Year 8 Science · Unit 2 · Lesson 19
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Learning Goals
1. Define a compound, and give the formula of one compound from this lesson.
Match each compound use to the property that makes it work
Write the matching letter in the middle column. Each letter is used once.
| Use | Letter | Property of the compound |
|---|---|---|
| Carbon dioxide (CO₂) in a fire extinguisher | A. Dissolves many different substances. | |
| Calcium carbonate (CaCO₃) in limestone building blocks | B. Safe to eat, dissolves easily and tastes salty. | |
| Sodium chloride (NaCl) as table salt | C. Dissolves in water under pressure, then bubbles out when the pressure drops. | |
| Silicon dioxide (SiO₂) in window glass | D. Does not burn, and is denser than air so it blankets a flame. | |
| Water (H₂O) for washing | E. Transparent, and can be melted and shaped. | |
| Carbon dioxide (CO₂) in soft drinks | F. A hard, stable solid that does not dissolve in water. |
Strong or weak reasoning?
Write S (strong) or W (weak) for each reason. For a weak reason, say what is missing or wrong.
| Reason given | S or W | What is missing or wrong |
|---|---|---|
| a. “Carbon dioxide is used in fire extinguishers because it is a gas.” | ||
| b. “Water is used to put out many fires because it does not burn and it cools the fuel.” | ||
| c. “Hydrogen is never used to put out fires, because hydrogen gas burns.” | ||
| d. “Sodium chloride is safe to eat because sodium is a useful metal.” |
Scenario: the calcium label
A tablet packet says in large letters: “Contains calcium, the bone-building element!” The small print shows the tablet is calcium carbonate. Maya wonders whether anything that contains calcium would be safe to swallow.
| Substance | Added to water | Added to acid | Safe to handle? |
|---|---|---|---|
| Calcium oxide (CaO) | Reacts and gets very hot | Reacts | No: corrosive to skin and eyes |
| Calcium (Ca) | Fizzes, giving off hydrogen | Reacts vigorously | No: reactive metal |
| Calcium carbonate (CaCO₃) | Does not dissolve | Fizzes gently and neutralises the acid | Yes: used in chalk and antacids |
Source: general chemistry data, simplified.
2. Which substance in Table 1 could be used in a tablet that settles an acid stomach? Give two pieces of evidence from the table.
3. Answer Maya’s question. Use two substances from Table 1 to explain why “contains calcium” tells you nothing about whether a substance is safe.
4. A classmate says: “Sea water is full of chlorine, and chlorine is a poisonous gas, so swimming in the ocean must be dangerous.” Evaluate this reasoning. Use the properties of sodium, chlorine and sodium chloride, and one other compound as a second example.
Warm Up
1. A compound is a substance made of two or more elements chemically bonded together; it is a new substance with its own properties. Any correct formula: NaCl, H₂O, CO₂, CaCO₃, SiO₂. [2 marks]
Match each compound use to the property that makes it work
Carbon dioxide (CO₂) in a fire extinguisher → D; Calcium carbonate (CaCO₃) in limestone building blocks → F; Sodium chloride (NaCl) as table salt → B; Silicon dioxide (SiO₂) in window glass → E; Water (H₂O) for washing → A; Carbon dioxide (CO₂) in soft drinks → C.
Strong or weak reasoning?
a. W: being a gas is not enough (hydrogen is a gas and burns); it must say CO₂ does not burn and is denser than air, so it smothers the flame. b. S. c. S. d. W: the reason uses sodium’s properties, not the compound’s; salt is safe because sodium chloride is a new substance that is safe to eat.
Go Deeper
2. Calcium carbonate. Evidence (any two): it is safe to handle; it neutralises acid; it reacts gently rather than vigorously; it does not dissolve or react with water; it is already used in antacids. [3 marks: 1 substance, 2 evidence]
3. No. All three contain calcium, yet calcium carbonate is safe while calcium oxide is corrosive and calcium metal reacts with water. Safety depends on the actual substance and its own properties, not on an element it contains. [3 marks: 1 answer, 1 two contrasting substances, 1 conclusion]
Show What You Know
4. Marking guide (points expected): 1 the faulty step: assuming a compound behaves like an element inside it; 1 the chlorine in sea water is not chlorine gas but part of dissolved sodium chloride (salt); 1 chlorine gas is toxic and sodium reacts violently with water, but sodium chloride is a safe white solid that dissolves harmlessly; 1 second example, e.g. water contains hydrogen but does not burn, or calcium carbonate contains reactive calcium yet is safe in antacids; 1 conclusion: judge a substance by its own properties. [5 marks]
Common error to watch for: in the match, pairing both carbon dioxide rows with the same letter. The same compound can have two uses, each resting on a different property.