BIO Lab 3: Measurements

Transcription

1 Measurements All Wisdom is from the Lord God and has been always with Him and is before all time. Who has numbered the sand of the sea, and the drops of rain, and the days of the world? Who has measured the height of heaven, and the breadth of earth, and the depth of the abyss? Ec.1:1-2 Introduction and Pre-Lab Discussion Biologists and medical health personnel carry out experiments or laboratory procedures in which they measure some parameter and record results. How well they perform this task is critical, especially when it involves the health of a patient. In this lab, you will practice measuring and accurately recording information using the tools provided in your kit. Significant Figures in Measurements Precision is a term that refers to how reproducible is a set of measurements. A precise instrument gives the same (or acceptably close) reading each time the same sample is measured. Precision also refers to the number of divisions between numerical values. In Figure 1, the lower ruler is more precise than the upper ruler. Before measuring, study the tool which you will use. The number of significant figures reported in the final measurement includes all the digits that are certain, plus a last digit that is estimated. Compare the number of recorded decimal places when a student measured the same pencil with two rulers with different levels of precision. With the upper ruler, there are 2 significant digits. The 4 is certain and the 3 is estimated. With the lower ruler, there are 3 significant digits. The 4.3 is certain and the 4 is estimated. It is important to record all significant digits. Note in the figure to the right, that the measurement was reported correctly as 4.30 cm. Since this ruler can measure to three decimal places, then three decimal places must be recorded. A measurement of 4.3 cm would not be correct. Reporting a measurement with all the digits that are known plus a last digit that is estimated applies to all measurements, including the ones you will be performing in this lab with the graduated cylinder, thermometer and scale. Accuracy of a measurement indicates how close that measurement is to the true value. It is subject to the reliability of the measuring device. If a student uses the span of his fingers to measure a piece of wood, the measurement may be close, but would not be an accurate one. GOOD RULER TECHNIQUE: When you wish to make accurate measurements with a ruler or tape measure, it is best NOT to begin at the end of the device, which may not be accurate. Instead, begin at the one (1) cm mark and remember to subtract one (1) cm from your final answer. Use good ruler technique for your measurements in this lab Catholic Initiatives in Math and Science, LLC All Rights Reserved 1

2 Measurement a Number and a Unit A measurement is a quantity with both a number and a unit. If a medical professional wrote an order in a chart to give your loved one 10 of glucose what does that mean? Did the medical staff mean 10 mg per milliliter of solution, or 10 grams per day, or 10 units per ml, or 10 units/kg weight/l solution? The possibilities are numerous and the life of your loved one hangs in the balance. So, in biology, whenever you record a measurement make sure you write BOTH a number AND the correct unit. Measuring Volume The volume of a substance measures the amount of space it occupies. The liter (L) is the unit for volume in the metric system. Prefixes are used to express smaller volumes, such as milliliters (ml), and are typically the volumes measured by your graduated cylinders. A meniscus is the curved upper surface of a nonturbulent liquid in a container. If the liquid wets the walls of the container the upper surface is concave, otherwise the upper surface is convex. The liquids you will be determining volumes on in biology lab may have convex or concave menisci. The picture (right) shows the correct way to read the measurement of the liquid with a concave surface. Place cylinder on steady surface View the meniscus at eye level Take the reading from the bottom of the meniscus (if concave as shown) Take the reading from the top of a convex meniscus Record the number and units, using the appropriate number of significant figures Measuring Mass The mass of an object indicates the amount of matter present in that object. In the metric system, the SI mass unit is the gram (g). The scale provided in your kit with which you will measure mass is a Spring Scale (Fig 3). There are a few important notes on your spring scale: The scale is divided in Newtons on the left and grams on the right. You will record all information in grams. Each mark on the grams side represents 2 grams. Final readings should therefore be given in whole numbers only. Before making any measurement, adjust the metal tab to the zero setting as shown. Note the hook on the lower end. In this lab, you will make several weigh boats to use during the class. A weigh boat is a container used for weighing samples. You should have at least two sizes: one for measuring small samples and one for measuring larger samples. Weigh boats can be constructed from plastic cups, small paper plates (dessert size), or even from aluminum foil. Choose durable materials as they will be used for the rest of the class Catholic Initiatives in Math and Science, LLC All Rights Reserved 2

