[Basic Chemistry] 7th Period: Detection Reactions

Good evening!
Are you brothers doing well?
Let's begin today's class.

[Review of the previous lesson]
[Mixtures]contain[Pure substances], and by performing the optimal
[Separation operation]based on theirrespective properties
, you can target and extract the[Pure substance]you want to retrieve.
One of those so-called "respective properties" was the topic of[Allotropes].

[Allotropes]will only become relevant to you brothers in the latter half of the
[Applied Chemistry]unit, specifically in[Inorganic Chemistry].

For those of you who don't need[Applied Chemistry]for your career path, it's
bye-bye to[Allotropes]after the next midterm exam.

So, there isabsolutely no needto get too worked up about it, like "I must not run away."
Let's just handle it efficiently, shall we?

Now, today we will learn about[Detection reactions].

[Detection reactions]are different from[Separation operations]in that they are not intended to recover large quantities of a specific substance in the first place.
Like
[Chromatography], the goal isto confirm the presenceof a specific substance.

In the[Basic Chemistry]you are studying, you will learn these
[Flame reactions],[Precipitation reactions], and[Gas evolution reactions],three typesin total.

So, let's get started right away.

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Flame reaction

Group 1Lupin,Group 2Bed of [Elements], when
compounds or aqueous solutions containing specific metal [elements] are [compounds] held over a flame, the
flame color will exhibit a color unique to that [element]. This is called
[flame reaction].

Group 1Lupin,Group 2Bed—are there any sheep who haven't memorized these yet?

It doesn't end with just saying, 'Oh, how pretty,' when the flame color changes.
What we are learning this time is [detection reactions], so you achieve the goal once you
understand which [element] is contained by looking at the flame color
.

[Basic Chemistry]—it is good to remember the following seven. LiLithium(red) NaSodium(yellow) KPotassium(purple)

Li-a-car, Na-ki, KK-mura

CuCopper(green) CaCalcium(orange) SrStrontium(crimson) BaBarium(yellow-green)
Do-ryoku, Ka-ru-tou, Su-ru-mo-ku-re-nai, Baryokupower

Now, let's actually see the colors in the video.
This time, we have the reliable and steady Hiroshi-sensei again.
Only CuCopper is at the very end, but
he will perform the experiments in the order of the mnemonic above, so watch closely.

[Video] For some reason, chemists who do unexpected things with a straight face are interesting.

Regarding the platinum PtPt that should be used, it is a substance that is
chemically very stable and a metal with very low reactivity.
Chemical reaction resistance is a given, and it is also resistant to physical changes caused by heat.
Its melting point is a staggering 1768°C.
Also, just like red phosphorus, it is also a type of reaction catalystenergy aid, so it also has the effect of
assisting the flame reactions of other metals, making the colors appear more vivid.

[Review] Red phosphorus's properties

That being said, in the [Flame reaction] experiment,
you would normally dip a platinum wire into a metal aqueous solution and stick it into the flame to observe the color, but...

Hiroshi-sensei really does things differently!

In the second half of the video, he briefly explained why it appears colored based on the
[Atom]'s [Energy] discussion.

Originally, [Atoms] are learned in the [Constituent particles of matter] unit, and
[Energy] is learned in [Applied Physics], so
for now, don't push yourself; just keep the 'I got it!' 35P in the back of your mind.

By the way, at the end, he burned chalk to see the color, but
did you figure out what metal [Element] is contained in the chalk?

I would hope you look back at the color and judge for yourself, but as reference information,
I will tell you that the main component of commercial chalk is calcium carbonate CaCO3.

[Aside] Speaking of flame reactions we see in our daily lives, fireworks are the most representative example.

Precipitation reaction

[Aqueous solution A] and [Aqueous solution B] are mixed, and a chemical reaction occurs.
The result of that reaction is known in advance.

Why is it known?
It is the result of the research of thousands of chemists.

The properties that substances possess, in other words,
who reacts with whom by burning, dissolving, or mixing,
is already known if it is a simple chemical reaction.

Therefore, by comparing it with the results of the actual mixture, you can
confirm its existence by saying, 'Ah, it was indeed in there.'

If those results were things like taste or smell, there would be individual differences in perception, leading to the possibility that 'it might be different.' However, if it is a

precipitate such as a solid, color, or cloudiness, it is objective and obvious at a glance.

The purpose of this

precipitation reaction is to confirm existence based on the presence or absence of a precipitate while comparing it with the pre-set recipe.

*Note*
When people hear 'precipitate,' they tend to strongly imagine solid objects. Like something chunky, powdery, or muddy. However, in the world of chemistry,

hazy cloudiness is also called a precipitate. It hasn't settled, but it's a precipitate—keep this in mind.

In short, if something that wasn't there before the reaction is produced as
something visible due to the reaction, it is considered a
precipitate.

Now, let's take a look at it in the [Video].

[Video] Confirming chlorine (Cℓ) and silver (Ag) through the precipitation of silver chloride (AgCℓ)

Sodium chloride is
dissociated into sodium ions and chloride ions in an aqueous solution.

$$
NaCℓ　➡　Na^＋　＋　Cℓ^－
$$

Silver nitrate is
dissociated into silver ions and nitrate ions in an aqueous solution.

$$
AgNO_{3}　➡　Ag^＋　＋　NO_{3}^－
$$

Adding the two together...
Sodium chloride + Silver nitrate ➡ Sodium nitrate + Silver chloride (↓)

$$
NaCℓ　＋　AgNO_{3}　➡　NaNO_{3}　＋　AgCℓ（↓）
$$

At this time, chloride ions and silver ions react to form a
silver chloride white precipitate, which is a known fact. Topics like

ionization or ions will be explained in detail in the [Ions] unit, so for now, just enjoy seeing the vivid
white precipitate form and confirming, 'This means it definitely contained
chlorine and silver.'

