Carbon, CO, and CO₂ · Intro Chemistry

One element, very different faces

The “lead” in a pencil is graphite, soft and slippery. The diamond in a ring is the hardest natural substance known. Both are pure carbon. Carbon’s two oxides are just as different: carbon monoxide is a deadly poison (which is why homes have CO alarms), while carbon dioxide is something you breathe out all day and the fizz in every soda.

This lesson has two parts:

Forms of carbon: diamond, graphite, and other forms of the element, and carbon’s chemical properties (stability, burning, and reducing power).
Carbon dioxide and carbon monoxide: the properties, uses, and lab preparation of the two oxides, and how to tell them apart.

A quick check follows Part 1; get it right before you start Part 2.

Part 1: Forms of carbon

	Diamond	Graphite
Appearance	Clear, colorless crystals	Dark gray, shiny, flaky solid
Hardness	The hardest natural substance	Very soft and slippery
Conducts electricity?	Almost not at all	Yes, well
Uses	Drill bits, glass cutters, jewelry	Pencil “lead,” electrodes, lubricants

Diamond and graphite are both made of carbon atoms, yet their physical properties are wildly different because the carbon atoms are arranged differently. Buckminsterfullerene, $\ce{C60}$ , is another form of carbon: a soccer-ball-shaped molecule of 60 carbon atoms. Charcoal and activated carbon are full of tiny pores, so they adsorb colors and odors very well.

Carbon’s chemical properties:

Stable at room temperature: ancient ink drawings made with soot are still dark after centuries.
It burns: with plenty of oxygen it forms carbon dioxide; with too little, carbon monoxide.

\ce{C(s) + O2(g) -> CO2(g)}

\ce{2C(s) + O2(g) -> 2CO(g)}

It’s a reducing agent: at high temperature, carbon pulls the oxygen out of copper(II) oxide. The black powder turns red (copper), and the gas produced turns limewater cloudy.

\ce{C(s) + 2CuO(s) ->[\Delta] 2Cu(s) + CO2(g)}

Part 1 check

Why are the physical properties of diamond and graphite so different?

Part 2: Carbon dioxide and carbon monoxide

Carbon dioxide

Physical properties: a colorless, odorless gas, denser than air, and fairly soluble in water. Solid carbon dioxide is dry ice, which absorbs a lot of heat as it sublimes.

Chemical properties:

It usually doesn’t burn or support burning. Pour carbon dioxide into a beaker holding a short and a tall burning candle, and the short candle goes out first.
It reacts with water to form carbonic acid, which turns litmus red. Carbonic acid is unstable and breaks down again when warmed:

\ce{CO2(g) + H2O(l) -> H2CO3(aq)}

\ce{H2CO3(aq) -> H2O(l) + CO2(g)}

It turns limewater cloudy. This is the test for carbon dioxide:

\ce{CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l)}

Uses: fire extinguishers, dry ice for cooling, a raw material for photosynthesis, greenhouse “fertilizer,” and carbonated drinks. Rising carbon dioxide in the atmosphere strengthens the greenhouse effect.

Lab preparation: react marble chips or limestone with dilute hydrochloric acid. Carbon dioxide dissolves in water and is denser than air, so collect it by upward displacement of air. A burning splint held at the mouth of the jar goes out when the jar is full.

\ce{CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)}

Carbon monoxide

Physical properties: a colorless, odorless gas, barely soluble in water, slightly less dense than air.

Chemical properties:

It burns with a blue flame and releases lots of heat, so it can be a fuel.
It’s a reducing agent: it reduces copper(II) oxide to copper, and it’s used to extract metals from their ores.
It’s toxic: it binds to hemoglobin in the blood so tightly that the hemoglobin can no longer carry oxygen. Because it’s colorless and odorless, it can poison people without warning. Fuel-burning heaters and stoves need good ventilation, and homes need CO alarms.

\ce{2CO(g) + O2(g) -> 2CO2(g)}

\ce{CO(g) + CuO(s) ->[\Delta] Cu(s) + CO2(g)}

Pick an example to see how the particles recombine in these reactions:

Try it: one event, three levels

Macroscopic: what you can see

Bubbling carbon dioxide through purple litmus solution turns it red.

