Sand and water: which one warms up faster?

Video: Sand and water at the beach

A little over a minute: hot sand at noon, cool water, the other way round at night, and the question why. Turn 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: same energy, different substances

The lesson starts with the question from the beach: Why is the sand hot and the water cool? Everyone draws their hypothesis (die Vermutung) as two curves on the worksheet: which sample, 50 g of sand or 50 g of water, warms up faster under the same lamp?

Every minute the groups first measure the temperature of the sand, then that of the water, until the sand shows 40 °C. The sand rises fast, the water only a little. The students explain it qualitatively: both samples get about the same energy, but water needs more of it for the same warming. The substance decides, not only the energy.

At the end the lesson returns to the beach. The term “specific heat capacity” is only named; calculating starts in the next lesson. The question from the video stays open: why do seas store so much energy?

Six steps

1. Start: beach question, video, hypothesis as two curves on the worksheet (slides 1 to 3).
2. Task and safety: assign roles, set-up, rules (slides 4 and 5).
3. Measuring: every minute alternately sand and water, until the sand shows 40 °C; write observation (die Beobachtung) and interpretation (die Deutung) separately (slide 6).
4. Evaluating: draw both curves, compare the groups (slides 7 and 8).
5. Explaining: same energy, different substances; sort the sentences, two sentences are wrong (slides 9 and 10).
6. Back to the beach: answer to the opening question (slide 11). Slide 12 is spare (cooling down).

Material per group (three people)

• a lamp, e.g. an incandescent bulb of about 50 W in a socket, with a template for 6 cm distance on the left and right
• two identical plastic cups: 50 g of dry sand (e.g. bird sand) and 50 g of water
• multimeter with type K probe, towel, clock or timer, safety goggles

Safety: the lamp glass gets hot and is not touched, do not point the lamp at paper or cables, tie back hair, switch the lamp off when the sand shows 40 °C, do not throw sand into the air, wash hands afterwards. The teacher switches the lamps off before clearing up.

What the lesson shows

• A fair comparison: same mass, same lamp, same distance, same starting temperature.
• Sand warms up faster than water. With the same energy, water needs more of it for the same warming; the substance decides.
• Not the whole truth: that the sea is warmer than the beach at night is not only due to the heat capacity; mixing, how deep the radiation penetrates and evaporation also play a role. The experiment separates only one part of it.
• Testing a statement: “Sand always warms up faster than water.” What would refute it? (For example wet sand.)

Notes for teachers

• The worksheet is available as a PDF with movable stickers (text puzzle, two of the eight sentences are wrong) and as a Goodnotes file; Goodnotes is optional.
• The times have not been tested: how fast the sand reaches 40 °C depends on the lamp and the distance. The water rises by only a few kelvin and the multimeter shows whole degrees. Try it first with one lamp and fix the distance (6 cm or more).
• The probe is wiped on the towel between the samples, always sand first, then water.
• Spare: cooling down, after both samples have been warmed to about 30 °C in a water bath.
• No script for the slides is published; the slides carry only what the students must see, the rest is spoken.

The worksheet

Two pages: diagram, roles and safety, measuring table, observation and interpretation, sorting sentences. Here you see the pages; open the PDF or the Goodnotes file to work.

📄 Worksheet (PDF) 📓 Goodnotes file

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