Weighing Water in Air — Gravimetric Humidity
Summary
You’ll literally weigh the water floating in a jar of air by trapping it in calcium chloride desiccant1 and reading the mass gained — after first proving the method on a known mass of water. You’ll be able to explain why a strong desiccant makes the measurement quantitative, why you calibrate before you trust it, and the difference between absolute and relative humidity.
This is a quantitative experiment. The point is not a color change but a measurement — and a measurement you can trust only if you calibrate it first. A scale that reads to 0.01 g (a cheap jewelry/kitchen “pocket” scale) is the key piece of equipment.
History
Long before electronic humidity sensors, chemists measured the water content of air by gravimetric absorption — pulling a known volume of air through a tube packed with a drying agent and weighing how much heavier the tube became. The mass gained is the water that was in the air. Justus von Liebig2’s combustion train (1830s) used exactly this principle: calcium chloride tubes captured the water produced when an organic compound was burned, and potassium hydroxide bulbs captured the carbon dioxide, letting him deduce the formula of the compound from two weighings.
The same chemistry shows up in everyday life: the little tubs of “moisture absorber” sold for damp closets and boats are calcium chloride. They visibly pull water out of the air and collect it as a brine in the bottom of the tub.
This experiment turns that absorption into a number. Because calcium chloride is such an aggressive desiccant, it captures essentially all the water vapor it is exposed to — which is what makes the method quantitative rather than approximate.
Hazards & preparation
PPE: safety glasses and gloves. This is a low-hazard experiment.
- Calcium chloride releases heat as it absorbs water and forms a caustic-ish brine — handle the pellets with gloves and don’t get the brine in your eyes.
- Use oven mitts for the hot drying tray, and let the desiccant cool in a sealed container.
Disposal: calcium chloride is a benign de-icing salt — dissolve and flush down the drain with water, or re-dry and reuse it. See the Safety page.
Materials
- Calcium chloride — anhydrous pellets or flakes, ~50 g (sold as desiccant or “damp-rid” moisture absorber)
- Scale reading to 0.01 g (0.1 g will work but is much less precise)
- An airtight container of known volume — a wide-mouth jar or a rigid food container, 2–10 L (larger is better; measure its volume by filling with water and pouring into a measuring jug)
- A small open dish or weighing boat that fits inside the container
- A shallow tray or foil boat to hold the desiccant inside the container
- Oven or hot plate (to dry the calcium chloride)
- A separate thermometer and, if you have one, a reference hygrometer (electronic humidity meter) for comparison
- A graduated syringe or dropper able to dispense ~0.5 mL of water
- Gloves and eye protection
Procedure
Part 1 — Prepare a dry, known starting mass (20 min + cooling)
- Spread the calcium chloride on a tray and dry it in an oven at 150–200 °C for 1–2 hours to drive off any water it has already absorbed. (Commercial anhydrous desiccant beads can be used straight from a fresh sealed pack and just need a short warm-up.)
- Let it cool in a closed container or a zip bag — if you let it cool in open air it immediately starts absorbing moisture and your “dry” mass will be wrong.
- Tip a portion into the foil boat and weigh the boat + desiccant. Record this as \(m_0\).
Part 2 — Calibrate with a known mass of water (overnight)
This is the step that makes the whole experiment trustworthy. Do not skip it. You are proving that the desiccant captures water quantitatively and that your scale reads true.
- Weigh the small dish empty, then add roughly 0.5 mL of water and weigh again. The difference is the exact mass of water added, \(m_\text{water}\) (about 0.50 g). Record it.
- Place both the open dish of water and the freshly weighed desiccant boat (\(m_0\)) into the airtight container and seal it. Do not let them touch.
- Leave it sealed for 12–24 hours at room temperature. The calcium chloride lowers the humidity inside the sealed container to nearly zero, so water continuously evaporates from the dish and is captured by the desiccant until the dish is dry.
