Video Transcript
In this video, we will learn about flame
tests, how to do them, why they work, and how they allow us to identify the metal ions in
some substances. We’ll look at the colours produced by
alkali, alkaline earth, and other metal ions.
A flame test is an experiment where a
substance is put into a flame. Some substances give the flame
colour. Understanding which substances produce
which colours, can help us identify unknown substances. Many substances will burn in a flame,
producing red, yellow, white, or blue hot flames, depending on the temperature.
For a flame test, the colour is pretty
consistent across the temperature range. For a flame test, the key factor is the
heating, not the burning. Only some substances would generate
visible light when heated in a flame. If a substance does produce a visible
flame colour, that colour will be unique. Although, sometimes, it is difficult to
tell the difference. Let’s have a look at how to do a flame
test.
There are many ways to do a flame test,
but one of the simplest is the use of a platinum loop. A loop of platinum wire is very
unreactive and does not colour the flame. In the first step, the loop is repeatedly
cleaned with hydrochloric acid and put in the flame until all contaminants are removed. It’s easy to tell when this has happened
because putting a clean loop in the flame will not change the colour of the flame.
The loop is then dipped in a test
substance, either a solution or a powder perhaps, and put into the frame. If the flame colour changes, the colour
is recorded. You could write down a description or
take a picture. It is important that the flame is hot and
does not produce a strong colour itself. If using a Bunsen burner, it is best to
use a blue roaring flame rather than bright yellow safety flame, so the colour from the ions
is easier to distinguish. To achieve a roaring flame, we just open
the air inlet at the base of the Bunsen burner.
Now let’s look at what a flame test might
look like for a common substance like lithium. When a substance containing lithium ions
is put into a flame, the lithium ions emit visible light, turning the flame red. A lithium ion Li⁺ has the electron
configuration of two, or one s two. When heated in a flame, the electrons are
excited, jumping to higher energy levels. When electrons fall back down, they can
emit photons of light. The bigger the gap, the higher the energy
of the photon, and the smaller its wavelength.
The colour we see can be the result of
many different wavelengths of light being emitted at the same time. But the wavelengths of light are unique
to each element, so the colour we see is unique as well. That’s not to say that the descriptions
of the colours are always unique. Here’s a table of ions and the colours of
flame they tend to produce along with a common description of the colour. The colours given are approximate.
As you can see, there are some ions which
produce very similar colours. Lithium and strontium deliver a similar
red. Sodium and calcium both produce an orangy
colour. Barium, boron, and copper produce
greens. And potassium, rubidium, and cesium all
produce purples. Telling these apart can be difficult. So, other chemical tests might be
needed. Or a diffraction grating or a
spectrometer can be used to analyse the light in more detail.
Some ions tend to produce more light than
others. Potassium ions, for instance, produce a
pale flame, while sodium ions produce a much stronger colour. If a sample is contaminated, the colour
from one ion may overpower that from another. So, it’s best to do flame tests with very
pure substances.
Sodium is quite a common contaminant. Filters like blue cobalt glass, which is
glass with added cobalt compounds, can be used to remove the bright orange yellow light from
sodium ions, allowing other colours to be seen more clearly. If you had a mixture of sodium and
potassium ions, you could use cobalt glass just to visualise the colours from the
potassium.
Now that we know what a flame test is,
and how to perform one, and which ions it can help us identify, let’s have some
practice.
In which of the following images does the
result of a flame test indicate the presence of barium?
In a flame test, we’re adding an ionic
solid, a powder perhaps, to a flame. And we’re looking at the colour
change. Each colour is characteristic of certain
ions. A is a red flame. B is an orange or yellow. C is green. D is blue. And E is purple. To answer this question, we’re going to
need to recall what flame colour is characteristic of the barium two plus ion.
A green flame is indicative of barium two
plus, boron three plus, or copper two plus. So, we can be pretty sure that the green
flame is the one most likely to indicate the presence of barium. A red flame is associated with lithium
plus or strontium two plus. An orange or yellow flame is indicative
of sodium plus or calcium two plus. A Blue flame like this is most likely
indicative of copper plus. While a purple flame, depending on the
depth of its colour, is more likely to indicate potassium plus, rubidium plus or cesium
plus.
