Video Transcript
In this video, we will practice
identifying measurement apparatus and describing accurate and reliable measurement
methods for given experiments. Before we begin looking in detail
at various types of apparatus, it’s important to make sure that we all understand
the difference between accuracy and precision in a scientific context.
Accuracy is a measure of how close
the value you obtain from an experiment is to the true value. On the other hand, precision is how
close your various measurements are to one another. Let’s illustrate the differences
between accuracy and precision with some examples.
A common way to explain these
concepts is with a dartboard. Imagine that you throw three darts
at a dartboard. Accuracy would be a measure of how
close your darts are to the bull’s-eye, which is in this center of the dartboard,
while precision would be how close your three darts are to one another.
On this first dartboard, you can
see that all three of our darts are very close to the center. This means our darts are
accurate. Moreover, the three darts are very
close to each other, which means that they are also precise. Let’s have a look at another
example. This time, when we look at our
three darts, we can see that none of them are close to the center. This means that our darts are not
accurate. However, the three darts are close
to one another, so they are precise.
Now let’s look at our third
example. Although our three darts aren’t
exactly on the center, they do average out at a point on the center. Because the average of our
measurements is on the center point, our results are accurate. However, our darts are not close to
one another, so these results are not precise. And finally, we have three darts
which are neither near the center nor are they close to one another. So these results are not accurate
and not precise.
Let’s visualize this in another
way. Let’s imagine that we’re performing
an experiment and the results that we’re recording are numbers between 1.3 and
1.6. It doesn’t really matter what it is
we’re recording for this. And let’s imagine that the true
value is 1.45, for argument’s sake. If we made three recordings like
this, we would say that our results are both accurate, because they’re close to the
true value, and precise, because they’re close to one another.
If we had results like this, we
would say that they are accurate because they average out to roughly the true value,
but they are not precise because our values are not similar to one another. Results like this, however, we
would consider not accurate since they’re nowhere near the true value. But they are precise because they
are close to one another. And results like this would be
neither accurate nor precise.
Now that we understand the
difference between accuracy and precision, let’s start looking at some scientific
apparatus. First, let’s look at different ways
we can measure a solid. Of course, the best way to measure
out a solid is by using a balance. In fact, there are actually
different types of balances depending on what you’re weighing out. The most common balances that you
might come across are balances which are accurate to two decimal places and
sometimes balances which are accurate to four decimal places. You may hear these referred to as a
two-figure and a four-figure balance.
The two-figure balance is the one
which is ideal for the majority of the work you’ll do. On the other hand, the four-figure
balance is used for very accurate work or when you need to weigh out a very small
amount of something. You can tell the difference by
looking at how many decimal places the display shows. You’ll also notice that a
four-figure balance will have some kind of housing around it. This is because when making such
accurate measurements of mass, any airflow over the top of the balance can upset the
reading. So to combat this, we put a house
on the top. This is, of course, see-through, so
you can still read the value on the balance display.
Now let’s look at how to use one of
these balances correctly. The first step is to place your
measuring container onto your balance. You should always use a wide-necked
vessel for weighing material out. A beaker would be okay, for
example, or a weighing boat is even better. But you shouldn’t use something
with a narrow neck like an Erlenmeyer flask, for example. This is to reduce the chance that
you’ll spill your solid over the bench, since this could create a hazard and is
wasteful. Weighing boats are specifically
designed for this purpose.
You can get aluminum foil boats,
for example. These come with a handy tab to
carry it with and are also flexible so that you can bend the boat in order to add
your solid to a reaction vessel easily. You can also get flexible plastic
boats. These are useful because you can
weigh out into a nice, wide-necked boat and then curl the boat into a straw shape so
that you can easily insert it into a narrow-necked reaction vessel and put all your
solid in without spilling it.
Once the container is on your
balance, you’ll need to set the display to zero. There will usually be a prominent
button saying zero or tare on the front of your balance to do this. Now we are able to weigh out our
substance. You could do this by simply putting
the solid into the weighing container whilst it’s on the balance using something
like a spatula. But this isn’t really the best
practice.
It’s better to take the weighing
boat off the balance to put your solid into it and then replace it back on the
balance. This prevents you from dropping any
solid onto the balance, which could damage the balance pan or the springs
underneath. You simply repeat this process of
taking the substance off the balance, adding a bit more, and then placing the boat
back on the balance until you reach the desired mass. Once you’ve reached your final
amount, make sure to record exactly what your final mass is according to the
balance. So this is how we measure out a
solid, but what about a liquid?
We have a variety of glassware
options if we want to measure out a liquid. Perhaps the most commonly used
apparatus for measuring a liquid is the graduated cylinder, sometimes called the
measuring cylinder. Graduated cylinders come in a
variety of sizes, for example, one milliliter, five milliliters, 15 milliliters, and
so on. Select the most appropriate size
for the amount of liquid that you want. If you need half a milliliter, for
example, you’d probably want the smallest cylinder, one milliliter. You would fill it to halfway to the
0.5-milliliter line and then empty it into your reaction vessel.
