Using the Lab Equipment

Purpose: The purpose of this exercise is to acquaint you with some of the apparatus you will be using throughout the semester;specifically the thermometer, the graduated cylinder, and the balance.

Discussion:Chemistry is an experimental science. It depends upon careful observation and the use of good laboratory techniques. In this experiment, you will become familiar with some basic operations that will help you throughout this course. Your success as well as your safety in future experiments will depend upon your mastering these fundamental operations.

Laboratory Balance

A laboratory balance is used to obtain the mass of various objects. There are several varieties of balances, with various limits on their accuracy and precision. Two common kinds of balances are shown in Figure 1. Generally, these balances are simple to use, but they are very delicate and expensive. Therefore, it is important to use proper technique when weighing out samples.

Figure 1: Digital electronic balances. The balance gives the mass directly when an object to be weighed is placed on the pan. (a) Analytical balance. (b) Top loader.

Image from Chemistry: The Central Science 10th edition by Brown LeMay and Bursten

Graduated Cylinders

Graduated cylinders are narrow glass or plastic open-ended tubes with volume markings (graduations) scribed along the side of the tube or cylinder. Since volumes are measured in these cylinders from the height of a column of liquid, the cylinder must have a uniform diameter. Obviously, a tall cylinder with a small diameter will be more accurate and precise than a short one with a large diameter, such as a beaker.

Thermometers

Most thermometers are based upon the principle that liquids expand when heated. Thermometers commonly use mercury or colored alcohol as the liquid. These thermometers are made with a thin capillary tube above a liquid reservoir. Thermometers are calibrated on the Celsius (°C) with 0 °C corresponding to the freezing point of water and 100 °C the boiling point of water. The length between these two points is separated into uniform divisions called degrees.

Limits of Measurement

The least significant digit of a numerical measurement is usually estimated. That is, some uncertainty is expected with every measurement that we make. Nevertheless, in terms of significant figures, the last digit is considered to be significant and so is recorded as such.

For instance, looking at the centimeter scale below the units are divided into 10 equal spaces. This means that the smallest increment measures 0.1 cm, which is 1 place past the decimal point. As a “rule of thumb” we will always estimate one more place than the unit division of the instrument we are using. An easy way to get the correct limit of measurement is to determine the smallest division (i.e. what does each line of the scale represent), then move the decimal place one place to the left. For example, since the smallest unit on the scale below is 0.1 cm, as mentioned earlier, so if we move the decimal one place to the left we get 0.01. Therefore we can estimate to the 0.01 units (in this case, centimeters)Therefore, the black line below the scale would have a length of 7.50 units. Since the measurement fell directly on the 7.5 mark we estimate the length to be 7.50 cm. Remember the last place is estimated so your number may be slightly different than mine. However, if your estimation is not around 7.5 cm check your measurement and if you still don’t see your mistake ask for help before performing this experiment.

Procedure:

  1. The Thermometer
  2. Remove your thermometer from your drawer. The unit of measure should be given at the top of your thermometer. Record this unit (milliliter, centimeter, degrees Celsius, grams, etc) on your data sheet.
  3. Using the “rule of thumb” discussed earlier record the smallest decimal place you will read (limit of measurement) on your data sheet. Be sure to include units!
  4. Set your thermometer out on the bench top. Be sure that it is secured so that it can not roll off the bench top. Also, make sure nothing is touching the bottom tip of the thermometer; this could throw off the temperature reading. As the thermometer sits the colored solution in the thermometer will rise. Since it is out on your bench top it will stop at the temperature of the air surrounding it (room temperature).
  5. After performing all other experiments and the colored liquid has stopped rising read the temperature by using the graduated scale on the thermometer. The scale increases from the bottom of the thermometer to the top. For example, in Figure 2, the colored liquid is between 20 and 30 degrees. There are ten units between 20 and 30 so each unit is 1 degree. Using the “rule of thumb” we will estimate one more place so we will read the temperature to 0.1 degrees. In figure 2, it looks as though the colored liquid is directly on the 1st line after 20°C so the temperature would be 21.0°C. However, if when you look at the figure below you think the colored line is slightly above the 1st line and estimate the temperature to be 21.1°C instead that is okay. The tenths place is estimated so two people may estimate it differently.
  6. Remember to record the temperature with the correct number of significant figures (using the “rule of thumb”) and units.