In the video, we also try a [Flame Test] by applying an aqueous solution containing chlorine to a copper wire, but for now, let's focus on the
[Precipitation Reaction].

I think this video showed the most vivid precipitation!

|Gas Evolution Reaction

When acid is added to a substance (solid), gas may be generated. By collecting this
gas, examining its properties, and identifying what the gas is, we confirm that the object contains the
gas's [Element].

[Example]
MarbleWhen dilute hydrochloric acid is added to , gas is generated. When this
gas is collected and passed through limewater, the limewater turned cloudy white. What
element is contained in this [marble]?

Marble is known to have calcium carbonate CaCO3 as its main component, and
limewater is calcium hydroxide Ca(OH)2.understood

[Video] Adding dilute hydrochloric acid HCℓ to calcium carbonate CaCO3

Explaining the reaction process.
❶calcium carbonate CaCO3 is added to dilute hydrochloric acid HCℓ.

$$
CaCO_{3} + 2HCℓ ➡ CaCℓ_{2} + H_{2}CO_{3}
$$

❷calcium chloride CaCℓ2 and carbonic acid H2CO3 are generated.

Carbonic acid H2CO3 is the source of the fizz in drinks like Cola and Fanta that you all know well. Yes, that is exactly what

carbonic acid is. Carbonic acid
is a very unstable substance that easily undergoes chemical change at room temperature and pressure.

$$
H_{2}CO_{3} ➡ H_{2}O + CO_{2}
$$

Therefore, the reaction equation for calcium carbonate CaCO3 and dilute hydrochloric acid HCℓ can be corrected as follows.

$$
CaCO_{3} + 2HCℓ ➡ CaCℓ_{2} + H_{2}O + CO_{2} (↑)
$$

So, looking only at the reaction equation, the
gas gas generated when adding dilute hydrochloric acid HCℓ to marble CaCO3 appears to be carbon dioxide
CO2.

❸ Identify the generated gas. Confirm if it is truly

carbon dioxide CO2. Since we have already predicted from the reaction equation that the gas is
carbon dioxide CO2, we will pass it through the standard test for carbon dioxide, limewater

, which is an aqueous solution of calcium hydroxide Ca(OH)2, and see if the solution turns cloudy white
.

$$
CO_{2} + Ca(OH)_{2} ➡ CaCO_{3} + H_{2}O
$$

Carbon dioxide CO2 reacts with limewater Ca(OH)2 to produce
calcium carbonate CaCO3.

One might tend to think the solution turned white because marble is originally a white ore, but it is not that simple. The reason it

appears white is due to the relationship between the molecular structure of calcium carbonate CaCO3 itself and light. You will learn the details in university subjects such as [Molecular Spectroscopy]
, so please just listen for now.

Why do substances appear to have color? How do the reflection and absorption of
light with infinite colors
relate to molecular structure? Is perfect
optical camouflage
possible? Why is it warm when exposed to sunlight? Can we

determine the properties of a substance from its reaction when exposed to light? For those of you who are interested, I think you should include a physics-related university as one of your career options.

Back to the main topic. When I passed the gas through

limewater Ca(OH)2, the
limewater Ca(OH)2 indeed turned cloudy, so we know the gas is
carbon dioxide CO2.

Carbon dioxide CO2 also means it contains carbon C, so we have
confirmed the presence of carbon C in the marble.

[Note]
In the video, I continued to pump carbon dioxide (CO2) even after it turned cloudy.
Then, I think many of you noticed that the aqueous solution eventually returned to being colorless and transparent.
I believe many of you noticed this.

Cloudiness is just a hazy look, but as you have already learned, in chemistry it is considered a precipitate.
Precipitate formation reactions include some that do not end with the formation of a precipitate, but rather develop into reactions where
the original color is restored by adding more of a specific substance.

$$
CaCO_{3} + CO_{2} + H_{2}O ➡ Ca(HCO_{3})_{2}
$$

Ca(HCO3)2 is called calcium hydrogen carbonate.
Calcium hydrogen carbonate is an ionic substance that dissolves easily in water.
Dissolving as ions means it cannot be visually identified as a solid.
It is not a substance that stays precipitated once it has formed.
As a result, the white haze disappeared and it returned to being colorless and transparent.

You will also learn about this in the [Basic Chemistry] unit on [Ions], so for now, just accept it as it is.

Well, how was that?
This time, we learned about three [Detection Reactions] covered in [Basic Chemistry].
Although this knowledge will truly shine in [Inorganic Chemistry] within [Applied Chemistry], it is also knowledge that will be used sufficiently in
[Basic Chemistry] from now on, so it would be good to have a moderate grasp of it. Besides, it will definitely appear on the

first semester midterm exam as well.

Next time, we will talk about [The Three States of Matter and Thermal Motion]. Once that is finished, it will finally be time for the
first semester midterm exam. Even though it is just the very beginning, the
first boss fight in the story, if you neglect leveling uppreparation
, you will easily lose, so please do your best.

For those sheep who are so excited for the midterm exam that they are trembling with anticipation,
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For those sheep who are trembling with fear instead of anticipation,
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It feels like spring and autumn have disappeared over the last few years,
leaving only memories of extreme heat and severe cold waves, so
everyone, please be very careful with your health management.

See you next time.
That concludes today's lesson.
Thank you for your hard work.