3 An initial reading is taken and recorded with an empty weigh boat. The object is placed in/on the weigh boat and the final reading is taken and recorded. The actual mass of object is obtained by subtracting the initial reading from the final reading. Learning Objectives: Prepare a weigh boat for Spring Scale and practice mass measurements Learn to use the measurement tools provided Correctly read a graduated cylinder and spring scale State the correct number of significant figures in a measurement Materials Required: From Biology Kit Graduated cylinders (10 & 100 ml) Pipette (eye dropper) Spring scale (100 g) Measuring spoons Measuring cups Student Supplied Ruler Plastic cup, plate or aluminum foil (choose more durable plastic/paper materials) String, yarn, or wire Scissors or sharp knife Safety Lab equipment is fragile. Handle with care. Return all equipment to box when finished. Handle glassware with care and keep out of reach of children Be careful when handling scissors and knives. Mass 1. Construct weigh boats one large and one small You will need weigh boats for weighing both larger and smaller quantities Either construct from foil (Figure 4B), plastic cups (Figure 4A) or small paper plates. With scissors, poke 3 evenly-spaced holes in the cup, paper plate or foil boat Thread wire, string or yard through the holes Connect the strings in a loop at the top Make sure that the strings are evenly placed 2. Prepare Spring Scale Hang spring scale from a secure and unobstructed location A hook may be constructed from a paperclip and used to hang the spring scale from a rod Apparatus should be away from wind disturbances Adjust metal tab to zero (Fig. 3). 3. Weigh 10 ml graduated cylinder from kit Place empty weigh boat on spring scale Record Initial Mass in Table 1 Place 10 ml graduated cylinder on spring scale 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 3

4 Record Final Mass in Table 1 Calculate the Mass of Object Record Mass of Object in Table 1 4. Weigh one empty test tube from kit Following instructions in Step 3, find mass of one empty test tube Record Mass of Object in Table 1 5. Weigh the set of plastic measuring spoons from kit Following instructions in Step 3, find mass of one empty test tube Record Mass of Object in Table 1 6. Continue to next part, and then perform the Data Analyses and Conclusions Measuring Volume 1. Compare accuracy between 100 ml cylinder and 1/8 th measuring cup Fill the 1/8 th measuring cup with water Look at the markings on cup and record the expected metric volume in Table 2A Carefully transfer water into 100 ml graduated cylinder Read the volume in the graduated cylinder and record to the appropriate number of significant figures (See Fig above). Record volume in Table 2A 2. Compare accuracy between 100 ml cylinder and 1/4 th measuring cup Fill the 1/4 th measuring cup with water Look at the markings on cup and record the expected metric volume in Table 2A Carefully transfer water into 100 ml graduated cylinder Read the volume in the graduated cylinder and record to the appropriate number of significant figures (See Fig above). Record volume in Table 2A 3. Compare accuracy between 100 ml cylinder and 1/3 th measuring cup Fill the 1/3 th measuring cup with water Look at the markings on cup and record the expected metric volume in Table 2A Carefully transfer water into 100 ml graduated cylinder Read the volume in the graduated cylinder and record to the appropriate number of significant figures (See Fig above). Record volume in Table 2A 4. Examine precision of multiple measurements Fill the 1/8 th measuring cup with water Carefully pour water into graduated cylinder Record volume in Table 2B Repeat two more times for a total of 3 trials Calculate the average and record in Table 2B Calculate the Percent Difference and record in Table 2B 5. Perform Data Analysis and Conclusions 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 4

5 Lab Report for: Table 1 Mass Object Graduated Cylinder, 10 ml Empty Test Tube Measuring Spoons Set Initial Mass Final Mass Mass of Object Measuring Cup Volume (English Units) 1th cup 8 Table 2A Volume Accuracy Measuring Cup Graduated Cylinder Volume (Metric) Volume (Metric) th cup th cup Table 2B Volume Precision Trial 1 Trial 2 Trial 3 AVERAGE %DIFFERENCE Data Analysis and Conclusions 1. The spring scale is a manual device for measuring mass. Identify at least two things that must be done manually before a true Mass of the Object can be obtained. 2. To what whole digit or decimal place can your spring scale measure? That is, what is the precision of your spring scale? 3. If you want to measure volumes up to 10mL, should you use the 10mL or 100mL graduated cylinder? Why? 4. Measuring cup accuracy: You were asked to measure water in the measuring cups and in the 100mL graduated cylinder. How accurate were the measuring cups? That is, how close was the stated value of the 3 measuring cups with that obtained using the graduated cylinder? Is there a change in technique, or something that you could do to improve the accuracy of the measuring cups? 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 5

6 5. When would you choose to use 10mL or 100mL graduated cylinders over the measuring cups? 6. When would you choose to use the measuring cups over the 10mL or 100mL graduated cylinders? 7. You made 3 measurements with the 1/8 th measuring cup which should deliver a volume of 29.5mL. Answer the following using your data: Based on the %Difference, how precise were your measurements? Name 3 factors that could contribute to the error. Be specific. Human error or instrument error is not sufficient. 8. One way to increase the measuring cup accuracy is to increase the number of repetitions. If you measure 10 times and take an average, then results will improve. Obviously, this is not desirable! What could you do instead to both sets of measuring cups in order to make future measurements more accurate? NOTE: This is highly recommended! Hint: think of using a permanent marker and either the 10mL or 100mL graduated cylinders. 9. Look at the figure to the right. What is the volume? Volume is: 10. The thermometer on the left records in Celsius ( o C). The temperature is: 11. The figure below shows a digital scale. Assume that the scale was set to zero prior to placing the object on the scale. Record the mass shown in the figure. Mass is: 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 6