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VCS Science Handbook Section 4.3

  • 4.3.0 Scientific instruments and measurement
    • Scientists use a variety of instruments to make observations and conduct experiments. Three basic types of instruments include those used to make measurements, various lab equipment used to help conduct experiments, and instruments used to extend our senses.

    • 4.3.1 Measurement instruments
      • Many scientific instruments are used for making measurements. Some measurement instruments provide examples of the thing being measured. For instance, a beaker measures volume because it provides an example of the volume being measured, and a ruler measures length because it provides an example of the length being measured. We can call these self-measurers (Table 4).

      • In contrast, non-self-measurers (Table 5) do not provide examples of the thing being measured. For example, an electronic scale does not weigh the same as the thing being measured but instead uses sensors to calculate weight.

Table 4: Self-measuring instruments
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Graduated Cylinder

Measure Volume

Graduated cylinders are used to measure exact volumes.
To use: handbook-meniscus_6.62.jpg
1. Keep the plastic ring near the top to prevent breakage in case the cylinder tips.
2. To measure a volume of liquid, choose a graduated cylinder that has a maximum capacity similar to the volume you want to measure.
3. Water in a glass graduated cylinder curves upward at the edges. This curvature is called the meniscus. Read the volume at the bottom of the meniscus when looking at the cylinder at eye level. The volume of water shown in the cylinder in the picture is 6.6 (each increment is 0.2).
4. If the volume is between two lines on the cylinder, then estimate the final digit in your measurement. For instance, if the water in the picture was about halfway between 7.0 mL and 7.2 mL, read a volume of 7.1 mL.
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Volumetric Flask

Measure One Volume

Volumetric flasks are used to measure a single, exact volume.
To use:
Fill the flask to the line on the neck to get the volume indicated on the flask. Pay attention to the meniscus.
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Graduated Pipette (left) and Pipette Filler (right)

Measure Small Volumes

Graduated pipettes are used to transfer small, exact volumes.
To use:
1. Attach a pipette filler to the pipette.
2. Pull liquid into the pipette by turning the dial on the filler.
3. Fill the pipette so that the meniscus is at the zero mark (usually more than halfway up the pipette).
4. Expel the desired volume into another container by reading the gradations on the side of the pipette while pressing the plunger on top of the filler.
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Erlenmeyer Flask

Transfer Liquids

Erlenmeyer flasks are used to hold and transfer liquids. Although they usually have a measurement scale on the side, they are not used to make exact measurements. Volumes are approximate.
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Beaker

Transfer Liquids

Beakers, like Erlenmeyer flasks, are used to hold and transfer liquids. Although they usually have a measurement scale on the side, they are not used to make exact measurements. Volumes are approximate. Use a beaker with about twice the capacity of the volume of liquid you need to hold.
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Measure Length

Rulers and Meter Sticks are used to measure length. Each marked number represents one centimeter (cm). The smaller lines between each centimeter represent millimeters (mm). There are 10 mm in one cm and 100 cm in one meter (see section III for more information on conversions).

Note: The picture of the ruler is not to scale.
Rulers and Meter Sticks
To use:
1. Place the ruler or meter stick such that the 0-cm mark lines up with the object you want to measure.
2. Make sure your eyes are perpendicular to the ruler and level with the object.
3. Estimate measurements between the lines.
4. For example, the object above the ruler measures between 7.1 mm and 7.2 mm. Therefore, the length should be estimated as 7.15 mm.

