Glauber’s Salt — Thermal Energy Storage

Sodium sulfate decahydrate as a phase change material: retrograde solubility, supersaturation, and latent heat
Intermediate🕐90 min●Low hazardthermochemistrycrystallization

Summary

Glauber’s salt melts and freezes at 32.4 °C, storing and releasing heat as a phase-change material1. You’ll see its unusual “retrograde” solubility (it gets less soluble above 32.4 °C), watch a supersaturated2 solution crystallize on cue, and measure the temperature plateau that reveals stored latent heat.

History

Johann Rudolf Glauber3 (1604–1670) was a German-Dutch chemist who isolated the decahydrate of sodium sulfate from mineral spring water and named it sal mirabile — the wonderful salt. He sold it as a panacea and laxative to wealthy clients across Europe, becoming moderately famous in his lifetime. Scientifically, he was among the first to systematically study the preparation and properties of mineral salts, bridging alchemy and early modern chemistry.

The unusual solubility of Glauber’s salt — rising sharply up to 32.4°C then falling as temperature continues to rise — puzzled chemists for two centuries. The explanation lies in the phase transition at 32.4°C: the decahydrate melts congruently at this temperature, dissolving in its own water of crystallisation. Above this point, the stable solid phase is the anhydrous salt (thenardite), which is far less soluble. The apparent decrease in solubility is therefore a change in which form of the salt is in equilibrium with the solution.

Modern interest in Glauber’s salt focuses on thermal energy storage. Its melting point of 32.4°C and high latent heat (~254 kJ/kg) make it suitable for solar thermal systems and passive building temperature regulation — storing heat during the day and releasing it at night when the building cools below 32°C.

Hazards & preparation

Warning

PPE: safety glasses; handle warm glassware with care.

  • The water baths here are gentle (~40 °C) — no scald risk, but don’t overheat.
  • Cleanliness matters for supersaturation (not a safety issue): a stray particle nucleates crystals early.

Disposal: sodium sulfate is low-hazard (a mild laxative) — flush dilute solutions down the drain, or keep the salt to reuse indefinitely. See the Safety page.

Materials

  • Sodium sulfate — 70 g anhydrous (or ~180 g Glauber’s salt decahydrate)
  • Distilled water — 100 mL
  • Small saucepan or heatproof beaker
  • Thermometer
  • Very clean glass jar with a tight lid (250 mL mason jar works well)
  • Seed crystal of sodium sulfate (save a few before starting)

Procedure

Part 1 — Making and Observing the Retrograde Solubility

  1. Dissolve 30 g anhydrous sodium sulfate in 100 mL water at room temperature (~20°C), stirring well. Some solid will remain — a saturated solution with excess.
  2. Warm the slurry gently while stirring. Note the temperature at which all solid just dissolves (~26°C) — near the solubility maximum.
  3. Continue warming past 35°C — solid reappears as the less-soluble anhydrous phase becomes stable (solubility decreases with heat).
  4. Cool to room temperature and watch crystals reform — the characteristic large flat plates of the decahydrate.

Part 2 — Supersaturation and Triggered Crystallisation

  1. Dissolve 70 g anhydrous sodium sulfate in 100 mL water at 40°C, stirring until fully clear.
  2. Filter through a clean paper towel or coffee filter directly into the very clean, pre-warmed jar — removing any dust that would nucleate crystals early.
  3. Cap the jar and cool slowly, undisturbed, to room temperature.
  4. The solution should stay liquid below 32.4°C — it is now supersaturated.
  5. Touch a seed crystal to the surface through a briefly opened lid. Crystallisation spreads from the point, and the solution warms noticeably as latent heat releases.

If premature crystallisation occurs while cooling, reheat gently to redissolve and try again.

Part 3 — Thermal Energy Storage Demo

  1. Fill a small sealed container with Glauber’s salt decahydrate.
  2. Melt it fully in a 40°C water bath (it becomes a clear liquid).
  3. Remove to a room-temperature spot.
  4. Record the temperature every 2–3 minutes with a thermometer taped to the outside.
  5. Plot temperature vs. time. A flat plateau near 32°C shows energy releasing as latent heat at constant temperature — the signature of a phase change material.

Variation — 18°C Eutectic with Table Salt

Adding sodium chloride to Glauber’s salt depresses the melting point, and at the eutectic composition the mixture freezes and melts at approximately 18°C — close to comfortable room temperature.

  1. Dissolve about 3 parts Glauber’s salt with 1 part table salt (by weight) in the minimum warm water needed for a homogeneous melt.
  2. Seal in a small container and cool.
  3. The mixture transitions between solid and liquid near 18°C, absorbing heat as it melts and releasing it as it freezes.

Because the phase change occurs near room temperature, this mixture is useful for passive cooling — packs that stay cold by absorbing heat at 18°C rather than requiring ice temperatures. Like pure Glauber’s salt, it tends to supercool, so seed with a small crystal to trigger solidification.

What you should see

In Part 1 the counter-intuitive moment is watching solid reappear as you heat past ~32 °C. In Part 2 the clear solution holds below its freezing point until a seed touches it, then crystallizes in a spreading, self-warming front of flat plates. In Part 3 the cooling curve stalls on a plateau near 32 °C — the salt paying back its stored heat.

