Measuring Boiling Points

Overview

Students investigate how adding salt changes the boiling point of water. They build on prior learning about temperature, particle models and fair testing, then use thermometers to record observations and explain how solution concentration affects an observable property.

Learning intentions

• WALT measure temperature accurately in degrees Celsius.
• WALT compare the boiling point of freshwater and a salt solution.
• WALT identify variables in a fair test.
• WALT explain how concentration can affect the boiling point of a solution.

Success criteria

• I can read a thermometer and record a temperature with the unit °C.
• I can identify the independent, dependent and controlled variables.
• I can use results to compare freshwater and salt solution.
• I can explain that increasing the amount of dissolved salt can raise the boiling point of water.

Curriculum links

• Physical Science — Solutions and concentration: observable properties of a solution can change with the solvent and the concentration of solute.
• Physical Science — Matter, heat and states of matter: freshwater boils at 100°C at sea level.
• Physical Science — Heat: measuring temperature with a thermometer and describing temperature in degrees Celsius.
• Science capabilities: participating and contributing, investigating in science, and communicating in science.

Lesson structure (45 minutes)

1. 0–5 min · Hook and prediction. Display the question “Does salty water boil at the same temperature as freshwater?” using the opening prediction slide; students make an individual prediction, then briefly justify it to a partner.

2. 5–12 min · Teach key ideas and safety. Use the concept and safety slides to review solvent, solute, solution, concentration, boiling point and °C; demonstrate safe thermometer use and explain that only the teacher handles hot equipment. Students identify the fair-test variables on the boiling-point investigation sheet.

3. 12–17 min · Set up the investigation. Prepare two labelled heatproof containers: freshwater and a measured salt solution, and place thermometers so they do not touch the container bases; students predict which sample will have the higher boiling point and record the independent, dependent and controlled variables on the boiling-point investigation sheet.

4. 17–30 min · Teacher demonstration and data collection. Heat both samples using separate hot plates, pausing at regular intervals for students to read and record temperatures in the boiling-point investigation sheet; students watch for sustained bubbling, record the temperature when each sample boils, and note observations such as bubbling and steam. Refer to the investigation instructions and results slide to keep the recording routine visible.

5. 30–37 min · Interpret results. Display the class results table on the comparison and discussion slide and invite pairs to compare readings, identify patterns and consider sources of error; students complete the questions on the boiling-point investigation sheet, including whether the evidence supports their prediction.

6. 37–45 min · Explain and assess. Use the final explanation and reflection slide to co-construct the conclusion: at sea level, freshwater boils at about 100°C, while dissolved salt can raise the boiling point; students complete the retrieval practice exit ticket slips with one recorded result, one explanation and a confidence rating.

Resources

• Teacher demonstration bench with two hot plates or a suitable heating source
• Two heatproof containers, lids or splash guards, stirring rods and labels
• Freshwater and measured salt solution, prepared before the lesson
• Two thermometers or temperature probes
• Safety goggles, heatproof gloves and heat-resistant mat
• the boiling-point investigation slide deck
• the boiling-point investigation sheet
• the retrieval practice exit ticket slips
• Timer, board and pens

Assessment

• Check predictions and questioning during the hook for prior understanding of boiling point and concentration.
• Circulate during setup and data collection, checking that students read thermometers at eye level, include °C and distinguish variables correctly.
• Use the exit ticket to assess whether students can state a result and link concentration with boiling point. Review misconceptions before the next lesson.

Differentiation

• Provide a word bank on the boiling-point investigation sheet with solvent, solute, solution, concentration, boiling point, independent variable and dependent variable.
• Support learners who need it with a partially completed results table, paired reading of thermometer scales, sentence starters such as “The salt solution boiled at…” and a labelled diagram of the setup on the investigation instructions and results slide.
• Use mixed-ability pairs and allow students to record observations orally before writing; explicitly model technical vocabulary and gestures for “dissolve”, “heat” and “boil” to support English language learners.
• Challenge students to explain why the investigation must keep the volume of water, heating equipment and thermometer position the same, and to suggest how repeated trials would improve confidence in the conclusion.
• Do not allow students to handle boiling liquids or hot equipment. Students who cannot safely view the demonstration from the front should be seated where the thermometer readings and display are clearly visible.