Year 8 Science · Unit 2 · Lesson 1
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s summary
All matter is made of tiny particles. In a solid, the particles are packed tightly and do not move at all. An element is a pure substance made of only one type of atom. Carbon dioxide is an element, because it is a pure substance. If you split a gold atom, the pieces are just smaller pieces of gold.
1. Write the three corrections.
Thought experiment: cutting gold
Imagine a strip of pure gold 1 mm wide. Each cut halves its width. Scientists measure tiny widths in nanometres (nm): 1 mm = 1 000 000 nm. One gold atom is about 0.3 nm across.
| Number of cuts | Width of strip (nm) |
|---|---|
| 0 | 1 000 000 |
| 10 | 977 |
| 20 | 0.95 |
| 21 | |
| 22 |
Source: calculated values, rounded. The width of a gold atom is approximate.
2. Complete Table 1 for 21 and 22 cuts. Give each width to two decimal places.
Answer nm
3. After which cut is the strip first narrower than one gold atom? Explain what that cut would really have to do, and whether the pieces would still be gold.
4. Gold leaf used to decorate picture frames is about 100 nm thick. About how many gold atoms thick is it? Show your working.
Answer atoms
Evaluate the claim
Someone claims
“Graphite in a pencil is soft, black and slippery. Diamond is hard, clear and sparkly. Things that look and behave that differently cannot possibly be the same element.”
5. State what decides which element a substance is, and name the element in both graphite and diamond.
6. Explain how graphite and diamond can have such different properties, and state what the example shows about how the arrangement of atoms affects a substance.
7. Passing electricity through water breaks it down into hydrogen gas and oxygen gas, always in the same ratio: two hydrogen atoms for every oxygen atom. A classmate says: “That proves water was always just a mixture of hydrogen and oxygen.” Evaluate this argument.
Find the errors
1. (i) Particles in a solid vibrate about fixed positions; they are never completely still. (ii) Carbon dioxide is a compound: carbon and oxygen atoms are chemically bonded. (iii) Splitting a gold atom gives protons, neutrons and electrons, which are not gold; the atom is the smallest unit that is still gold. [3 marks: 1 per correction]
Your Turn
2. 21 cuts: 0.95 ÷ 2 = 0.48 nm (0.475 is also accepted). 22 cuts: 0.48 ÷ 2 = 0.24 nm. [2 marks]
3. After the 22nd cut (0.24 nm is less than 0.3 nm; after 21 cuts the strip is still about 0.48 nm wide). That cut would have to split gold atoms apart. The pieces would be protons, neutrons and electrons, which do not have gold’s properties, so they would no longer be gold. [3 marks: 1 cut number; 1 splits atoms; 1 no longer gold]
4. 100 ÷ 0.3 ≈ 333, so about 330 atoms thick (accept 300 to 350). [2 marks: 1 working, 1 answer]
Evaluate the claim
5. The element is decided by the type of atom it contains (the number of protons in each atom), not by how it looks. Both are carbon. [2 marks]
6. The carbon atoms are arranged differently: in graphite they are in layers that slide over each other (soft, slippery); in diamond they are locked in a rigid 3D lattice (very hard). The same atoms in a different arrangement give different properties, so appearance alone cannot tell you which element something is. [3 marks: 1 graphite layers; 1 diamond rigid lattice; 1 conclusion]
Show What You Know
7. Marking guide (points expected): 1 water is a compound, not a mixture; 1 water’s properties are different from both elements (water is a liquid at room temperature and puts out fires; hydrogen is a flammable gas; oxygen is a gas that helps things burn); 1 in a mixture each substance keeps its own properties, so a hydrogen and oxygen mixture would still behave like those gases; 1 the fixed 2 : 1 ratio every time shows the atoms are bonded in fixed proportions, which is true of compounds, while a mixture can have any proportions; 1 water had to be broken down chemically using electricity, whereas a mixture can be separated by physical means; 1 the flaw: being able to get elements out of a substance does not mean they were only mixed in it; every compound can be broken down into its elements. [6 marks]
Common error to watch for: in Q3, saying the strip “gets too small to cut”. The point is not the tool; a piece smaller than one atom would no longer be gold at all.