Welcome to Gas Chemistry!
Gases are all around us. We breathe them in, use them to power cars, carbonate our favourite fizzy drinks, and even use them to weld metals together. In this chapter, we explore the gases that make up our atmosphere and take a deep dive into the preparation, properties, and chemical tests for three essential gases: hydrogen, oxygen, and carbon dioxide.
Don't worry if chemical equations seem tricky at first! We will break each gas down step-by-step with simple analogies, clear reactions, and easy memory tricks so you feel completely confident for your exam.
1. Composition of the Earth's Atmosphere
Our atmosphere is a mixture of different gases surrounding the Earth. Clean, dry air has a very specific composition that you need to know for your exam:
• Nitrogen (\(\text{N}_2\)): approximately \(78\%\)
• Oxygen (\(\text{O}_2\)): approximately \(21\%\)
• Argon (\(\text{Ar}\)) and other noble gases: approximately \(0.9\%\)
• Carbon dioxide (\(\text{CO}_2\)): approximately \(0.04\%\)
• Water vapour (\(\text{H}_2\text{O}\)): variable amounts depending on temperature and weather.
Memory Aid: The Air Pie
Think of the air as a pie with \(100\) slices: about \(78\) slices are nitrogen, \(21\) slices are oxygen, and only \(1\) slice is shared between argon, carbon dioxide, and other trace gases!
Did you know? Even though carbon dioxide makes up only about \(0.04\%\) of the atmosphere, it plays a massive role in regulating the Earth's temperature through the greenhouse effect.
Key Takeaway: Nitrogen is the most abundant gas (\(78\%\)), followed by oxygen (\(21\%\)), argon (\(0.9\%\)), and carbon dioxide (\(0.04\%\)).
2. Hydrogen Gas (\(\text{H}_2\))
Hydrogen is the lightest and most abundant element in the universe. On Earth, it is a diatomic gas made of two hydrogen atoms bonded together (\(\text{H}_2\)).
A. Laboratory Preparation
In the school laboratory, hydrogen gas is prepared by reacting a reactive metal (such as zinc or magnesium) with a dilute acid (such as dilute hydrochloric acid):
Word Equation:
\(\text{Zinc} + \text{hydrochloric acid} \rightarrow \text{zinc chloride} + \text{hydrogen}\)
Symbol Equation:
\(\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}\)
If magnesium is used instead:
\(\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}\)
B. Physical Properties
• Colour: Colourless
• Odour: Odourless
• Density: Much less dense than air (it is the lightest gas known)
• Solubility: Insoluble (or sparingly soluble) in water
C. Collection Method
Hydrogen is usually collected over water using a trough and an inverted measuring cylinder/gas jar, or by upward delivery (downward displacement of air) into an inverted test tube because it is lighter than air.
D. Chemical Test for Hydrogen
• Procedure: Apply a lighted (burning) wooden splint to the mouth of the test tube containing the gas.
• Positive Result: It burns with a distinctive squeaky pop sound.
• Equation for test: \(2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}\)
E. Uses of Hydrogen
• Clean fuel: When hydrogen burns, it produces only water vapour as a waste product, making it an environmentally friendly fuel for fuel-cell vehicles.
• Weather balloons: Because it is less dense than air (though helium is often preferred nowadays because helium is non-flammable).
• Hardening of oils: Used in the food industry to convert liquid vegetable oils into solid fats (margarine).
Common Mistake to Avoid: Confusing the test for hydrogen with the test for oxygen! Remember: Hydrogen pops with a burning splint; Oxygen relights a glowing splint.
Key Takeaway: Hydrogen is prepared from metal + acid, is lighter than air, and gives a squeaky pop with a lighted splint.
3. Oxygen Gas (\(\text{O}_2\))
Oxygen is essential for the respiration of living organisms and supports combustion (burning).
A. Laboratory Preparation
Oxygen is prepared by the catalytic decomposition of hydrogen peroxide (\(\text{H}_2\text{O}_2\)). Hydrogen peroxide breaks down very slowly on its own, so we add a black powder catalyst called manganese(IV) oxide (\(\text{MnO}_2\)) to speed up the reaction without being used up.
Word Equation:
\(\text{Hydrogen peroxide} \xrightarrow{\text{manganese(IV) oxide}} \text{water} + \text{oxygen}\)
Symbol Equation:
\(2\text{H}_2\text{O}_2\text{(aq)} \xrightarrow{\text{MnO}_2} 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}\)
B. Physical Properties
• Colour: Colourless
• Odour: Odourless
• Density: Slightly denser than air
• Solubility: Slightly soluble in water (this small amount of dissolved oxygen allows aquatic organisms like fish to breathe!)
C. Chemical Test for Oxygen
• Procedure: Place a glowing wooden splint (a splint that has been lit and blown out so it is just glowing red) into the gas.
• Positive Result: The splint relights.
D. Combustion of Elements in Oxygen
When substances burn in pure oxygen, they burn much more vigorously and brightly than in air.
1. Carbon (Non-metal)
• Observation: Burns with a bright orange/yellow glow, producing a colourless gas (\(\text{CO}_2\)).
• Equation: \(\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}\)
• Nature of oxide: Non-metal oxide \(\rightarrow\) acidic (turns moist blue litmus paper red).
