Lesson Explainer: Measuring Substances Chemistry

In this explainer, we will learn how to identify measurement apparatus and describe accurate and reliable measurement methods for given experiments.

It is important to understand the difference between accuracy and precision. Accuracy describes how close a measurement is to the true or accepted value. Precision is an entirely different concept, and it describes how close measurements of the same item are to each other. It is possible to be both precise and inaccurate and it is possible to be both accurate and imprecise.

The distinction can be understood by considering where darts land on a dartboard. We will assume that the bull’s-eye (center) is the true or accepted value of the dartboard. Accurate throws hit the bull’s-eye, and inaccurate throws miss the center of the dartboard by a significant amount.

Darts players can be both precise and inaccurate if they repeatedly hit a point that is far away from the bull’s-eye. The following figure uses red-colored darts to show how a darts player can be both precise and inaccurate.

The differences between precision and accuracy can also be understood by considering measurements that are made during scientific experiments. Let us assume that 15.0 mL of an aqueous acidic solution is poured into a graduated cylinder (measuring cylinder). We will also assume that some students make measurements of this aqueous acidic solution one after the other.

Students would be making accurate measurements if they measured a volume that is exactly or almost exactly equal to a value of 15.0 mL. Students would be making inaccurate measurements if they measured a volume that is significantly less or significantly more than 15.0 mL.

The students could be described as being both precise and inaccurate if their measured values were all clustered around one value that is significantly different from 15.0 mL. The following image uses red arrows to show how a set of measurements can be both precise and inaccurate. The blue arrows are used to show how a set of measurements can be both accurate and imprecise. The blue arrows have an average value that is almost exactly equal to 15.0 mL, but the measurements are not all clustered around the same point on the number scale.

It is important that we do not mix up or freely exchange the terms accuracy and precision when we are talking about scientific experiments and scientific measurements. The terms have entirely different meanings, and we should always be careful how we talk about and how we practice science.

Example 1: Identifying Data Which Is Accurate and Precise

The following image shows the position of three points on a set of target boards. The true value is represented by the center of the target, the red circle.

1. Which target board shows poor accuracy but good precision?
2. Which target board shows good accuracy and good precision?
3. Which target board shows poor accuracy and poor precision?

Answer

Part 1

Accuracy, when used in science, relates to a measurement being close to the true value. In the case of a target such as the one in the question, this would mean the dots are near the red central target. As such, target board B will not be the correct answer. Good precision relates to repeated measurements being close together, and as such, the correct answer is target board C.

Part 2

The second part of this question asks us which board displays good accuracy and good precision. We are looking for results that are close to or coincident with the central red circle and also closely grouped together. This means the correct answer is target board B.

Part 3

In the final part of this question, we are looking for a board that displays poor accuracy and poor precision. In scientific terms, that would mean the three black marks are not closely grouped together and not near the central red circle. We can see this illustrated in target board A, the correct answer.

Scientists usually use a balance to measure the mass of an object because a balance is a very precise machine. There are different types of electronic balancing machines that can be used to measure the mass of a substance and they have different readability values.

The most commonly used type of balance is one that is accurate to two decimal places. This is colloquially known as a two-figure balance. It consists of a metal pan above the main body and a digital display that shows values to two decimal places. This type of balance is ideal for most experiments that are conducted in classroom settings.

There are other types of balances that are colloquially known as four-figure balances, because they show mass values to the nearest 0.0001 grams (g). Four-figure balances resemble two-figure balance machines, but they also tend to be surrounded by some kind of protective housing. The protective housing is designed to stop any air currents in the room affecting the operation of the analytical balance machine.

Four-figure balances can be trickier to use, and they are almost always more expensive than two-figure balances. Four-figure balances are usually reserved for work that needs to be incredibly precise or for measuring very low-mass substances. For example, we would need to use a four-figure balance to measure 0.0255 g of a sample substance because two-figure balances could only show values of 0.02 or 0.03 grams.

The following picture shows how a four-figure balance can be used to measure the mass of an orange-colored solid. It is important to realize here that the protective housing is made with one transparent wall that can be slid back and forth. The sliding door of any balance should always be closed before the tare button is pressed and before any measurements of mass values are made.

The following guide explains how the mass of a solid substance is measured with an analytical balance machine.

You might get the false impression that steps 3 and 5 are unnecessary and that solids can be weighed without first removing and replacing containers from a balance pan. However, this is not the best practice. Firstly, you can end up spilling material onto the balance pan. Secondly, the shock waves caused by dropping solid directly onto the pan can damage the mechanism inside.

There is a wide range of scientific glassware that can be used to measure the volume of a liquid. Many beakers and flasks have crude measuring scales etched into their surface, and we may initially think that they would be appropriate pieces of equipment to use for measuring liquids. However, these types of glassware are not intended for accurately measuring out liquids and should not be used to do so.

