Science and Technology K–6 · Year 6

Dissolving and getting it back: separating sand and salt

Science understanding: Chemical sciences

PracticalLow risk

The idea

Dissolving is a reversible change: salt dissolved in water passes through a filter that stops sand and is recovered unchanged when the water evaporates, with its mass back on the balance.

What you need

10.0 g clean dry washed sand
10.0 g table salt, plus about 25 g more for the saturation test
tap water: 50 mL for dissolving, 20 mL for rinsing and 50 mL for the saturation test
a kitchen balance reading to 0.1 g
a funnel and a coffee filter paper
2 cups, a spoon and a stirring stick
a shallow dark dish about 15 cm across
a half-teaspoon measure
a sunny windowsill

How to do it

Saturation test: weigh one level half-teaspoon of salt. Add level half-teaspoons one at a time to 50 mL of room-temperature water, stirring for one minute each time, until some salt stays undissolved. Record the number of spoonfuls and the mass dissolved.
Mix 10.0 g of sand with 10.0 g of salt and describe the mixture.
Add 50 mL of water and stir for 2 minutes.
Pour the mixture through the filter paper in the funnel. Rinse the sand in the filter twice with 10 mL of water, collecting all the liquid.
Dry the sand and filter paper on a tray overnight and weigh the sand.
Pour all the collected liquid into the dark dish and leave it on the sunny windowsill until dry. Scrape up the crystals and weigh them.
Work out the percentage of the salt recovered and explain where any missing salt went.

What you should see

Saturation: 50 mL (50 g) of water at 20 degrees Celsius dissolves at most 18.0 g of salt, because the solubility of sodium chloride is 36.0 g per 100 g of water; with half-teaspoons of about 3 g that is about six, and the seventh stays as crystals on the bottom. Hot water would dissolve hardly any more (39 g per 100 g at 100 degrees Celsius), unlike sugar. Separation: the dried sand comes back at close to 10.0 g, and the 70 mL of liquid, a layer only 4 mm deep in a 15 cm dish, dries in the sun over one to a few days, leaving white cube-shaped crystals. Nine to ten grams of salt are recovered; the missing fraction of a gram stayed in the filter paper and on the sand. Getting the same salt back shows dissolving is reversible: the salt was in the water all along, not destroyed.

What changes

What you change: the separation step used (filtering, evaporating)
What you measure: mass of sand and of salt recovered
What you keep the same: starting masses
water volumes
same balance

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Salt disappears when it dissolves.
Dissolving is the same as melting.
Hot water always dissolves a lot more of anything.

Safety card

Low riskLearners carry it out

Hazards

salt stings eyes and cuts
fine sand dust

Controls

washed sand only
no tasting
wash hands afterwards

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-6MW-S ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01 ST3-DAT-01
Australian Curriculum v9AC9S6U04AC9S6I02AC9S6I03AC9S6I05

Sources

The pages the author read to write this activity.

All Concept Studio activities