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.
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.