Year 8 Science · Unit 2 · Lesson 17
Challenge Worksheet
Learning Goals
Find the errors
This summary has exactly four errors. Underline them and correct them below.
A student’s summary
Gold is used on the contact points inside a phone because it conducts electricity better than copper. Being reactive is always a bad property for an element to have. Freshly cut sodium is bright and silvery, but it reacts with the moisture in air, so it could never be used for jewellery. Lead would make good aircraft wings because it is so dense. A malleable metal is one that can be drawn out into a thin wire.
1. Write the four corrections.
Scenario: the wax test
A class compared how well three metals conduct heat. Each group stuck a small bead of wax to one end of a 20 cm rod, then heated the other end in a Bunsen flame and timed how long the wax took to melt. Each metal was tested three times.
| Metal rod | Trial 1 | Trial 2 | Trial 3 | Mean |
|---|---|---|---|---|
| Copper | 44 | 46 | 45 | |
| Aluminium | 52 | 80 | 55 | |
| Iron | 190 | 205 | 198 |
Source: sample class results.
2. One result in Table 1 does not fit the others. Identify it and suggest one reason it might have happened. Then fill in the Mean column, leaving the odd result out, to the nearest whole second.
Odd result: s
Check the test was fair
Use the wax test above to fill in each part.
Variables
One improvement that would make the results more reliable, and why
A company proposes
“Copper won the wax test, so from now on every saucepan we sell will be made of solid copper.” Copper melts at 1085 °C and aluminium at 660 °C. Copper has a density of 9.0 g/cm³; aluminium 2.7 g/cm³. Copper costs more than aluminium per kilogram, and its surface slowly darkens in damp air.
3. Evaluate the company’s proposal. Name the properties a saucepan needs, use Table 1 and the facts above to compare copper with aluminium on each one, and explain why winning a test of one property does not settle the choice. End with a recommendation.
Find the errors
1. (i) Gold is a worse conductor than copper; it is used on contacts because it does not corrode. (ii) Being reactive can be useful: iodine kills bacteria and the phosphorus on a matchbox strip catches fire from one strike. (iii) Lead would make poor wings: it is dense and weak, so the wing would be heavy and bend. (iv) Drawn into wire is ductile; malleable means it can be hammered into sheets without breaking. (The sodium sentence is correct.) [4 marks: 1 per correction]
Your Turn
2. Odd result: aluminium trial 2, 80 s. Reason (any one): a larger wax bead, the rod further from the hottest part of the flame, or the timer started late. Means: copper (44 + 46 + 45) ÷ 3 = 45 s; aluminium (52 + 55) ÷ 2 = 53.5, so 54 s (accept 53 or 54); iron (190 + 205 + 198) ÷ 3 = 197.7, so 198 s. [4 marks: 1 odd result, 1 reason, 2 means (1 if two correct)]
Check the test was fair
Independent: the metal the rod is made of. Dependent: time for the wax to melt. Controlled (any three): rod length (20 cm); rod thickness; size of the wax bead; flame setting and position of the rod in the flame; starting temperature of the rods. Improvement, for example: identical wax beads and every rod clamped in the same part of the flame, or more repeats, so differences come from the metal, not the method.
Show What You Know
3. Marking guide (points expected): 1 heat conduction: copper is best (45 s) but aluminium is close (about 54 s); 1 melting point: both far above cooking temperatures (1085 and 660 °C); 1 density: copper is over three times as dense (9.0 against 2.7 g/cm³), so the pan is much heavier; 1 cost and surface: copper costs more and slowly darkens, so it needs more care; 1 one test does not settle it: the job needs several properties, and the winner is good enough on all of them; 1 reasoned recommendation, for example aluminium for everyday pans. [6 marks]
Common error to watch for: in question 2, including the 80 s result in the aluminium mean, or dividing by 3 after leaving it out.