Water of crystallisation
- CBSE · NCERT Class 10, Ch 2 Acids, Bases and Salts
Water of crystallisation is water locked into a salt's crystals in a set amount, such as five water molecules for each unit of copper sulphate.
Some salt crystals look dry but hold water inside their structure. This water is part of the formula. It is written after a dot, as in CuSO₄·5H₂O.
Heat such a crystal and the water leaves. The salt can change colour or form. Add water back, and the first form can return.
A small example
Blue copper sulphate crystals are CuSO₄·5H₂O: 5 water molecules for each unit of salt. Heat them in a dry tube. Drops of water collect on the cool part of the tube, and the salt turns white. Add 2 or 3 drops of water, and the blue colour comes back.
Gypsum is CaSO₄·2H₂O. Heated to 373 K, it keeps only half a water molecule per unit and becomes Plaster of Paris, CaSO₄·½H₂O.
Where marks go
Question 20 of CBSE's 2026-27 Science sample paper asks which of four salts hold water of crystallisation. The 2026-27 Science marking scheme answers with the formulas. Plaster of Paris, washing soda (Na₂CO₃·10H₂O) and gypsum do. Baking soda, NaHCO₃, does not.
So knowing the formula settles the question. Baking soda and washing soda sound alike, but only one has water in its formula. It is 1 mark, with no part credit. See where the marks go in CBSE Science.
Sources
Facts last checked against these sources on 30 September 2026.
About the author
Founder Member & Academic Lead
An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.
Deepti Sardana on LinkedIn
