Mixing water: what is a scientific statement?

Video: What is a scientific statement?

Three minutes: Galileo Galilei tests the claim “ice floats because it is flat”. Switch on the English subtitles in the video.

The presentation

The slides from the lesson. Use the arrows at the bottom right or the arrow keys; many pictures build up step by step. The presentation works without a connection to outside servers.

The key idea: mix and predict

The lesson starts with a single question: How warm does the water get when I pour warm and cold water together? Every group writes down a number before anything is measured. That is the guess (die Vermutung), and it has to be worded so that a measured value can refute it.

In three runs the groups mix different amounts: 100 ml warm and 100 ml cold, then 200 ml warm and 100 ml cold, then (as a reserve) 100 ml warm and 200 ml cold. They measure in insulated double cups; the measured values go into the table on the worksheet.

At the end the lesson returns to the opening question. Did the guess hold up, or did a measured value refute it? A guess that has not been refuted is not proven, it has only held up (bewährt).

Six steps

1. Start: the mixing question, every group writes down its number (slides 1 and 2).
2. Video: Galileo and the question of how to test a claim (slide 3).
3. Make a guess: text puzzle and task 1 on the worksheet, plus the question whether the guess can be refuted (slides 4 and 5).
4. Plan: choose the material, decide amounts and roles (slides 6 to 8).
5. Experiment and measure: runs 1 and 2 with the table; write observation and measured value separately (slide 9).
6. Evaluate and return: was the guess refuted? The groups compare their results, then the key sentence (slides 10 and 11). Slide 12 opens the next question.

Material per group

• warm water (tap water, below 50 °C) and cold water
• measuring cup with markings, stirring rod
• insulated double cups
• multimeter with thermocouple, or a thermometer
• stopwatch, towel

The water does not have to be heated; tap water below 50 °C is enough and safe.

What the lesson shows about science

• Refutable, not confirmed: A statement is scientific if an experiment could refute it (widerlegen). It can never be finally confirmed, because you cannot test all cases.
• Held up (bewährt): If a statement is not refuted, we say: it has held up.
• Record (das Protokoll): What was measured goes into the table. Others must be able to repeat the experiment.
• Observation and interpretation kept apart (die Beobachtung, die Deutung): “I see …” stands on its own, the explanation comes afterwards.

Notes for teachers

• The worksheet is a PDF with movable stickers (text puzzle). In Goodnotes the stickers are easy to drag; the PDF also works in any PDF app and printed out, Goodnotes is optional.
• The lesson is planned as a double lesson with reserve: the core path ends after run 2, run 3 and further questions are reserve.
• No script for the slides is published; the slides carry only what the students must see, the rest is spoken.
• Slide 12 shows a QR code that leads to this page.

The worksheet

Three pages: text puzzle and guess, material and measuring table, evaluation. Here you see the pages; open the PDF to work on it.

📄 Worksheet (PDF) 📓 Goodnotes file

The same worksheet as a Goodnotes file (with movable stickers); open or import it in Goodnotes.