Table 5: Non-self-measuring instruments
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Electronic Scale

Measure Mass

Electronic scales are used to quickly and easily measure the mass or weight of a substance. The Ohaus Scout has two control buttons. The first is used to turn the scale on or off or to reset it, and the second is used to change the units of measurement to grams or ounces.
To use: handbook-scout-close.jpg
1. To make a measurement, first turn on the scale and wait for the display to read 0.00. If the scale does not read 0.00, press the ON/ZERO button. What you do next depends on what you are measuring.
For a solid object
2. Put the object on the scale and read the mass when the display has stopped changing.
3. Since the scale is sensitive to air movement, try to restrict movement around the scale when using it.
For liquids
2. Place an empty container on the scale. When the display stops changing, press the ON/ZERO button (this is called zeroing the scale).
3. Remove the container from the scale. (The scale will show a negative number.)
4. Fill the container with the desired volume of liquid, and put it on the scale. The scale will now display the mass of the liquid.
5. Alternatively, you can find the mass of the container and then find the mass of the container with the liquid inside. To find the mass of the liquid by itself, subtract the mass of the empty container from the mass of the container with liquid. The mass of the empty container is known as the tare weight. The formula for this calculation follows:
mass (container + liquid) – mass (empty container) = mass (liquid)
For dry powders or crystals
2. Place a piece of weighing paper or a shallow container called a boat on the scale, and press ON/ZERO to reset the scale with the paper still on it.
3. Add the desired mass of substance to the paper.
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Alcohol Thermometer

Measure Temperature

Alcohol thermometers are used to measure temperature. They are based on the principle that changing the temperature of a liquid changes its volume, therefore causing the liquid to rise or fall in the tube. The alcohol is the red liquid. Many thermometers have both Celsius and Fahrenheit scales, so be sure you are reading the correct numbers when making a measurement. Do not use a thermometer to measure temperatures beyond its range.
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Stopwatch

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Wall Clock

Measure Time

Stopwatches and wall clocks can be used to measure elapsed time.

Make Measurements with Electronic Probes and Sensors

The Vernier LabQuest allows students to obtain and manipulate laboratory data digitally. Follow instructions carefully when using this equipment.
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Vernier LabQuest
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pH Sensor
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Dissolved Oxygen Probe
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Temperature Probe
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Gas Pressure Sensor

Samples of Sensors Used in VCS Science Classes

    • 4.3.2 Accuracy, precision, and significant figures
      • When making measurements, two factors that must be considered are accuracy and precision. In addition, when recording measurements it is important to indicate how precise the values are.

      • Accuracy refers to how close a measurement is to the true value. For example, suppose you measure the length of a microscope slide as 74 mm using a ruler and your lab partner measures the same slide as 77 mm. If the actual length of the slide is 75 mm, then your measurement is more accurate than your partner’s because yours is closer to the true value. Inaccurate measurements can mean that an instrument is not properly calibrated.

      • Precision refers to how closely several measurements of the same quantity agree with one another. For instance, if you use an electronic scale to measure the mass of a quarter four times and get readings of 5.69 g, 5.69 g, 5.70 g, and 5.68 g, you could say that your measurements are precise because they closely agree with one another. Imprecise measurements can mean that the person doing the measuring is not careful to take measurements in the same way each time.
        • It is possible for measurements to be precise without being accurate (see Figure 3). For example, the actual mass of a quarter is 5.67 g, not 5.69 g. Possible sources of error in measurement include not resetting the electronic scale or not calibrating the scale before use.

handbook-imprecise-inaccurate.gif
handbook-precise-inaccurate.gif
handbook-imprecise-accurate.gif
handbook-precise-accurate.gif
The darts are both far apart and far from the center. They represent measurements that are imprecise and inaccurate.
Possible cause:
Sloppy lab techniques and improperly set up equipment.
The darts clustered together far from the center represent measurements that are precise but inaccurate.

Possible cause:
Good lab techniques but improperly set up equipment.
These darts are far from one another but close to the center. They represent imprecise but accurate measurements (on average).
Possible cause:
Sloppy lab techniques but properly set up equipment.
The darts clustered together in the center represent measurements that are both precise and accurate.

Possible cause:
Good lab techniques and properly set up equipment.
Figure 3 The difference between accuracy and precision in measurements illustrated by analogy of darts on a dartboard. Possible causes of each pattern are given underneath its description.
      • When making a measurement, all digits that are known with certainty plus one estimated digit are called significant figures. Scientists use significant figures to indicate the precision of their measurements. Measurements must be recorded using the correct number of significant digits (Myers, et. al. (2000). Chemistry.). Let’s consider some examples.

