Efflorescence and deliquescence

Why water moves at all. Every hydrated crystal has water molecules locked into its structure, and that locked-in water exerts its own small vapour pressure — a tendency to escape as vapour, exactly like water in an open dish does. The surrounding air also has a vapour pressure of water in it, which we feel as humidity. Whichever side has the higher vapour pressure "wins": water moves from high vapour pressure to low, just as heat flows from hot to cold. This one idea explains all three phenomena below — only the direction of the imbalance differs.

Efflorescence happens when a hydrated salt's own water vapour pressure is higher than the air's, so the crystal continuously loses water of crystallisation to the atmosphere. The classic example is washing soda, $Na_2CO_3\cdot10H_2O$, which effloresces on standing in ordinary room air, crumbling from clear crystals into a dull white powder of a less-hydrated form:

where: the decahydrate on the left loses nine of its ten water molecules to the air, leaving the monohydrate as a crumbly solid on the right — only one water molecule of crystallisation is retained, not zero.

Deliquescence happens when a solid's own vapour pressure is lower than the air's, so the solid keeps absorbing atmospheric moisture — and absorbs so much of it that it dissolves entirely in the very water it has taken in, ending up as a liquid solution of itself. Ferric chloride, $FeCl_3$, is the standard example: left exposed to moist air, it draws in water vapour until nothing solid remains, only a puddle of $FeCl_3$ solution.

Hygroscopy is the broader case of a solid absorbing atmospheric moisture without necessarily going all the way to a self-liquefied solution. A hygroscopic substance becomes damp, or reacts chemically with the water it absorbs, but it need not turn into a liquid pool of itself. Quicklime, $CaO$, is hygroscopic and reacts with the moisture it absorbs to form calcium hydroxide:

Concentrated sulphuric acid is also hygroscopic — it absorbs atmospheric water and simply becomes a progressively more dilute acid, again without ever forming a solid-to-liquid pool the way a deliquescent solid does.

Holding the three apart. Efflorescence is water leaving a solid. Deliquescence and hygroscopy are both water entering a solid, but deliquescence always finishes as a liquid solution of the original substance, while hygroscopy is the wider umbrella term for moisture uptake that may stop at "damp solid" or "reacted solid" without ever becoming a puddle. Every deliquescent substance is therefore also hygroscopic, but not every hygroscopic substance is deliquescent.

Efflorescence, deliquescence, and hygroscopy are all descriptive, definition-based observations for this syllabus, not phenomena that need a derived equation — but the mass change when washing soda effloresces can be calculated exactly, since we know precisely which and how many water molecules are lost. The goal of the calculation below is to find what fraction of a sample of washing soda crystals is lost as vapour when the decahydrate effloresces completely down to the monohydrate, using nothing beyond molar masses and the balanced equation already given above.

Step 1. Find the molar mass of the decahydrate, $Na_2CO_3\cdot10H_2O$. Sodium carbonate itself, $Na_2CO_3$, has molar mass $2(23) + 12 + 3(16) = 46 + 12 + 48 = 106\ g/mol$. Ten water molecules add $10 \times 18 = 180\ g/mol$. So $M(Na_2CO_3\cdot10H_2O) = 106 + 180 = 286\ g/mol$.

Step 2. Find the molar mass of the monohydrate left behind, $Na_2CO_3\cdot H_2O$. This is $106 + 18 = 124\ g/mol$ — the sodium carbonate part is unchanged, but only one water molecule of crystallisation now remains attached.

Step 3. Find the mass of water lost per mole of decahydrate. Nine water molecules escape to the air, so the mass lost is $9 \times 18 = 162\ g$ per mole. As a check, $286 - 162 = 124\ g$, matching the monohydrate's molar mass from Step 2 exactly — mass is conserved.

Step 4. Express the loss as a percentage of the original mass:

where: $162\ g$ is the mass of water vapour released and $286\ g$ is the starting mass of one mole of washing soda crystals. So a little over half the original mass of washing soda crystals is lost as water vapour by the time efflorescence is complete — which is why a jar of washing soda left open for weeks ends up noticeably lighter and visibly shrunken into powder.

Question: A sample of washing soda crystals, $Na_2CO_3\cdot10H_2O$, weighs $28.6\ g$ when freshly prepared. It is left open on a shelf and effloresces completely to the monohydrate, $Na_2CO_3\cdot H_2O$. Calculate (a) the number of moles of decahydrate present initially, (b) the mass of water lost to the air, and (c) the final mass of the powder remaining.

Step 1. Moles of decahydrate initially: using $M(Na_2CO_3\cdot10H_2O) = 286\ g/mol$ from the derivation above, $n = 28.6\ g \div 286\ g/mol = 0.1\ mol$.

Step 2. Mass of water lost: each mole of decahydrate loses $162\ g$ of water on full efflorescence (Step 3 above), so $0.1\ mol \times 162\ g/mol = 16.2\ g$ of water vapour escapes to the air.

Step 3. Final mass of the powder: $28.6\ g - 16.2\ g = 12.4\ g$. As a check, $0.1\ mol \times 124\ g/mol$ (the monohydrate's molar mass) also gives $12.4\ g$, confirming the answer.

Answer: starting from $28.6\ g$ ($0.1\ mol$) of washing soda decahydrate, $16.2\ g$ of water is lost to the air, leaving $12.4\ g$ of monohydrate powder behind — a loss of $56.6\%$ of the original mass.

• Confusing efflorescence with deliquescence — efflorescence is a hydrated solid losing water to the air (crystal to powder); deliquescence is a solid gaining water from the air until it becomes liquid. They are opposite directions of water movement.
• Treating hygroscopy and deliquescence as identical — every deliquescent substance is hygroscopic (it absorbs moisture), but not every hygroscopic substance is deliquescent (it does not always end up as a liquid solution of itself).
• Assuming efflorescence always removes all the water of crystallisation — often only part of the water is lost, leaving a lower (but still nonzero) hydrate, as with washing soda losing nine of its ten water molecules, not all ten.
• Forgetting these are physical processes tied to atmospheric water vapour specifically, not general dissolving of a solid when directly immersed in liquid water.

Ask the tutor: Want to try a quick-fire round identifying efflorescence, deliquescence, or hygroscopy from new example substances, or work through another water-of-crystallisation mass calculation?