Year 7 Science · Unit 2 · Lesson 15
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Learning Goals
Scenario
A chemistry student is reading the formula Al₂O₃, which is the main compound in aluminium ore (bauxite) mined extensively at Weipa, Queensland. She knows that oxygen always forms the O²⁻ ion (charge of minus two) in this type of compound.
(a) Predict: Using the subscripts in Al₂O₃, how many aluminium atoms and how many oxygen atoms are in one formula unit? Show your reading step by step.
(b) There are 3 oxide ions (O²⁻), each with a charge of −2. The total negative charge is therefore 3 × (−2) = −6. If the formula is electrically neutral overall, what must the charge on each Al ion be? Show your reasoning.
(c) Is Al₂O₃ a binary compound? Justify your answer using the definition from the lesson.
Real-world context
Calcium hypochlorite, Ca(OCl)₂, is the active chemical in pool-sanitising granules widely used in Australian backyard pools and in public pools such as those managed by local councils in Sydney and Melbourne. It kills bacteria and algae by releasing chlorine into the water. The compound is made by reacting calcium hydroxide with chlorine gas — a purely chemical process, not a simple mixing of elements.
(a) Read the formula Ca(OCl)₂. Identify each element present and count how many atoms of each element appear in one formula unit. (Remember: the subscript 2 outside the bracket multiplies everything inside.)
(b) Explain why Ca(OCl)₂ is a compound and not a mixture. Use the lesson definition of "compound" in your answer.
1. How many atoms are in one formula unit of NaHCO₃ (sodium bicarbonate — bicarb soda)? List each element and its count, then give the total.
2. A student says chlorine gas (Cl₂) and sodium chloride (NaCl) both contain chlorine, so they must be the same substance. Explain why this reasoning is wrong, using the terms element, compound and diatomic.
Wrap Up
In one sentence, what was the main idea of this lesson?