        • Example 1: Look at the picture of the graduated cylinder in Table 4. The gradations on the cylinder are in increments of 0.2. Thus, we can know with certainty the volume of the liquid to 0.2 mL, but the hundredths place will be an estimate. The water in the picture is just a little above 6.6 mL but for sure less than 6.7 mL. Therefore, we could estimate the volume as 6.62 mL. The measurement is accurate to two places, but the third place is estimated. This means that the actual volume is probably between 6.60 mL and 6.64 mL. If we settled for a measurement of 6.6 mL, we would be communicating that the volume is between 6.4 mL and 6.8 mL, but we know the volume more accurately than that; that is why we used three significant figures instead.

        • Example 2: Consider the ruler in Table 4. The ruler is accurate to 1 mm, but it is possible to estimate increments of 0.5 mm. That is why the length of the object is reported as 7.15 mm instead of 7.1 mm or 7.2 mm.

      • There are rules for determining the number of significant figures in a measurement (see Table 6) and special rules for arithmetic involving significant figures (see Table 7).

Table 6: Rules for determining the number of significant figures in a measurement
Rule
Examples
1. All nonzero digits are always significant in a measurement.
1 481 L has four significant figures.
4.6 mm has two significant figures.
2. Exact numbers can be thought of as having an infinite number of significant figures.
28 students is equivalent to 28.000000000...
There are exactly 100 (100.000...) cm in 1 m.
3. Zeros between nonzero digits are significant.
1 002 km has four significant figures.
2.07 has three significant figures.
4. Leading zeros in front of nonzero digits are not significant;
they are merely placeholders.
0.0082 has two significant figures.
0.1 has one significant figure.
5. Trailing zeros that are also right of the decimal point are significant.
10.10 has four significant figures.
0.87340000 has eight significant figures
6. Trailing zeros at the end of a number with no decimal point may or may
not be significant. If the zero has been measured or estimated, then it is
significant and should have a decimal placed after it (it can also be written
in scientific notation). If the zero is just a placeholder, then it is not
significant.
5300 m might have two, three, or four significant figures.
5300. m has four significant figures.
5.3 × 103 m has two significant figures.
5.30 × 103 m has three significant figures.
5.300 × 103 m has four significant figures.

Table 7: Rules for arithmetic involving significant figures
Rule
Examples
Multiplication and Division
The answer can have no more significant figures than the measurement with the fewest significant figures.
3.94 × 2.0 = 7.88, which is rounded to 7.9
0.123 × 0.123 = 0.015129, which is rounded to 0.0151
7.560/0.05829 = 129.69635, which is rounded to 129.7
23 students × 5.673 = 130.479, which is rounded to 130.5 (23 is considered to have an infinite number of significant figures here, so 5.673 has the least at four; see Rule 2 in Table 6)
Addition and Subtraction
The answer can have no more precision than the least precise measurement. The answer has the same number of decimal places as the measurement with the fewest decimal places. Thus, it is the position of significant figures that is important in addition and subtraction, while it is the number of significant figures that are important in multiplication and division.
When using a calculator, do not round until after the last calculation.
1.203
0.12
+26.567
26.890 → 26.89

7.93
+0.001
7.931 → 7.93

5.7
+4.3
10.0 (don’t round)

100
+110
210 → 200 because the first number is only known to the hundreds place; the answer cannot have more precision than the measurement with the least precision.

100.
+110.
210. because both numbers are known to the ones place.

    • 4.3.3 Other lab equipment
      • There are many different types of scientific instruments used to help conduct experiments. The few shown in Table 8 are some of the types you are most likely to use at VCS. They have been grouped by whether they are more often used in chemistry or biology labs.