- Re-weigh the desiccant boat. Call the gain \(\Delta m_\text{cal} = m_\text{cal,final} - m_0\).
- Compute recovery:
\[\text{Recovery} = \frac{\Delta m_\text{cal}}{m_\text{water}} \times 100\%\]
A good run gives 95–102 %. If the dish came out completely dry and recovery is near 100 %, your method is validated. If recovery is low, the container is leaking, the desiccant wasn’t fully dry, or it didn’t sit long enough.
Part 3 — Measure the humidity of room air (overnight)
- Dry and re-weigh a fresh desiccant boat (\(m_0\), as in Part 1).
- Note the room temperature \(T\) and, if you have one, the reading on your reference hygrometer.
- Open the airtight container to flush it with room air (fan it a few times), then quickly place the desiccant boat inside and seal it. The air now trapped is a sample of known volume \(V\) at known temperature.
- Leave sealed 12–24 hours, then re-weigh the boat. The gain is the mass of water vapor that was in that air: \(\Delta m = m_\text{final} - m_0\).
Part 4 — Calculate the humidity
Absolute humidity (mass of water per cubic metre of air):
\[\text{AH} = \frac{\Delta m}{V}\]
with \(\Delta m\) in grams and \(V\) in cubic metres (1 litre = 0.001 m³).
Relative humidity — divide by the maximum the air could hold at that temperature (the saturation vapour density, from the table below) and multiply by 100:
\[\text{RH} = \frac{\text{AH}}{\rho_\text{sat}(T)} \times 100\%\]
| Temperature | Saturation vapour density \(\rho_\text{sat}\) |
|---|---|
| 10 °C | 9.4 g/m³ |
| 15 °C | 12.8 g/m³ |
| 20 °C | 17.3 g/m³ |
| 25 °C | 23.0 g/m³ |
| 30 °C | 30.4 g/m³ |
Worked example: a 5.0 L container (\(V = 0.005\) m³) at 22 °C. The desiccant gains \(\Delta m = 0.058\) g.
\[\text{AH} = \frac{0.058}{0.005} = 11.6 \text{ g/m}^3\]
Interpolating the table to 22 °C gives \(\rho_\text{sat} \approx 19.4\) g/m³, so
\[\text{RH} = \frac{11.6}{19.4} \times 100\% \approx 60\%\]
Compare this to your reference hygrometer. Agreement within a few percent is an excellent result for a kitchen-scale measurement.
What you should see
Nothing dramatic to the eye — that’s the point. The desiccant that went in dry comes out a fraction of a gram heavier, and at high humidity the pellets clump and glisten with brine they’ve pulled from the air. The drama is in the numbers: your calibration recovers ~100 % of a known water mass, and your room-air measurement lands within a few percent of a hygrometer.
| Symptom | Likely cause | Fix |
|---|---|---|
| Mass gain too tiny to trust | Container too small | Use a bigger (5–10 L) jar so there’s more air to sample |
| Desiccant gains weight as you weigh it | Absorbing room moisture during handling | Work fast; keep it covered between jar and scale |
| Recovery above 100 % | Picked up extra moisture, or wasn’t fully dry at \(m_0\) | Re-dry and cool sealed before weighing |
| Recovery well below 100 % | Container leaking, or not left long enough | Check the seal/gasket; give it another night |
The reactions
Calcium chloride binds water in a stepwise series of hydrates, releasing heat at each step:
\[\ce{CaCl2 (s) + 6 H2O (g) -> CaCl2.6H2O (s)}\]
Beyond the hexahydrate it deliquesces — it dissolves in the very water it has pulled from the air, forming a concentrated brine:
\[\ce{CaCl2.6H2O (s) + H2O (g) -> CaCl2 (aq)}\]
the desiccant grabs water vapour so aggressively it drives the air nearly dry — so the weight it gains equals the water that was in the air.
