Year 7 Science · Unit 2 · Lesson 20
Apply Worksheet
Learning Goals
Compare two (and three!)
Complete the table to compare an element, a compound and a mixture. Use your lesson notes to fill in all cells.
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Made of… | |||
| Example | |||
| Can be separated physically? | |||
| Fixed or variable composition? | |||
| Pure substance or not? |
Predict + reason
Scenario
A scientist analyses an unknown white solid labelled "Substance X." The tests show:
• It melts at exactly 801 °C every time it is tested.
• When dissolved in water, the solution conducts electricity.
• It cannot be separated into simpler parts by filtering or evaporation.
• Its formula is NaCl.
(a) Predict whether Substance X is an element, a compound, or a mixture. Write your prediction clearly.
(b) Justify your prediction using at least three pieces of evidence from the scenario. For each piece of evidence, name the Unit 2 concept it relates to.
(c) Sodium-23 has an atomic number of 11. Using atomic number and mass number, calculate how many protons and neutrons are in one sodium atom. Show your working.
1. Explain why dissolving salt in water is a physical change, but reacting sodium with water is a chemical change. Use the particle model in your answer.
Wrap Up
In one sentence, what was the main idea of this lesson?