Table 8: Lab equipment used in VCS chemistry and biology labs
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Ring Stand
Ring stands provide structural support for various attachments that are usually used to hold glassware. See the pictures at right for examples. The individual parts are described below.
To use:
1. Attach clamps securely to the metal rod of the ring stand.
2. Make sure that the setup is balanced to prevent tipping and that there is enough room for burners, glassware, etc.
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Ring stand with Bunsen burner, support ring, clamp, and flask
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Ring stands with various attachments
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Support Ring
Support rings hold a wire gauze, triangle, or funnel on the ring stand.
To use:
Attach securely to the metal rod of the ring stand using the thumbscrew.
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Wire Gauze
Wire gauze covers the support ring to provide a flat surface for beakers and other glassware, usually when heating them.
To use:
Center on ring support.
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Triangle
Triangles rest on support rings and are used to hold funnels during filtering or crucibles during heating.
To use:
Center on support ring.
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Funnel
Funnels can be used with filter paper to filter fluids. They can also be used for transferring fluids.
To use:
Place inside triangle.
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Crucible with Lid
Crucibles hold small amounts of a substance to be heated and dehydrated. Made of porcelain or ceramic, they can withstand high temperatures.
To use:
Place on triangle with Bunsen burner below.
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Crucible Tongs
Crucible tongs are used to hold crucibles and crucible lids.
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Burette Clamp
Burette clamps, and other clamps like it, are used to hold burettes, test tubes, flasks, and other glassware on the ring stand.
To use:
1. Attach the clamp to the ring stand at an appropriate height and tighten the thumbscrew.
2. Place the glassware between the rubber tongs and gently tighten them.
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Burette (50 mL)
Burettes dispense exact volumes of liquid. They are used during a process called titration, in which the exact concentration of a substance is determined.
To use:
1. Use one or two clamps to hold the burette.
2. Fill to the “0” line.
3. Release some liquid.
4. Read the amount of liquid released.
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Test Tube
Test tubes are used to carry out chemical reactions. To mix solutions in the test tube, swirl it gently. When not in a test tube holder or clamp, put test tubes in a test tube rack.
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Test Tube Holder
Test tube holders are used to temporarily hold and move test tubes.
To use:
1. Squeeze the handle so that the clamp opens.
2. Release the handle so that the clamp closes around a test tube.
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Bunsen Burner
Bunsen burners produce a hot flame used to heat, dehydrate, or sterilize substances in the lab. Ours run on natural gas.
To use:
1. Make sure all gas tubes are tightly sealed. Check for gas leaks before lighting. Make sure there are no flammable substances or gases nearby.
2. Turn gas lever to low to start the flow of gas.
3. Use a flint striker to light the burner.
4. Once lit, adjust the amount of gas to adjust the size of the flame and the amount of heat produced.
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Flint Striker
Flint strikers produce sparks to light Bunsen burners. The flint striker consists of a movable, metal wire with a piece of flint screwed onto the end. The flint moves across a textured, metal surface to produce sparks.
To use:
1. Squeeze the striker in much the same way as you would squeeze a safety pin, making sure the flint is dragged across the metal as you do.
2. To light a Bunsen burner, hold the striker over the burner so that some gas collects under the cap. The gas will ignite when sparks are produced.
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Hot Plate-Stirrer
Hot plate-stirrers simultaneously heat solutions and stir them.
To use:
1. To stir a solution (in a beaker or flask), put a small magnetic stirrer into the solution.
2. Turn the stirrer dial (a light will turn on). A magnet in the hot plate then spins, causing the stirrer in the solution to spin as well.
3. The solution can be heated by turning the heating dial (a light will turn on).
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Hot Plate
Hot plates are used to heat solutions. Never touch the top of a hot plate with your fingers or any other combustible or flammable material.
To use:
1. Put a beaker or flask containing a solution to be heated onto the hot plate, and then turn the unit on.
2. To prevent shattering, do not put room temperature glassware onto a hot hot plate, and do not put hot glassware into cool water.
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Mortar and Pestle
Mortars and pestles are used to grind solids into powders.
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Dissecting Pan
Dissecting pans hold specimens to be dissected. Specimens can be pinned to the rubber bottom of the tray.
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Scissors
Scissors are the main tool used in dissections at VCS. They are used to cut skin and tissues. Keep the blades at a shallow angle when cutting into an organism to prevent damage to internal structures.
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Scalpel
Scalpels are used for precision cutting. They are useful not only in dissections, but also when trying to obtain a thin biological sample for examination in the microscope.
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Probe
Probes are used to manipulate specimens and probe openings.
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Teasing Needle
Teasing needles are multi-purpose tools that can be used to manipulate, probe, or clean parts of specimens. Like the scalpel, they can also be used to obtain specimens (such as fungal spores or pollen) for viewing in the microscope.
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Dissecting Pins, T-Type
Dissecting pins are used to position or identify parts of a dissected organism.
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Forceps
Forceps are used for grasping and manipulating parts of an organism during a dissection. They have many other uses as well.

