Evaporation and Crystallisation

Overview

In this lesson 6 of 18, students build on earlier work on properties of matter and separating mixtures by focusing on two related processes: evaporation (a change of state) and crystallisation (using differences in solubility and water loss to form solid crystals). Students will describe how evaporation works and how it helps form crystals, linking substance properties to separation techniques.

Learning intentions

• WALT understand evaporation and how it aids crystallisation.
• Students will explain what evaporation is in terms of water changing from liquid to gas.
• Students will describe how evaporation concentration leads to crystallisation.
• Students will identify real-life applications where evaporation and crystallisation are useful.

Success criteria

• I can describe evaporation as water changing from liquid to gas.
• I can explain how evaporation can increase the concentration of a solution.
• I can describe how crystallisation happens when a concentrated solution forms crystals.
• I can use evidence (observations or a diagram) to justify my explanation.

Curriculum links

• SC4-SOL-01: Students explain how the properties of substances enable separation in a range of techniques.
• SCLS-SOL-01: Students describe the properties of familiar substances and materials (including water and dissolving).
• SC4-WS-01: Students use scientific tools and instruments for observations (make careful observations using simple recording).
• SC4-WS-04: Students follow a planned procedure to undertake safe and valid investigations (teacher-led demonstration routine and observation method).

Lesson structure (30 minutes)

1. 0–3 min · Visual hook. Teacher shows a short teacher-made image/sequence (wet towel drying, puddle shrinking, salt crystals forming) and asks: “What do you think happens to the water when a salty solution ‘dries’?” Students write one prediction on the worksheet (multiple choice first response).

2. 3–8 min · Introduction with demonstration (modelled observation). Teacher performs a safe demonstration: place a small amount of coloured water/salty water in a clear shallow dish under a heat lamp or near a warm area (teacher manages heat and distance; no student contact with heat). Students observe changes and complete a quick multiple-choice check: “What is happening to the liquid level?” and “Where could the missing water have gone?” using provided options.

3. 8–14 min · Guided explanation: evaporation → concentration. Teacher models thinking aloud: “As water evaporates, less liquid remains, but dissolved particles stay. This makes the solution more concentrated.” Show a simple labelled diagram of particles with fewer water molecules as time passes. Students complete a 4-question multiple-choice “Explain the cause” section (e.g., “Why does the solution become more concentrated?”).

4. 14–19 min · Guided discussion: crystallisation examples. Teacher leads discussion using prompts and examples: salt on roads, sugar crystals, rock candy, seawater/evaporation ponds. Focus on the link to evaporation and crystal formation rather than naming every process detail. Students discuss in pairs then choose the correct statement on the board/worksheet: “Crystals form when the solution becomes concentrated enough.”

5. 19–26 min · YouTube clip (with purposeful pause points). Teacher plays the video “Evaporation and Crystallization” and pauses at two points: one showing evaporation and one showing crystal formation. Students answer two multiple-choice questions as the clip plays: “Which part shows evaporation?” and “Which part shows crystallisation?”

6. 26–30 min · Independent worksheet close + exit check. Students complete the remaining independent section (multiple choice, plus one short response sentence with a sentence starter) explaining evaporation effects in one context (e.g., salty water left in a dish). Teacher collects/marks quickly for understanding and notes misconceptions to revisit next lesson.

Resources

• Teacher demonstration supplies: clear shallow dish, water + food colouring, salt (optional), labelled diagram poster, paper towels
• Heat lamp or warm spot access (teacher-only use for safety)
• Student observation sheets (multiple choice)
• Coloured particle diagram handout (optional printed)
• Whiteboard/markers for key terms: evaporation, gas, concentrated, crystals
• YouTube audio/visual setup for the clip
• Timer for timed pauses and transitions
• Seating support for autistic students (predictable routine chart on board)
• Sentence starters strip (e.g., “Evaporation is when…”, “Crystals form because…”)

Assessment

• Formative during demonstration: check multiple-choice responses about where the “missing water” goes.
• Formative during discussion/diagram: correct selection of the concentration idea and crystal formation statement.
• Exit ticket (final 4 minutes): multiple-choice “evaporation effects” question + one sentence using a starter; teacher scans for SC4-SOL-01 alignment (properties enabling separation).

Differentiation

• Provide step-by-step visual routine on the board (Hook → Observe → Explain → Discuss → Video → Worksheet).
• Use multiple-choice worksheets throughout to reduce writing load while still assessing reasoning through “best explanation” options.
• Offer sentence starters and word bank (evaporation, gas, concentrate, solution, crystals, water).
• For autistic students: minimise unexpected movement, use clear times, and allow brief movement breaks if needed; keep demonstration consistent and calm.
• Extension within the lesson (for early finishers): ask them to choose an additional real-life example and match it to “evaporation” or “crystallisation” on the worksheet.

Independent worksheet (what students complete)

• Q1 (MC): Evaporation is… (liquid → gas / solid → liquid / gas → solid)
• Q2 (MC): During evaporation, the water… (leaves as gas / turns into salt / disappears instantly without change)
• Q3 (MC): Evaporation makes the solution… (more concentrated / less concentrated / always the same)
• Q4 (MC): Crystals form when… (the concentrated solution can no longer keep particles dissolved / evaporation stops completely / the dish becomes empty)
• Q5 (MC): Salt crystals on a road can be explained by… (water evaporating and leaving dissolved salt to crystallise / salt magically appearing / salt dissolving then evaporating completely)
• Q6 (MC): Best explanation of what happened to the coloured salty water: (evaporation reduced water; crystals formed)
• Final short response: “Evaporation is when ___. This helps crystallisation because ___.” (choose from starter options if needed)