Beginner Topic 723 of 1102 Inorganic Chemistry
Hydrates
Crystalline salts that contain a definite number of water molecules, such as CuSO4 5H2O.
In-depth explanation of Hydrates
Hydrates are crystalline salts that contain water molecules in a fixed ratio as water of crystallization, written with a dot as in CuSO4 x 5H2O. Heating a hydrate drives off the water, often with a colour change, so copper(II) sulfate pentahydrate is blue and anhydrous copper(II) sulfate is white. The water content is found from the mass lost on heating, which gives the formula. Some salts are deliquescent and absorb enough water from air to dissolve, while others are efflorescent and lose water. Gypsum is a hydrate of calcium sulfate.
Going deeper: Hydrates
Key ideas: Salts with fixed water of crystallization, written CuSO4.5H2O, and named copper(II) sulfate pentahydrate. Find x by heating to constant mass: x = (moles of water lost) divided by (moles of anhydrous salt). Example: 2.50 g CuSO4.5H2O loses 0.90 g, so 5 H2O per salt (molar mass 249.7 g per mol). Efflorescent salts (Na2CO3.10H2O) lose water in air, deliquescent ones (CaCl2, NaOH) absorb it until they dissolve. Anhydrous CoCl2 blue turns pink when wet.
Common mistakes and connections: Students include water mass wrongly in molar mass or forget that the dot is not multiplication. Heating may decompose the salt, so avoid strong heat for those. Percent water = 18x divided by molar mass of hydrate. Anhydrous CuSO4 white serves as a water test. Links to stoichiometry, plaster of Paris, alums and desiccants.
Visualization of Hydrates
Molar mass of copper sulfate hydrates as water is lost on heating, falling from 249.7 for the blue pentahydrate to 159.6 for the white anhydrous salt. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Heating 2.50 g of CuSO4 x 5H2O to drive off the water leaves about 1.60 g of anhydrous CuSO4.
Frequently asked questions about Hydrates
What is Hydrates?
Crystalline salts that contain a definite number of water molecules, such as CuSO4 5H2O.
Is Hydrates a beginner, intermediate or advanced topic?
Hydrates is a beginner topic in Inorganic Chemistry. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Hydrates?
Study these earlier topics in Inorganic Chemistry first: Inorganic Nomenclature, Polyatomic Ions, Chemical Formula Types.
What comes after Hydrates?
The next topics in Inorganic Chemistry are Types of Chemical Reactions and Activity Series of Metals.
Can I practise Hydrates with quizzes?
Yes. The interactive lesson includes a self-check question on Hydrates, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Hydrates fits in Inorganic Chemistry
Hydrates is topic 108 of 113 in Inorganic Chemistry.