Graduated cylinders have lots of etchings that run along their long axis, and they can be used to measure a volume of liquid more accurately. Graduated pipets and burets also provide a high level of accuracy and they are frequently used in highly precise titration experiments. Volumetric pipets are manual liquid-handling devices that are used to dispense a single, specific quantity of liquid to a very high degree of accuracy. Some of this sophisticated scientific glassware is depicted in the following figure.

Example 3: Selecting the Most Accurate Piece of Glassware

The picture below shows the top parts of several graduated cylinders. Which of the following graduated cylinders can give the most accurate measurement?

Answer

When discussing accuracy in science, we are referring to a measurement that is very close to the true or accepted value. Cylinder B has the most graduation marks out of the four graduated cylinders (measuring cylinders). Cylinder B can be used to make measurements that have a smaller error margin. Cylinder B can be used to determine a volume that is closer to the actual or true volume of liquid in the measuring cylinder. We can use this line of reasoning to determine that option B is the correct answer for this question.

The buret is a relatively sophisticated piece of scientific equipment that is used in titration experiments. The buret is ideally suited for titration experiments because it has a stopcock or tap that can be used to control the outflow of liquid from the buret into a cylinder or flask. The buret can be used to slowly add a liquid to a conical flask and determine the right amount of one reactant liquid that needs to be added to another reactant liquid. This statement could be reworded to say that the buret can be used to add just the right amount of one liquid to another.

The buret is a very thin and fragile piece of scientific equipment. The apparatus is so thin that the water surface cannot form a single flat line. The water surface forms a concave meniscus and scientists have to measure buret volumes by examining the bottom position of the meniscus. Burets also have the rather unusual tendency to form hanging drops of titrant at their end point. It is important that the buret nozzle is gently flicked or nudged so that any hanging drops move from the buret nozzle into conical flasks or reaction vessels.

Technically, we could also measure our liquids by weighing them on a balance as we do for solids. However, this can be trickier than weighing out solids, since you have to take into account the density of the liquid as well. It is easier to measure volumes directly rather than measure mass values first and subsequently convert them into volume values.

Example 4: Stating What the Volume Is and How the Volume Should Be Measured

The image below shows a volume of liquid in a graduated cylinder.

1. Where should the volume of the liquid be read from?
2. What is the volume of the liquid in the graduated cylinder?

Answer

Part 1

The meniscus is the upward or downward curve seen at the top of a liquid in a container. The meniscus is formed due to surface tension effects in the upper surface of the liquid. Volume values should always be determined from the position of the bottom of the meniscus. Volumes will be determined incorrectly if they are not determined from the lowest point of the meniscus. Thus, the correct answer is the bottom of the meniscus of the liquid.

Part 2

The graduated cylinder in the diagram has two sets of graduation marks. It has larger, numbered graduated marks in fives measuring from 5 mL to 50 mL. The second set of graduation marks measures in 1 mL increments between each of the larger marks. This being the case, we can see that we have more than 45 mL of liquid in the graduated cylinder, and by looking at the smaller set of graduation marks, you can see that the volume of liquid is 47 mL.

It is important to always appreciate that even the most sophisticated glassware has its limitations. No single piece of scientific equipment can be used to make any one measurement that is perfect and is entirely void of any type of error whatsoever. The following figure shows how scientific equipment is designed to show its users that the equipment is not perfect and that it has a known accuracy grade. The figure shows a volumetric pipet that has an accuracy grade of 0.04 mL. The volumetric pipet can be used to measure exactly of liquid, but it cannot be used to measure exactly of liquid.

The volume of a product gas can be measured with a gas syringe. It can also be measured by placing an upturned graduated cylinder into a beaker that contains an unreactive aqueous solution.

In either instance, we use rubber tubing to attach the reaction vessel to the measurement apparatus. We then monitor gas formation by examining how far the plunger in the gas syringe moves or by watching how far a column of liquid moves in the upturned graduated cylinder. Both setups work in a similar way, though the gas syringe is perhaps a little easier to work with. Gas syringes are generally the preferred option for measuring gas formation during a reaction because they can usually provide a higher level of accuracy.

Scientific equipment is designed for a particular purpose, and it is up to use to determine the most appropriate equipment for our scientific experiments. Imagine, for example, that we wanted to measure the volume of carbon dioxide gas that is produced when calcium carbonate reacts with hydrochloric acid. We could initially determine the mass of the calcium carbonate with a two-figure balance. We could then use a graduated cylinder to measure out a known volume of hydrochloric acid that would react with all of the calcium carbonate solid. We could then use a gas syringe to determine how much carbon dioxide gas is produced when our calcium carbonate rocks are made to react with our known volume of hydrochloric acid. The apparatus would enable us to understand how many reactant molecules there were before the chemical reaction and how many product molecules there were after the chemical reaction. More sophisticated scientific equipment could be used if we wanted to produce more accurate data.