. By using the application of a chemical reaction with bleach, students see that measuring volume with drops is important. 1 B: Volume and Chemistry 17