Year 8 Science · Unit 2 · Lesson 14
Challenge Worksheet
Learning Goals
Find the errors
This answer contains exactly four errors. Underline each one and write the correction below.
A student’s test answer
All metals are dense, so a piece of any metal will sink in water. Solid non-metals are usually dull and brittle, and most of them are poor conductors. Non-metals are always gases or liquids at room temperature. Metalloids are mixtures made by melting a metal together with a non-metal. Silicon is a metalloid, so it is made of metal atoms joined to non-metal atoms.
1. Write the four corrections.
| Set of elements | Metals | Metalloids | Non-metals | Total |
|---|---|---|---|---|
| First 20 elements (hydrogen to calcium) | 7 | 2 | 11 | 20 |
| All 118 known elements | 95 | 6 | 17 | 118 |
Source: standard classification; metalloids counted are B, Si, Ge, As, Sb and Te.
2. What percentage of the first 20 elements are metals? What percentage of all 118 elements? Give whole numbers.
First 20: % All 118: %
3. The two answers are very different. Think about which rows the first 20 elements come from, and explain why they give a misleading picture of the whole table.
Scenario: does heat help silicon conduct?
The lesson says silicon conducts electricity only weakly, but better when it is heated. A student has a small silicon wafer, a battery, wires, an ammeter (which measures current) and a hot plate.
Hypothesis (If the silicon is heated, then …)
Variables
Method and one safety step
4. Mercury is a metal that is liquid at room temperature, sodium is a metal that floats on water, and graphite is a non-metal that conducts electricity. A student claims: “These exceptions prove the metal and non-metal categories are useless and should be abandoned.” Evaluate this claim. Use evidence from this sheet and the lesson, and finish with a judgement.
Find the errors
1. (i) Not all metals are dense: group 1 metals such as sodium float on water. (ii) Non-metals are not always gases or liquids: carbon, sulfur, phosphorus and iodine are solids. (iii) Metalloids are elements, not mixtures: they have some metal-like and some non-metal-like properties. (iv) Silicon is an element, so it contains only silicon atoms. (The second sentence is correct.) [4 marks: 1 per correction]
Your Turn
2. First 20: 7 ÷ 20 × 100 = 35%. All 118: 95 ÷ 118 × 100 = 80.5, so about 81% (matches the lesson’s “about 80%”). [2 marks]
3. Non-metals sit on the right, and in the short top rows (periods 1 to 3) they fill most boxes. Metals fill the left and the middle, but the middle block starts only in period 4, after calcium, so it is missing from the first 20. The long lower rows are mostly metals. [3 marks: 1 non-metals on the right fill the short top rows, 1 middle block starts after the first 20, 1 lower rows mostly metals]
Go Deeper: investigation plan
Hypothesis: if the temperature of the silicon increases, the current through it will increase, because silicon conducts better when heated. IV: temperature of the silicon. DV: current (ammeter reading). Controlled (any two): same wafer, battery, wires, probe positions. Method: connect wafer and ammeter in the circuit; record the current at room temperature; heat the wafer to several set temperatures and record the current at each; repeat readings. Safety: move the hot wafer only with tongs; do not touch the hot plate. [6 marks: 1 hypothesis, 2 variables (IV and DV, 1 two controls), 2 method, 1 safety]
Show What You Know
4. Marking guide (points expected): 1 mercury conducts and is shiny, so it fits the metals on other properties; 1 sodium conducts, is shiny when cut and is in group 1, so it is a metal; 1 graphite is brittle and carbon is on the non-metal side, so it stays a non-metal; 1 the categories are still useful: position gives a first clue to properties and uses (conductors for wiring); 1 metalloids show the boundary is already accepted as not sharp; 1 judgement: the claim is wrong, because categories describe broad patterns judged from several properties. [6 marks]
Common error to watch for: treating a metalloid as a mixture or compound of a metal and a non-metal. Metalloids are elements, each made of one kind of atom.