2. Sulfur (Non-metal)
• Observation: Melts and burns with a bright blue flame, producing a pungent, choking gas (sulfur dioxide, \(\text{SO}_2\)).
• Equation: \(\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}\)
• Nature of oxide: Non-metal oxide \(\rightarrow\) acidic.
3. Magnesium (Metal)
• Observation: Burns with an intense, blinding white light, leaving behind a white ash/powder (magnesium oxide, \(\text{MgO}\)).
• Equation: \(2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}\)
• Nature of oxide: Metal oxide \(\rightarrow\) basic (alkaline when dissolved; turns red litmus blue).
4. Iron (Metal)
• Observation: Iron wool or filings glow bright orange and give off bright orange/yellow sparks, leaving a dark grey/black solid (iron oxide, \(\text{Fe}_3\text{O}_4\)).
• Equation: \(3\text{Fe(s)} + 2\text{O}_2\text{(g)} \rightarrow \text{Fe}_3\text{O}_4\text{(s)}\)
5. Copper (Metal)
• Observation: Does not burn with a flame; the shiny reddish-brown metal surface turns into a black coating of copper(II) oxide.
• Equation: \(2\text{Cu(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CuO(s)}\)
E. Uses of Oxygen
• Medical / Respiration: In oxygen masks and cylinders in hospitals to assist breathing.
• Oxy-acetylene torches: Mixed with acetylene gas to create extremely hot flames for cutting and welding steel.
Key Takeaway: Oxygen is prepared from hydrogen peroxide using \(\text{MnO}_2\) catalyst, relights a glowing splint, and reacts with elements to form basic metal oxides or acidic non-metal oxides.
4. Carbon Dioxide Gas (\(\text{CO}_2\))
Carbon dioxide is a compound consisting of one carbon atom chemically bonded to two oxygen atoms.
A. Laboratory Preparation
Carbon dioxide can be prepared by reacting a metal carbonate (usually calcium carbonate / marble chips) with dilute hydrochloric acid:
Word Equation:
\(\text{Calcium carbonate} + \text{hydrochloric acid} \rightarrow \text{calcium chloride} + \text{water} + \text{carbon dioxide}\)
Symbol Equation:
\(\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)
Alternative Preparation (Thermal Decomposition):
Heating certain metal carbonates also produces carbon dioxide. For example, green copper(II) carbonate turns into black copper(II) oxide:
\(\text{CuCO}_3\text{(s)} \xrightarrow{\text{heat}} \text{CuO(s)} + \text{CO}_2\text{(g)}\)
B. Physical Properties
• Colour: Colourless
• Odour: Odourless
• Density: Denser than air (it sinks and can be poured like a liquid!)
• Solubility: Slightly soluble in water, forming a weakly acidic solution called carbonic acid (\(\text{H}_2\text{CO}_3\)):
\(\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{H}_2\text{CO}_3\text{(aq)}\)
C. Chemical Test for Carbon Dioxide
• Procedure: Bubble the gas through limewater (calcium hydroxide solution, \(\text{Ca(OH)}_2\)).
• Positive Result: The limewater changes from colourless to milky / cloudy white.
Why does limewater turn cloudy?
The carbon dioxide reacts with calcium hydroxide to form an insoluble white precipitate of calcium carbonate:
\(\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}\)
What happens if you keep bubbling \(\text{CO}_2\) for a long time?
If excess carbon dioxide is bubbled through the cloudy mixture, it eventually becomes clear again. This is because the insoluble calcium carbonate reacts further to form soluble calcium hydrogencarbonate:
\(\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3)_2\text{(aq)}\)
D. Uses of Carbon Dioxide
• Fire extinguishers: Because it is denser than air and does not support combustion, it settles over flames, smothering the fire by depriving it of oxygen.
• Carbonated fizzy drinks: Carbon dioxide is dissolved in drinks under pressure. When the bottle is opened, the pressure drops and bubbles of \(\text{CO}_2\) escape.
Key Takeaway: Carbon dioxide is prepared by acid + carbonate, is denser than air, turns limewater milky (and clear with excess), and is used in fire extinguishers and fizzy drinks.
5. Quick Summary Table for Gas Tests
Keep these straight in your mind for quick marks in exam questions:
• Hydrogen (\(\text{H}_2\)): Lighted splint \(\rightarrow\) Burns with a squeaky pop.
• Oxygen (\(\text{O}_2\)): Glowing splint \(\rightarrow\) Relights the glowing splint.
• Carbon Dioxide (\(\text{CO}_2\)): Bubble through limewater \(\rightarrow\) Turns limewater milky/cloudy.
6. Common Exam Pitfalls & Tips
• Splint Confusion: Always write "lighted splint" for hydrogen and "glowing splint" for oxygen. Mixing these up is an easy way to lose marks.
• Observations vs Inferences: If an exam asks for an observation when carbon dioxide is added to limewater, write "the solution turns milky/cloudy", not "carbon dioxide is formed" or "calcium carbonate forms".
• Catalyst Definition: Manganese(IV) oxide is a catalyst in the preparation of oxygen — it speeds up the reaction without being consumed chemically.