Symptom Likely cause Fix
Crystallizes while cooling (Part 2) Dust or undissolved grains Filter into a spotless jar; reheat and retry
No temperature plateau (Part 3) Too little material, or already crystallized Use more salt; make sure it was fully melted first
Won’t supercool at all Seed particles present Rinse glass with distilled water; filter the hot solution

The Science

Congruent Melting and Retrograde Solubility

At exactly 32.4°C, sodium sulfate decahydrate (Na₂SO₄·10H₂O) melts into a solution that has exactly the composition needed to re-form the decahydrate on cooling — this is congruent melting. Above 32.4°C the equilibrium solid is anhydrous sodium sulfate, which has a much lower solubility. The apparent reversal in solubility reflects this shift in equilibrium phase, not a decrease in the intrinsic solubility of the salt.

\[\ce{Na2SO4 \cdot 10H2O(s) <=> Na2SO4(aq) + 10H2O(l)} \quad T_\text{m} = 32.4°C\]

Supersaturation

Like sodium acetate in the “hot ice” experiment, sodium sulfate solution can be cooled below 32.4°C without crystallising if the solution is clean and undisturbed. The supersaturated state is metastable — the solution “wants” to crystallise but lacks a nucleation site. A seed crystal provides the template needed to start the cascade.

Latent Heat

Crystallisation releases the latent heat of fusion — energy stored in the disordered liquid arrangement that is released as ions settle into the ordered crystal lattice and as water molecules bind into the hydrate structure. For Glauber’s salt this is ~254 kJ/kg, comparable to many commercially used phase change materials. The temperature plateau in Part 3 is direct evidence of this stored energy: the system maintains 32°C against the temperature gradient of the surroundings for an extended period.

Tips for Success

  • Cleanliness is critical for supersaturation — rinse the jar with distilled water; any particle nucleates crystals prematurely.
  • If using pharmacy Glauber’s salt, check it’s pure sodium sulfate decahydrate (some formulations contain additives).
  • Supersaturation is more reliable if you heat to ~45°C and let it cool inside an insulated box — slow, even cooling minimises disturbance.
  • For the storage demo, an inexpensive digital thermometer with a wire probe tracks the plateau best.
  • Glauber’s salt can be reused indefinitely — just remelt and repeat.

Questions to Explore

  1. Why does solubility decrease above 32.4°C? Most salts get more soluble when heated; Glauber’s salt gets less. Is the salt itself becoming less soluble, or is something else changing?

    Hint / answer

    Above 32.4 °C the solid in equilibrium switches from the very soluble decahydrate to the much less soluble anhydrous form. The intrinsic salt isn’t becoming less soluble — you’re now measuring a different solid phase, and that one dissolves less.

  2. What is “congruent melting” and why does it matter? At 32.4°C the decahydrate melts into a solution of exactly the right composition to re-form it. What if the melt composition were different?

    Hint / answer

    Congruent melting means solid and melt have the same composition, so cooling cleanly re-forms the decahydrate — perfect for reversible heat storage. If the composition were off (incongruent), part would separate out as a different phase and the material would gradually “fatigue,” storing less each cycle.

  3. Why does the supersaturated solution stay liquid? Below 32.4°C it “should” crystallize but doesn’t. What prevents it, and why does one seed break the stalemate?

    Hint / answer

    The same nucleation barrier as in hot ice: forming the first crystal cluster costs surface energy that a still, clean solution can’t muster. A seed supplies a ready surface, letting the dissolved ions pile on immediately and crystallize in a cascade.

  4. What makes Glauber’s salt useful for storing solar heat? Its 32.4°C phase change stores ~254 kJ/kg. What advantage does a phase-change material have over just using a large mass of water or rock?

    Hint / answer

    A phase change stores a lot of heat in a small mass at a nearly constant temperature (the melt/freeze point), whereas water or rock must swing through a big temperature range to hold the same energy. So a PCM buffers a building near a set temperature compactly.

  5. Why does adding table salt lower the melting point to 18°C? The NaCl/Na₂SO₄ eutectic melts at 18°C. What is a eutectic, and why does mixing two salts melt lower than either alone?

    Hint / answer

    A eutectic is the mixture composition with the lowest possible melting point for that pair. Each salt disrupts the other’s ability to crystallize cleanly, so the blend stays liquid to a lower temperature than either pure salt — the same reason salt melts ice on roads.

Going further

  • Plot the plateau. Log temperature vs. time carefully in Part 3 and estimate how long the salt holds ~32 °C — a direct measure of its usefulness for storage.
  • Tune the temperature. Try the table-salt eutectic to shift the transition to ~18 °C for a room-temperature cold pack.
  • Next in the Crystals & Solutions track: grow large single crystals and build a microphone in Rochelle Salt.

Footnotes

  1. Phase-change material — A substance that stores and releases heat as it melts and freezes at a fixed temperature.↩︎

  2. Supersaturation — A solution holding more dissolved substance than it normally could — an unstable state that crystallizes when disturbed.↩︎

  3. Johann Rudolf Glauber — German-Dutch chemist (1604–1670) who first made sodium sulfate (‘Glauber’s salt’) and potassium permanganate.↩︎