So, of the images given, the one that
indicates the presence of barium is flame C, the one with the green flame.
Now we’ve looked at a visual example,
let’s look at one that requires more memory.
Which alkaline earth metal most resembles
copper when analysed using a flame test?
A flame test produces a colour depending
on the ion. The alkaline earth metals are otherwise
known as group two on the periodic table. The elements in group two are beryllium,
magnesium, calcium, strontium, barium, and radium. Radium is rare and radioactive, so for
the purposes of this question, we can ignore it.
Each of these elements tends to form ions
with a two plus charge. To answer this question, we need to
recall what colour flame copper produces in a flame test and compare it to those of
beryllium, magnesium, calcium, strontium, and barium. Depending on the compound of copper will
produce a blue-green flame. Beryllium and magnesium have no
characteristic flame test colour. Calcium ions turn flames orange-red. Strontium ions turn them bright red. And the flame test colour for barium ions
is green.
The closest colour of these to blue-green
is the green of barium. So, the alkaline earth metal whose flame
test colour most resembles that of copper is barium.
That’s two examples where we looked at
particular colours of flame. Now let’s have a look at the practical
aspect.
A student uses a flame test to determine
whether the major product of a reaction is a potassium salt. Which of the following would not improve
the quality of the test? A) Closing the inlet of the Bunsen
burner. B) Dissolving the product in hydrochloric
acid. C) Viewing the flame through cobalt
glass. D) Washing the sample loop in
hydrochloric acid before use. Or E) Recrystallizing the product.
A flame test is a way of checking for the
presence of certain ions by the production of a particular colour of flame. The potassium salt will contain potassium
plus ions. The addition of potassium salts to a
roaring blue Bunsen flame will turn it a pale pink.
If when the student does the test the
flame colour is pink, then there’s definitely potassium present. However, if the flame colour is not pink,
it’s not necessarily clear whether potassium is there or not. Our job is to look at the statements and
see which one would not improve the quality of the test. A better quality test is more likely to
produce a positive result.
What about closing the inlet of the
Bunsen burner? If the air inlet of a Bunsen burner is
open, more oxygen gets to the flame. It gets hotter. And it becomes more blue. If you close the inlet of a Bunsen
burner, you’ll produce a safety flame which is a bright yellow. A bright yellow flame is likely to mask
the pink of any potassium. So closing the inlet of the Bunsen burner
would definitely not improve the quality of the test.
Let’s have a look at the other options
just to be safe. Dissolving the product in hydrochloric
acid first will make the solid easier to vaporise. In solution, the ions are already
separated, so when they’re heated up, they’ll all go into the air and start emitting
light. Depending on the concentration, there’s
an opportunity to vastly increase the brightness of the pink colour and improve the quality
of the test. So, this is not a correct answer.
What about viewing the flame through
cobalt glass? Cobalt glass is a blue filter. Viewing the flame through cobalt glass
will remove the orange-yellow bright light from sodium. This will allow the pink colour from
potassium ions to come through more clearly, improving the quality of the test. Sodium ions are quite common
contaminants, so this is likely to be a good idea.
Washing the sample loop in hydrochloric
acid is another way of improving the quality of the test. Washing the loop removes other
contaminants, which might colour the flame differently and lead to an ambiguous result.
Finally, we have recrystallizing the
product. Recrystallization will purify the
product. A more concentrated product means fewer
contaminants that are likely to interfere with the results of the test. So, recrystallizing the product is a good
way of improving the quality of the test, and, therefore, is not our answer. So, of the five options given, a student
doing a flame test to check if potassium is present in the product should probably not close
the inlet of the Bunsen burner in order to improve the quality of the test.
Now that we’ve looked at a few examples,
let’s wrap up. In this video, we’ve learnt that
substances produce light when put in a frame. Some of these substances produce visible
light with a distinctive colour due to the specific electron transitions. And the distinctive colours can be used
to help identify specific ions in substances.