If you needed to measure out 3
milliliters, you could use the smallest, the one-milliliter, cylinder three times,
but that might get a bit tedious. You might be better off using the
5-milliliter cylinder. You could also use the
15-milliliter cylinder. And that wouldn’t be wrong, but it
is more difficult. Because you have such a large
cylinder and you’re only adding a small amount of liquid, it’s more difficult to
read the measurements. So the five-milliliter cylinder
would be better.
Let’s have a look at how we read
the measurements. Here’s a close-up of a measuring
cylinder. Now let’s add our liquid. How would we read this volume? The rule that we use is that we
always read from the bottom of our meniscus. The meniscus is the skin-like
surface of our liquid. You can see that our meniscus is
higher at the edges of our cylinder, but it’s the very bottom that we need to read
from. In this diagram, we can see that
the bottom of our meniscus is in line with this graduation, so this volume would be
1.2 milliliters.
We use the same technique when
measuring volume using a pipet. There are two main types of glass
pipet: volumetric and graduated. Volumetric pipets come in various
sizes and are good for measuring out set volumes, for example, 25 milliliters of a
liquid. Graduated pipets, on the other
hand, are useful in a similar way to graduated cylinders. You can measure out a wide variety
of different volumes with a graduated pipet. You simply fill the pipet to the
marking that you need, measuring from the bottom of the meniscus, remember, and then
empty your pipet into the reaction vessel.
Glass pipets like these can come
with different accuracy gradings. This tells you how accurately that
particular piece of glassware will measure the volume that it says. You can use higher-accuracy-grading
glassware for more accurate work. And of course, you can also use a
buret to measure out a liquid. To do this, you simply add your
liquid to the buret, making sure not to add any more than the top line, and then
dispense the required amount using the tap at the bottom.
You might ask, can we use an
Erlenmeyer flask, sometimes called a volumetric flask, to also measure liquids? The answer is “Not really.” They’re in no way accurate. The same is true of beakers. The markings on the side of these
bits of glassware are really just a rough guide and shouldn’t be used for
measuring. You might also ask, well, could we
not weigh the liquid in a similar way that we weighed out our solid? It isn’t wrong to weigh out a
liquid, but it is a bit more tricky than weighing out a solid.
When you weigh out a liquid, you
also need to take into account the density of your particular liquid. So most of the time it’s easier to
work with volumes.
Now that we’ve measured both
liquids and solids, let’s look at gases. Perhaps the easiest way to measure
a gas is with a gas syringe. You attach one end of the gas
syringe to the gas that you’d like to draw up or to your reaction vessel, if you’re
measuring gas production. And then you draw or watch the
plunger move and measure using the markings on the side. Another way to measure gas produced
by a reaction is with an upturned, graduated cylinder. The tubing from your reaction
carries the gas into the upturned cylinder, which is initially full of water. As bubbles are produced and enter
the upturned cylinder, they displace the water and the water level drops. You can then use the markings on
the side of the cylinder to work out how much gas has been produced. This works just as well but is
slightly more fiddly to set up.
Now that we’ve learned how to
measure gasses, liquids, and solids. Let’s put this to use in an
experiment.
Let’s imagine that we’re carrying
out this reaction. We have calcium carbonate reacting
with hydrochloric acid to form carbon dioxide, water, and calcium chloride. Note that this equation is not
balanced. We are told that we need to measure
the production of this particular product, CO2 gas. So let’s work out which apparatus
we’re going to need.
Our first reactant is calcium
carbonate, which we’re told is a solid. To measure out this solid, we’re
going to need to use a balance. The first thing we’ll need to do is
select a weighing container. We’re going to want something wide
necked, like a weighing boat. Now that we’ve placed our container
on the balance, we need to set the display to zero. This is sometimes called taring the
balance.
Next, we need to weigh out our
calcium carbonate. Remember that the safest way to do
this is to take your container off the balance, add some of your solid, and then put
it back on the balance, repeating this process until you get the mass that you
require. Once you’ve weighed out the correct
mass, remember to record the value.
Next, we need to measure out our
hydrochloric acid. Because our acid is aqueous, this
means measuring out a liquid. To measure a liquid, we could use a
graduated cylinder, pipet, or buret. The experiment tells us that we
need 25 milliliters of hydrochloric acid. So we could use a 25-milliliter
graduated cylinder, a 25-milliliter volumetric pipet, or a 25-milliliter graduated
pipet. We could use a buret, but it’s not
the best piece of equipment for this. Burets are more useful when you’re
not sure quite how much you’re going to need to add, so you add a little bit at a
time until you reach the point you need, for example, in a titration experiment. Remember to always read from the
bottom of the liquid’s meniscus.
Finally, we need to measure the
production of our carbon dioxide gas. The easiest way to do this is with
a gas syringe. You simply attach your gas syringe
to tubing connected to your reaction vessel and watch the plunger of the gas syringe
move.
Now that we’ve put this into
practice, let’s summarize what we’ve learned. Accuracy is how close you are to a
true value, while precision is how close your measurements are to each other. We measure solids using a
balance. Remember to always use a
wide-necked container to measure your solid out in. To measure a solid, you first place
your container onto the balance, then you set the display to zero, weigh out your
substance, and record the final mass.
When measuring liquids, we tend to
use graduated cylinders, pipets, and burets. To measure out a gas, we use gas
syringes or water displacement equipment. And always remember to measure from
the bottom of your meniscus.