Reading Instruments

Reading Instruments

Before you perform an experiment you should be comfortable with certain apparatus that you will be using. This lesson gives some commonly used apparatus and how to use them.

Most rulers you find have two sets of lines on them. You can ignore those with the numbers spaced further apart. We only work in the metric system and those are for the imperial system. The closest together lines are for millimetres, the thicker lines are for \(\text{5}\) \(\text{mm}\) and the thicker, longer lines with numbers next to them mark off every \(\text{10}\) \(\text{mm}\) (\(\text{1}\) \(\text{cm}\)).

A thermometer can have one, or two sets of numbers on it. If it has two sets of numbers one will be in Celsius, and one will be in Fahrenheit. We use Celsius, so you can ignore the side with a larger temperature range. In the figure below, you can ignore the right-hand side. Looking on the left you can see that the red line (coloured ethanol here) is next to the fourth line above \(\text{0}\) \(\text{℃}\). Each small line is \(\text{1}\) \(\text{℃}\), so the temperature is \(\text{4}\) \(\text{℃}\).

Laboratory thermometers will go to much higher temperatures than those used for measuring the temperature outside, or your body temperature. It is important to make sure that the thermometer you are using can handle the temperature you will be measuring too. If not, do not use that thermometer as you will break it. Make sure your thermometer is upright whenever you use it in an experiment, to avoid incorrect results.

Different scales have different functions. However, a basic function of all scales is a tare button. This zeros the balance. It is important that you zero the balance before you take any measurements. If you are weighing something on a piece of paper you should tare the balance with the piece of paper on it, and weigh the substance. Make sure you check the units that your scale is weighing in. If you want your value to be accurate to \(\text{.00}\) \(\text{g}\) then the scale must measure to that accuracy. A scale that measures in \(\text{mg}\) would be best.

A burette is used to accurately measure the volume of a liquid added in an experiment. The valve at the bottom allows the liquid to be added drop-by-drop, and the initial and final volume can be measured so that the total volume added is known. More information about burettes is given to you in your first titration experiment this year in Chapter 9.

The surface of the water (the meniscus) is slightly higher at the edges of a container than in the middle. This is due to surface tension and the interaction between the water and the edge of the container (see the figure above). When measuring the volume in a burette (or measuring cylinder or pipette) you should look at the bottom of the meniscus. Where that lies is where you measure the volume. So in this example the meniscus is on the fifth line below the large line that represents \(\text{1}\) \(\text{ml}\). Therefore the volume is \(\text{1.5}\) \(\text{ml}\).

It is also possible that the liquid being measured has greater internal forces than those between it and the container. Then the meniscus would be higher in the middle than at the sides, and you would use the top of the meniscus to measure your volume.

A measuring cylinder is used to measure volumes that you want accurate to the nearest millilitre or so. It is not a highly accurate way of measuring volumes. The volume in a measuring cylinder is measured in the same way as for a burette, the difference is that in a measuring cylinder the smallest volume would be at the bottom, while the largest would be at the top.

There are two types of pipettes you might encounter this year. A volumetric pipette has a large bulb, marked with the set volume it can measure. Above the bulb on these pipettes there is a line. For a \(\text{5}\) \(\text{ml}\) volumetric pipette, when the meniscus of your liquid sits on the line, then the volume in that pipette is \(\text{5}\) \(\text{ml}\).

A graduated pipette has the same type of marking you see on a burette. The top is \(\text{0}\) \(\text{ml}\), and the volume increases as you move down the pipette. In this pipette you should fill the pipette to near the \(\text{0}\) \(\text{ml}\) line and make a note of the volume. You can then add the desired volume, stopping when the volume in the pipette has decreased by the required amount.

Video: Using Laboratory Thermometer

This elementary science video talks about laboratory thermometer which is commonly used in labs and its calibration.