Biological Principles Lab: Scientific Measurements

Transcription

1 Biological Principles Lab: Scientific Measurements Name: PURPOSE To become familiar with the reference units and prefixes in the metric system. To become familiar with some common laboratory equipment. INTRODUCTION Biology students, as part of their laboratory experience, will be asked to make observations. Observations may be either qualitative or quantitative. A qualitative observation describes a characteristic and is not numerical; a quantitative observation is numerical. Scientific observations are usually made as quantitative as possible so that they are easier to evaluate. For example, if you were trying to find the insect with the world's largest wingspan, quantitative observations are needed. An insect's wingspan may be qualitatively described as huge, but a quantitative description of an insect s wing span as 20 centimeters would be much more useful for this project. The metric system is universally used in science because it is a decimal system of measurement and is easy to convert from one unit of related measurement to another (e.g., volume to mass). The metric reference units are the meter (m) for length, the liter (l) for volume, and the gram (g) for mass. Prefixes are used as part of a unit to indicate a portion of or a multiple of a reference unit. For example, the prefix milli (m) indicates.001 ( 1 / 1000 ). A millimeter (mm) is ( 1 / 1000 ) of a meter. A milliliter (ml) is ( 1 / 1000 ) of a liter. A milligram (mg) is ( 1 / 1000 ) of a gram. Note that the same prefixes are used throughout the metric system, regardless of whether we re discussing length, volume or mass. The table below lists the prefixes we will be using most often in this course. Prefixes of the Metric System Prefix Abbrev. Part of Reference Unit kilo k 1000 or 10 3 centi c 1 / 100 or 0.01 or 10-2 milli m 1 / 1000 or or 10-3 micro µ 1 / 1,000,000 or or 10-6 nano n 1 / 1,000,000,000 or or 10-9 MATERIALS metric ruler microscope slide 10-ml graduated cylinder 100-ml graduated cylinder ungraduated pipette (dropper) weigh boat sand dollar tap water 1-ml pipette 10-ml pipette green pipette pump blue pipette pump digital balance pinecone dollar bill 1

2 PROCEDURE A. Length Length is the measurement of a line from end to end. The reference unit of length is the meter (m). Here is an example of a ruler with a metric scale: 1 cm The numbered lines count off the centimeters (cm). The unnumbered lines count off the millimeters (mm). Answer the following questions. How many millimeters are there in a centimeter? How many centimeters are there in a meter? Which is larger, a millimeter or a centimeter? Estimating and visualizing lengths is a useful skill when reading and making observations. Using the metric scale of the ruler provided, find some part of your hand that is 1 cm long. What part of your hand did you measure? Find some part of your hand that is 10 cm long. What part of your hand did you measure? Do the following work to gain an ability to visualize lengths in the metric system. First, estimate the measurement of the first item on the list using only your eyes and hands but no rulers. Record your estimate, including units. Second, actually measure the item with your ruler and record your measurement, including units. How well does your estimate compare to the actual measurement? Continue in the same way with each item on the list. Estimate Actual Measurement Length of your pinecone Diameter of your sand dollar Diameter of this letter "e" Width of a microscope slide Length of a microscope slide Scientific Measurements 2

3 B. Volume Volume is the space that a given object occupies. The reference unit of volume is the liter (l). beakers graduated cylinders Erlenmeyer flasks Graduated cylinders and graduated pipettes are measuring instruments. Beakers, flasks and droppers are not accurate enough for measuring, so they are used primarily for moving, mixing or storing. Answer the following questions. 1.4 L = ml 0.2 L = ml 550 ml = L Pour some water into a 100-ml graduated cylinder. Observe the curved surface of the water. This is called the meniscus. The conventional reading of the volume in the graduated cylinder is taken by looking at the lowest point of the meniscus. Use an ungraduated pipette (dropper) to find out how many drops there are in 1 ml of water. Count the number of drops needed to fill a 10-ml graduated cylinder to the 1 ml mark. There are approximately drops in 1 ml. Pipette Pumps: Pipettes are hollow glass tubes that are marked (or graduated) and allow for precise measurements of volume. Liquids are moved through pipettes using devices called pipette pumps. Use the blue pipette pump with the 1 ml graduated pipette. Use the green pipette with the 10 ml pipette. Very gently push the pump onto the top of the pipette. Make sure that the white plunger on the top of the pipette is pushed down completely. Scientific Measurements 3

4 Fill the 10-ml pipette to the 10-ml mark with water and dispense the water into the 10-ml graduated cylinder. Determine the volume of water in the graduated cylinder. Remember to look for the bottom of the meniscus. Does the water come exactly to the 10 ml mark on the graduated cylinder? If you don't have exactly 10 ml of water in the graduated cylinder, what might account for the discrepancy? Use the 1-ml graduated pipette to dispense 0.4 ml water on the table. Measure the diameter of the drop of water to the nearest mm. Which do you think would be more accurate for measuring 0.8 ml water, a graduated cylinder or a graduated pipette? From the instruments that you have used in this lab, indicate which instrument you would use and the best size of that instrument to choose. To Measure Instrument Size 50 ml water 4.2 ml water 15 ml water 0.4 ml water Scientific Measurements 4

5 C. Mass The metric reference unit for mass (weight) is the gram (g). Using the digital balance, weigh each item and record your measurement, including units. Make sure you set the balance to 0 grams before each measurement. This is accomplished by pushing the button marked TARE until the balance is zeroed. Mass of pinecone Mass of sand dollar Mass of dollar bill Which weighs more, the sand dollar or the real dollar? How much more? How much would 1,000 dollar bills weigh? How much do you think a $1,000 bill would weigh? Place a weigh boat on the digital balance and press the TARE button. Again, this will set the balance to zero, taking into account the weight of the weigh boat. Add 20 ml of water to the weigh boat (while it is still on the balance). Mass of 20 ml of water Dump out the water and repeat the procedure with the following volumes of water. 8 ml 78 ml 60 ml 45 ml Graph your five data points as a line graph on the grid supplied. Your independent variable (the volume) should go on the X-axis (horizontal axis); the dependent variable (the mass) should go on the Y-axis. Make sure you label the axes correctly! Based on your graph, how much does 50 ml of water weigh? CLEAN UP Empty any tap water in the sink. Return your materials to the instructor s bench. Wipe down your lab bench with cleanser and paper towels. Push in your chair when you leave. Scientific Measurements 5

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