The Science
Why calcium chloride works for this
A desiccant is only useful for a quantitative measurement if it drives the humidity in the sealed container down to nearly zero — otherwise it stops absorbing while some water is still left in the air, and you under-measure. Calcium chloride is deliquescent: its affinity for water is so strong that it keeps absorbing until the air above it is extremely dry (equilibrium relative humidity below ~5–10 % over the solid hydrates). That near-zero endpoint is what lets you assume all the water has been captured, so the mass gain equals the water that was present.
Why calibration comes first
Any measuring method can be wrong in ways you won’t notice unless you test it against a known. In Part 2 you put in a water mass you measured directly, and check that the same mass comes back out as weight gained by the desiccant. This single check tests several things at once: that the scale is linear and accurate, that the container doesn’t leak, that the desiccant captures water completely, and that you waited long enough for equilibrium. Only after that check passes do the humidity numbers in Part 3 mean anything. This is the everyday logic of analytical chemistry — verify the method on a known sample before trusting it on an unknown.
Absolute vs. relative humidity
The desiccant measures absolute humidity directly — actual grams of water per cubic metre, no temperature assumptions needed. Relative humidity is that amount expressed as a fraction of the maximum the air could hold at the current temperature, which is why warm air at 50 % RH holds far more water than cold air at 50 % RH. The conversion between them needs the saturation table, and the saturation density roughly doubles for every 10 °C rise — the reason a cold window fogs up when warm room air touches it.
Questions to Explore
Why must the desiccant drive humidity nearly to zero to be quantitative? What if a weaker desiccant stopped at 30 % RH — over- or under-estimate?
Hint / answer
If it stops absorbing while air is still at 30 % RH, it leaves some water uncaptured, so the mass gain is short of the true amount — you under-estimate. Calcium chloride pulls the air to near 0 % RH, so essentially all the water is caught and the number is right.
What does calibration actually prove? List everything that must be right for recovery to hit 100 %.
Hint / answer
That the scale is accurate and linear, the container is airtight (no leaks), the desiccant captures water completely, and you waited long enough for equilibrium — all at once. Several of these (a slow leak, incomplete capture) you’d never notice without the known-mass check.
Why does saturation vapour density roughly double every 10 °C? Is the air really “holding” the water?
Hint / answer
It’s not the air holding water — it’s how readily water molecules escape the liquid. Warmer molecules escape faster, so the vapour pressure (and thus the water air can carry before condensing) rises steeply with temperature. Cool that vapour on a cold window and it condenses as dew.
Absolute vs. relative — which does your method measure, and which does a forecast quote? Why no temperature needed for one?
Hint / answer
Your desiccant weighs the actual water present, so it gives absolute humidity with no temperature assumption. Weather reports quote relative humidity (a percentage of the max at that temperature), which is why converting your result to RH requires knowing the temperature.
Where does the captured water go, and why does the boat get warm? Relate it to the oven regeneration.
Hint / answer
Water binds into the solid hydrates (and then brine), and forming those bonds releases energy as heat — which is why the desiccant warms. Regenerating it in the oven puts that same energy back in to break the bonds and drive the water off — a reversible cycle.
Going further
- Track a day. Measure morning vs. afternoon, or a bathroom before and after a shower, and watch the absolute humidity change.
- Build a Liebig train. Pack calcium chloride in a tube and pull a measured air volume through it (siphon air by draining a jug of water) — the classic flow-through method.
- Compare desiccants. Repeat the calibration with silica gel or table salt — which recover the full water mass, and which leave the dish damp?
Related experiments and chemicals:
- Calcium Chloride
- Cobalt Humidity Indicator — the qualitative counterpart: see moisture instead of weighing it
- Dissolution Thermochemistry — calcium chloride’s exothermic dissolving
- Simple Distillation — another quantitative separation of water from a mixture
Footnotes
Desiccant — A hygroscopic material used to keep its surroundings dry by absorbing water vapour.↩︎
Justus von Liebig — German chemist (1803–1873), a founder of organic chemistry, who devised a practical silvering process.↩︎