Drying and desiccation are methods for removing water from gases, liquids, and solids. In chemistry, even small amounts of water can change a reaction rate, shift an equilibrium, or ruin a moisture sensitive product. A laboratory desiccator creates a low humidity space around a sample so it can dry without being exposed to open air.
Drying agents make this possible by capturing water molecules through absorption, adsorption, or chemical hydration.
Understanding Chemistry: Drying and Desiccation
Water leaves a material when molecules at its surface have enough energy to enter the gas phase. Some water molecules return from the air to the surface at the same time. Drying is fastest when more molecules leave than return.
Air movement, warmth, exposed surface area, and low water content in the surrounding air all help. A crushed solid usually dries faster than a large crystal because it has more surface exposed.
Drying slows near the end because the remaining water may be trapped in pores or held tightly to ions, proteins, or other polar parts of the sample. This is why a sample can feel dry yet still contain measurable water.
Choosing a drying method requires more than choosing the strongest water absorber. The drying material must not dissolve in, react with, or contaminate the sample. A drying agent for a gas may be unsuitable for a liquid solvent.
Some agents remove water by becoming part of a hydrate. Others hold water on internal surfaces or inside pores. Molecular sieves are useful when a solvent needs to be very dry, since their pores can admit small water molecules while excluding larger molecules.
Heat can speed drying, but it may decompose a chemical, drive off a valuable solvent, or change the crystal form of a solid. Vacuum drying lowers pressure and can work at lower temperatures, though volatile samples may evaporate too quickly.
Students meet moisture control outside the laboratory in many ordinary products. Silica gel packets protect electronics, medicines, shoes, and food during storage. Powdered drink mixes and washing powders clump when they absorb water from humid air.
Dry air systems matter in compressed air tools because water can cause corrosion or freeze in cold valves. In chemical manufacturing, wet reactants can reduce product yield or create unwanted side products. Water is especially important in reactions involving reactive metals, acid chlorides, or certain catalysts.
Clean glassware can still carry a thin film of water after washing. For moisture sensitive work, glassware is often heated, cooled away from humid air, then used promptly.
Careful measurement shows whether drying has actually finished. A common approach is to weigh a sample, dry it, cool it in a dry container, then weigh it again. Repeating this cycle until the mass stays nearly unchanged gives a more reliable result than judging by appearance.
Cooling matters because a hot object creates air currents that disturb a balance reading. It can also absorb moisture quickly if left on the bench. When measuring water captured by a drying agent, the mass increase is useful only if the agent has absorbed water alone.
Leaks, spilled material, or absorption of solvent vapor can give a misleading result. Students should record drying time, temperature, sample mass, and the condition of the drying agent.
Key Facts
- Relative humidity = actual water vapor pressure / saturation vapor pressure x 100%
- Anhydrous salts remove water by forming hydrates, such as CuSO4 + 5H2O = CuSO4·5H2O
- A desiccator lowers the partial pressure of water vapor around a sample, which increases evaporation from the sample.
- Calcium chloride is a strong drying agent for many gases and nonreactive liquids, but it can react with alcohols and amines.
- Molecular sieves trap water in tiny pores and are useful for drying solvents because their pore size can be selected.
- Mass of water removed = final mass of hydrated desiccant - initial mass of dry desiccant, if only water is absorbed.
Vocabulary
- Desiccant
- A desiccant is a substance that removes water from its surroundings by absorbing, adsorbing, or chemically binding it.
- Desiccator
- A desiccator is a sealed container that holds samples above a drying agent to maintain a low humidity environment.
- Anhydrous
- Anhydrous means containing no water, especially no water of crystallization in a salt or reagent.
- Hydrate
- A hydrate is a compound that includes water molecules as part of its crystal structure.
- Adsorption
- Adsorption is the sticking of molecules to the surface of a solid rather than entering its bulk.
Common Mistakes to Avoid
- Using any drying agent with any solvent, which is wrong because some desiccants react with certain functional groups or dissolve in the liquid.
- Leaving the desiccator lid ungreased or unsealed, which is wrong because humid room air can leak in and reduce the drying effect.
- Assuming a sample is dry because it looks dry, which is wrong because solids and crystals can hold adsorbed water or water of hydration that is not visible.
- Adding too little drying agent to a wet solvent, which is wrong because the desiccant can become saturated before all the water is removed.
Practice Questions
- 1 A drying tube contains 12.50 g of anhydrous calcium chloride before use and 13.04 g after drying a gas stream. Assuming the mass gain is only water, how many grams of water were removed?
- 2 An anhydrous salt sample has a mass of 5.00 g. After sitting in a humid desiccator, its mass becomes 5.72 g. Calculate the percent increase in mass due to water uptake.
- 3 A student needs to dry an alcohol solvent before a reaction. Explain why choosing calcium chloride without checking compatibility may be a poor choice, and suggest what property a better drying agent should have.