Particulate: what the particles do

COOCOOHHOHHO

→

HHCOOOHHCOOO

Symbolic: how chemists write it

$\ce{CO2 + H2O -> H2CO3}$

It's the carbonic acid formed, not the carbon dioxide itself, that turns litmus red.

Try it: identify four gases

The four jars below hold oxygen, carbon dioxide, carbon monoxide, and nitrogen in random order. Which tests can tell them apart?

Try it: run tests to collect evidence, then identify the unlabeled samples

Click a test above. The results will be recorded in this table.

Example 1: Carbon monoxide or carbon dioxide?

You have two jars of colorless gas, one carbon monoxide and one carbon dioxide. Give several ways to tell them apart.

Step-by-step solution

Limewater. Shake each with limewater; the one that turns it cloudy is carbon dioxide.
Litmus. Bubble each through purple litmus solution; the one that turns it red is carbon dioxide.
A burning splint. The gas that catches fire with a blue flame is carbon monoxide; the one that puts the splint out is carbon dioxide.
Hot copper(II) oxide. The gas that turns black copper(II) oxide red is carbon monoxide. Never try smelling: both are odorless, and carbon monoxide is poisonous.

Example 2: Why marble chips and dilute hydrochloric acid?

Step-by-step solution

Not sulfuric acid. Sulfuric acid reacts with marble to form calcium sulfate, which is only slightly soluble. It coats the marble and soon stops the reaction.
Not powdered sodium carbonate. It reacts with acid too fast to collect the gas properly.
Not concentrated hydrochloric acid. It gives off hydrogen chloride gas, which would contaminate the carbon dioxide.
Conclusion. Marble or limestone chips with dilute hydrochloric acid react at a steady rate, and both are cheap and easy to get.

Practice: carbon monoxide, carbon dioxide, or both?

Toxic: it starves the body of oxygen
Turns limewater cloudy
Burns with a blue flame
Used to put out fires
Turns purple litmus solution red
Colorless and odorless
Reduces copper(II) oxide to copper
Its solid form is used to keep things frozen
Made of the elements carbon and oxygen

Common mistakes

Mistake 1: “Diamond and graphite are different because they’re made of different elements.” Both are pure carbon. They’re different because their carbon atoms are arranged differently.

Mistake 2: “A bowl of water in the room prevents carbon monoxide poisoning.” Carbon monoxide barely dissolves in water, so that does nothing. Ventilation and a working CO alarm are what protect you.

Mistake 3: “Carbon dioxide turns litmus red.” The carbonic acid formed when carbon dioxide reacts with water does. Dry litmus paper in dry carbon dioxide doesn’t change color.

Quick check

What's the best way to test whether a jar of gas is carbon dioxide?

Summary

Diamond is the hardest natural substance; graphite is soft and conducts electricity. The difference comes from how the carbon atoms are arranged. Carbon is stable at room temperature, burns, and reduces metal oxides.
Carbon dioxide doesn’t burn or support burning, is denser than air, reacts with water to form carbonic acid, and turns limewater cloudy (the test). It’s made from marble and dilute hydrochloric acid and collected by upward air displacement.
Carbon monoxide burns, reduces metal oxides, and is toxic. It barely dissolves in water.

On to AP (for later)

In topic 3.2, diamond is a covalent network solid: every carbon atom is bonded to four others, which makes it extremely hard. Graphite has layers held together only weakly, so it’s soft and slippery. Carbon and carbon monoxide reducing metal oxides to metals are classic redox reactions (topic 4.9), and carbon dioxide forming carbonic acid in water comes back in the acid-base equilibria of Unit 8.

Carbon, CO, and CO₂碳单质、CO 与 CO₂

Learning goals

Before you start

One element, very different faces

Part 1: Forms of carbon

Part 1 check

Part 2: Carbon dioxide and carbon monoxide

Carbon dioxide

Carbon monoxide

Macroscopic: what you can see

Particulate: what the particles do

Symbolic: how chemists write it

Macroscopic: what you can see

Particulate: what the particles do

Symbolic: how chemists write it

Macroscopic: what you can see

Particulate: what the particles do

Symbolic: how chemists write it

Try it: identify four gases

Example 1: Carbon monoxide or carbon dioxide?

Example 2: Why marble chips and dilute hydrochloric acid?

Common mistakes

Quick check

Summary

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