    • 4.3.4 Microscopes Hubble_Deep_Field-1996-640.jpg
      • Microscopes extend our senses by helping us to see things that our own senses could not otherwise detect. Types of microscopes include compound light microscopes, dissecting microscopes, and electron microscopes, but only the first two are used at Village. Table 9 explains how to use the compound microscope and how to do some basic slide preparation techniques.

      • Other scientific instruments used to extend our senses, in other words, to help us see, feel, hear, or otherwise sense things that our own senses are incapable of detecting, include telescopes (optical telescopes, radio telescopes, etc.), hand lenses, motion detectors, and amplifiers (see Figure 4).

Table 9: Microscopy
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Compound Light Microscope

Use a Compound Light Microscope

Compound light microscopes magnify specimens hundreds of times. Much about the compound light microscope is revealed by its name. The word “compound” indicates that it uses several lenses to magnify objects, “light” means that light illuminates the object being examined, and “microscope” reveals that the instrument is used to view small objects (“micro-” = small; “-scope” = viewing instrument). See the following page (Figure 5) to become familiar with the various parts of the microscope and their functions.
To Use:
1. Always carry the microscope by the arm with one hand and supporting the base with the other hand.
2. Position the microscope so that the arm is facing toward you, and turn on the light.
3. Make sure that the low-power objective is in position.
4. Place a slide on the stage such that the specimen is in the field of view.
5. Move the stage as close to the low-power objective as it will go. Next, while you are looking through the eyepiece, slowly turn the coarse focus knob until the slide comes into focus. Since your specimen might not be centered on the slide, keep one hand on the slide to move it while you are focusing. You will only see the movement through the eyepiece when the slide is nearly in focus.
6. Only after the specimen is in focus can the high-power objective be used (otherwise, the lens could hit the slide). Use only the fine focus knob when using the high-power objective.
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Make a Wet Mount

Wet mounts are microscope slides that contain a specimen mounted in water. A cover slip (or cover glass) is used to flatten the specimen and the water drop. This makes focusing easier, dampens vibrations, prevents fogging of lenses, and protects the high-power objective from touching the sample.
1. Obtain a clean microscope slide and cover slip. Hold cover slips only by the edges.
2. Add a small drop of water to the slide.
3. Place the specimen in the drop of water. If you are looking at an aqueous sample (like pond water), skip this step.
4. Hold the cover slip at an angle next to the drop of water and slowly lower it onto the water. This helps prevent air bubbles. (See the diagrams below.)
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Stain a Slide

Stains are used to make certain substances, organisms, or parts of organisms more visible.
1. Prepare a wet mount as described above.
2. Using a dropper, place a drop of the stain next to one end of the cover slip.
3. Place a piece of paper towel at the end of the cover slip opposite the stain. The paper towel will absorb water, drawing the stain under the cover slip and staining the specimen. (See diagram below.)
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