Figure 2: How to read a thermometer

Image from

  1. The Graduated Cylinder
  2. Examine the 10 mL graduated cylinder in your drawer. The unit of measure should be given at the top of your graduated cylinder. Record this unit [mL (milliliter), cm (centimeter), °C (degrees Celsius), g (grams), etc] on your data sheet.
  3. Using the “rule of thumb” discussed earlier record the record the smallest decimal place you will read (limit of measurement) on your data sheet.
  4. Obtain a 10 X 75 mm test tube from your drawer. Fill the test tube to the top with water. Pour the water into the graduated cylinder making sure that none is spilled. If some water is spilled empty out the graduated cylinder and start again. Notice that the meniscus (curved surface of the water) is concave up (Figure 3). The volume is read using the bottom of the water meniscus. To obtain the most accurate measurement ensure that your eyes are perpendicular to the scale.

Figure 3: Proper eye position for taking volume readings.

Image from Chemistry: The Central Science 10th edition by Brown LeMay and Bursten

  1. Record the volume. Remember to record all volumes with the correct number of significant figures and units. For example, in Figure 4 (next page), the liquid level is between 50 and 60. There are ten units between 20 and 30 so each unit is 1 degree. Using the “rule of thumb” we will estimate one more place so we will read the volume to the nearest 0.1 mL. In Figure 4, it looks as though the bottom of the meniscus is slightly below the third line from 50mL. So the volume is at least 52mL. But remember, we need to estimate one more decimal place. So the volume could be estimated as 52.8 mL.

Figure 4: How to read a graduated cylinder.

Image from ChemPages Laboratory Resources

  1. Measure the volume and record the volume of red liquid in the graduated cylinders at the front of each bench top. Remember to record all volumes with the correct number of significant figures and units.
  1. The Balance
  2. When using the balances be sure to observe the following:
  3. Do not drop an object on the pan.
  4. Center the object on the pan.
  5. Do not place chemicals directly on the pan; use a beaker, watch glass, weigh boat, or weighing paper.
  6. Do not weight hot or warm objects; objects must be at room temperature.
  7. Return all weights to the zero position after weighing.
  8. Clean up any chemical spill in the balance area using the paintbrush in the balance box.
  9. Do not remove the round plastic draft protector unless the object to be weighed can not fit with it in place.
  10. Inform your instructor if the balance is not operating correctly; do not attempt to repair it yourself.
  11. A weigh boat is a plastic dish that is used to hold a solid that is being measured. Place the weigh boat on the center of the balance pan and hit the button that says tare. This rezeros the balance to ignore the mass of the weigh boat and therefore, gives only the mass of the solid being weighed.
  12. The instructor will post a list with all students’ names and a number. Record this number on your data sheet. This number is the amount of solid you are to weigh out.
  13. Using the spatula to transfer the solid, carefully weigh the correct number of grams of sucrose in the weigh boat. The weight does not have to be exact, but get as close as possible. Do not put excess solid back into the reagent bottle after leaving the balance box. Instead dispose of any extra in the correct waste container as mentioned by the instructor at the beginning of the lab period. Record the actual weight of the sucrose on the data sheet (with units!). You do not need to use the “rule of thumb” here since the instrument is doing the estimation for you. Record all numbers shown on the balance. Do not round this number!
  14. Transfer the sucrose into a 10 X 75 mm test tube. Do not use your fingers to transfer any reagents; bend the weigh boat and slide the solid into the test tube. You can use your spatula to release any solid that may be stuck to you weight boat.
  15. Using a new weigh boat repeat the procedure used for the sucrose and weigh out the correct number of grams of sodium chloride. As before, the weight does not have to be exact, but get within 0.1 grams. Record the actual weight of the sodium chloride on the data sheet (with units!). Transfer the sodium chloride to a second 10 X 75 mm test tube. Notice that the same weight of sucrose and sodium chloride fill the test tubes to different volumes. This is because these two substances have different densities (mass to volume ratio). You will learn more about density in lecture.
  16. Obtain two labels from the front of lab and label each test tube with your name, section number, and contents (sodium chloride or sucrose).
  17. Show the labeled test tubes to your instructor or TA and have them sign your data sheet. Place your test-tubes in the box in the fume hood.
  18. Clean all utensils used today in lab with soapy water, dry them, and put them back in the appropriate lab drawer.

Using the Lab Equipment Data Sheet

Name______Date______Section______

  1. The Thermometer

1. Thermometer units………………………………………………______

2. Limit of measurement using “rule of thumb”…………………...______

6. Room temperature (using the limit of measurement)……………______

  1. The Graduated Cylinder

1. Graduated cylinder units………………………………………..______

2. Limit of measurement using “rule of thumb”…………………...______

5. Volume of water in a 10 X 75 mm test tube…………….……...______

6. Volume of red liquid……………………………………………______

  1. The Balance

2. Number of grams of sodium chloride or sucrose weighed (from

alphabetical sheet)…………………………………………______

4. Actual weight of sucrose………………………………………______

6. Actual weight of sodium chloride……………………………..______

8. Signature of Instructor ______

Source:Chemistry: The Central Science 10th edition by Brown LeMay and Bursten