S3 Chemistry Practice Book · Unit 1: Carbon and its inorganic compounds

Unit 1 Carbon and its inorganic compounds

Key unit competence: Relate the physical and chemical properties of carbon and its inorganic compounds to their preparation, uses and effects on the environment.

Key ideas

Allotropy is the existence of an element in two or more forms in the same physical state. Crystalline allotropes of carbon: diamond (hard, non-conductor, used in cutting tools and jewellery), graphite (soft, conducts, used in pencils, electrodes, lubricants) and fullerenes. Amorphous forms: charcoal, coke, soot (lamp black).
Chemical properties of carbon: burns in plenty of oxygen to CO₂ and in limited oxygen to CO; it is a reducing agent (2CuO + C → 2Cu + CO₂, 2Fe₂O₃ + 3C → 4Fe + 3CO₂); it reacts with steam to give water gas (C + H₂O → CO + H₂).
Carbon dioxide: prepared by CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; colourless, denser than air, slightly soluble (forms carbonic acid); does not support burning; turns limewater milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O); excess CO₂ makes it clear again (CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂). Uses: fire extinguishers, fizzy drinks, dry ice, photosynthesis.
Carbon monoxide: formed by incomplete combustion; colourless, odourless and very poisonous (binds to haemoglobin); a neutral oxide; a reducing agent in extraction of iron; burns with a blue flame to form CO₂.
Carbonates: most are insoluble except those of Na, K and NH₄⁺; most decompose on heating (CuCO₃ → CuO + CO₂) except Na and K carbonates; all react with acids to give CO₂. Hydrogencarbonates decompose on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
Hard water contains dissolved Ca²⁺ and Mg²⁺ ions and does not easily lather with soap. Temporary hardness (Ca(HCO₃)₂) is removed by boiling; permanent hardness (CaSO₄, MgSO₄) by adding washing soda (Na₂CO₃) or ion exchange. The carbon cycle balances CO₂ through photosynthesis, respiration, combustion and decay; excess CO₂ and CO cause global warming and poisoning.

Common misconceptions

"Diamond and graphite are different elements." No: They are allotropes of the same element, carbon, with different arrangements of atoms.
"Boiling removes all hardness of water." No: Boiling removes only temporary hardness caused by hydrogencarbonates.
"Carbon monoxide can be detected by its smell." No: CO is odourless and colourless, which makes it very dangerous.

Worked examples

Example 1. Describe the laboratory preparation of carbon dioxide and two tests or properties shown by the gas.

Solution

Add dilute hydrochloric acid to marble chips (calcium carbonate) in a flask: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Collect the gas by downward delivery (upward displacement of air) since it is denser than air, or over water.

Test: it turns limewater milky.

Property: it puts out a burning splint (does not support combustion).

Example 2. Explain the difference between temporary and permanent hardness of water and how each is removed.

Solution

Temporary hardness is caused by calcium/magnesium hydrogencarbonates; boiling removes it: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂.

Permanent hardness is caused by calcium/magnesium sulfates or chlorides; boiling does not remove it.

Permanent hardness is removed by washing soda (Ca²⁺ + CO₃²⁻ → CaCO₃) or by ion exchange.

Practice questions

Define allotropy and name two allotropes of carbon.
Show answer
Existence of an element in different forms in the same physical state; diamond and graphite.
Common mistake: Naming carbon dioxide as an allotrope.
Give two uses of graphite.
Show answer
Pencils, electrodes, lubricant (any two).
Common mistake: Saying jewellery.
Write the equation for carbon burning in limited oxygen.
Show answer
2C + O₂ → 2CO.
Common mistake: Writing C + O₂ → CO.
Describe the test for carbon dioxide.
Show answer
Bubble the gas through limewater: it turns milky.
Common mistake: Using a glowing splint.
State two physical properties of carbon dioxide.
Show answer
Colourless, odourless, denser than air, slightly soluble in water (any two).
Common mistake: Saying it burns.
Why is carbon monoxide called a 'silent killer'?
Show answer
It is colourless and odourless, so people do not notice it before being poisoned.
Common mistake: Saying it makes no sound when burning.
Write the equation for the action of heat on copper(II) carbonate.
Show answer
CuCO₃ → CuO + CO₂.
Common mistake: Writing copper and oxygen.
Give two disadvantages of hard water.
Show answer
Wastes soap (scum), forms scale in kettles and boilers, blocks pipes (any two).
Common mistake: Saying it causes cholera.
Explain why carbon is used in extracting iron.
Show answer
Carbon (as coke/CO) is a reducing agent that removes oxygen from iron oxide.
Common mistake: Saying carbon adds iron.
Calculate the volume of CO₂ at STP from burning 3 g of carbon completely.
Show answer
0.25 mol × 22.4 = 5.6 dm³.
Common mistake: Using 24 dm³ when STP is given.

Matching and drag-and-drop

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. Heating sodium hydrogencarbonate produces:	a. combines with haemoglobin and stops oxygen transport
2. Washing soda (Na₂CO₃) softens permanently hard water by:	b. sodium carbonate, water and carbon dioxide
3. Carbon monoxide is poisonous because it:	c. precipitating calcium ions as calcium carbonate
4. Which process removes carbon dioxide from the atmosphere?	d. Photosynthesis

Show answer

1 → b (sodium carbonate, water and carbon dioxide) · 2 → c (precipitating calcium ions as calcium carbonate) · 3 → a (combines with haemoglobin and stops oxygen transport) · 4 → d (Photosynthesis)

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
Ca²⁺ + CO₃²⁻ → CaCO₃.
CO forms carboxyhaemoglobin.
Plants use CO₂ to make glucose.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
carbonic acid · the hardest natural substance · allotropes · carbon monoxide
1. Diamond is used in cutting tools because it is .
2. Different forms of the same element in the same physical state are called .
3. Carbon burns in limited oxygen to form .
Show answer
1: the hardest natural substance · 2: allotropes · 3: carbon monoxide
- Each carbon atom is strongly bonded to four others.
- Diamond and graphite are allotropes of carbon.
- 2C + O₂ → 2CO.

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. Carbon dioxide is prepared in the laboratory from:	a. copper and carbon dioxide
2. Which is an amorphous form of carbon?	b. calcium carbonate and dilute hydrochloric acid
3. Which human activity adds most carbon dioxide to the atmosphere?	c. Charcoal
4. Carbon reduces copper(II) oxide when heated. The products are:	d. Burning fossil fuels and firewood

Show answer

1 → b (calcium carbonate and dilute hydrochloric acid) · 2 → c (Charcoal) · 3 → d (Burning fossil fuels and firewood) · 4 → a (copper and carbon dioxide)

Marble chips react with the acid to give CO₂.
Charcoal has no regular crystal structure.
Combustion releases CO₂.
2CuO + C → 2Cu + CO₂.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
soluble calcium hydrogencarbonate forms · turns limewater milky · the water evaporates · boiling
1. Temporary hardness is removed by .
2. The test for carbon dioxide is that it .
3. Excess carbon dioxide passed into limewater makes it clear again because .
Show answer
1: boiling · 2: turns limewater milky · 3: soluble calcium hydrogencarbonate forms
- Boiling decomposes Ca(HCO₃)₂ into insoluble CaCO₃.
- A white precipitate of CaCO₃ forms.
- CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂.

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