What it does
Many salts form crystals with water built into them, called water of crystallisation. Blue copper sulfate is really CuSO₄·5H₂O: five water molecules for every CuSO₄. Heat the crystals and the water is driven off, leaving a white powder. Weigh before and after, and you can work out how many water molecules there were. This page runs that experiment:
- pick one of six hydrated salts and the mass of crystals,
- Heat 1 minute and weigh: the crystals change colour as they lose water, and the balance shows the new mass,
- keep going until two weighings in a row agree (constant mass), or press Heat to constant mass,
- the working shows the mass of water lost, the moles of water and of salt, and x = moles of water ÷ moles of salt,
- Add water back shows the reverse: white copper sulfate turns blue again and warms up,
- type your own three weighings from a real practical to get x.
It goes with the Class 10 chapter on acids, bases and salts (water of crystallisation, copper sulfate, washing soda, gypsum and plaster of Paris) and the Class 11 mole concept.
The mole steps are the same as in the mole bridge; formulas of salts come from the formula writer, and how acidic or basic salt solutions are is shown on the pH scale explorer.
How to use it
- Choose the Crystals and type the Mass of crystals.
- Press Heat 1 minute and weigh several times. Each weighing goes into the table with the change from the one before.
- When two weighings agree, the page says constant mass reached and gives x as a whole number.
- Under Work out x from your own results, type the empty crucible, crucible with crystals, and crucible after heating to constant mass.
A worked example
The page starts with 2.495 g of copper sulfate crystals in a 20.000 g crucible, so the first weighing is 22.495 g. After heating to constant mass the crucible weighs about 21.595 g.
- Water lost = 22.495 − 21.595 = 0.900 g, which is 0.900 ÷ 18 = 0.0500 mol.
- CuSO₄ left = 21.595 − 20.000 = 1.595 g, which is 1.595 ÷ 159.5 = 0.0100 mol.
- x = 0.0500 ÷ 0.0100 = 5, so the crystals are CuSO₄·5H₂O.
Stop after only two minutes of heating and x comes out near 3, too small, because water is still in the crystals. That is why you heat to constant mass.
Tips
- Let the crucible cool before weighing (in a real lab, in a desiccator), or it will pick up water from the air again.
- Heat gently: too strong and some salts break down further (iron(II) sulfate gives off choking gases), which makes x too big.
- Washing soda (Na₂CO₃·10H₂O) is about 63 % water by mass.
Check yourself
Four fresh questions each time: the molar mass of a hydrate, its percentage of water, grams of water in a sample, and x from results. Nothing is saved.
FAQ
Do real crystals lose water smoothly like this? No: copper sulfate loses its water in steps at different temperatures. The page uses a smooth made-up curve so the weighings level off; the final answer is the same.
Is this a safe experiment to do? Only in a school lab with a teacher, eye protection and gentle heating. Some of these salts are harmful if swallowed.
Is this from the NCERT book? It covers the same topic with fibub’s own numbers. fibub is not part of NCERT.
Where the numbers come from
- Formulas of the common hydrates (CuSO₄·5H₂O, FeSO₄·7H₂O, Na₂CO₃·10H₂O, MgSO₄·7H₂O, CaSO₄·2H₂O, CoCl₂·6H₂O) and the rounded atomic masses used in school books are general chemistry. The heating curve is a simple made-up model so that the mass levels off; real crystals lose water in steps. Written for fibub.
- Checked against Python: 2.495 g of CuSO₄·5H₂O (249.5 g/mol) holds 0.900 g of water and leaves 1.595 g of CuSO₄, giving x = 5; and for every salt, heating to constant mass gives back its x.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Read more on the Blog
Last updated 2026-11-09.