Hydrated and anhydrous substances
Distinguish chemically combined water from dampness and connect hydration with the required colour examples.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Describe the copper sulfate and cobalt chloride reversible colour changes.
- Distinguish a chemical change from a physical state change.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which colour pair belongs to the copper sulfate example?
What you will learn
- Define hydrated and anhydrous substances using chemically combined water.
- Distinguish a hydrate from a wet sample or an ordinary aqueous solution.
Water can be part of the substance
A hydrated substance is chemically combined with water. An anhydrous substance contains no water in its composition. This is different from water merely adhering to the outside of a crystal or serving as the solvent around dissolved ions. A dried-looking crystal can still be hydrated, and a visibly damp sample is not proven to be a hydrate from dampness alone.
In the required examples, blue hydrated copper(II) sulfate can lose combined water under appropriate supervised heating to become white anhydrous copper(II) sulfate. Hydrated cobalt(II) chloride is pink and its anhydrous form blue in the stated school model. Adding water can restore hydration. These colour pairs belong to different salts; colour alone without substance identity cannot establish one unique hydrate.
Drying and dehydrating ask different questions
Removing adhering solution when drying recovered crystals need not remove their chemically combined water. Stronger or unsuitable heating can change the desired hydrated product, so drying and purification methods must preserve the intended substance. Anhydrous does not mean the substance contains no hydrogen or oxygen atoms anywhere in its formula; it means water is not present as part of its composition.
When asked to define the terms, state chemically combined water rather than only describing appearance. Extended work counts water molecules in hydrate formulas such as CuSO₄·5H₂O. In this Core lesson, the key distinction is compositional: surface moisture, solvent and combined water have different roles and cannot be treated as one category.
Pause and explain
Which definition correctly identifies a hydrated substance?
Worked example
A blue copper sulfate crystal is blotted dry, yet remains the hydrated form. Why is removing surface liquid not proof that it became anhydrous?
Show the worked solution
- Blotting removes adhering solution or moisture from the outside.
- The hydrate's chemically combined water is a separate part of its composition.
- A dry surface can therefore coexist with a hydrated crystal; anhydrous requires no water in the substance's composition.
Answer Surface drying does not itself remove chemically combined water; the crystal can remain hydrated.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define hydrated and anhydrous and relate each to the copper sulfate colour example.
Give me a hint
State composition before describing colour.
Compare my reasoning
- Hydrated means chemically combined with water; the copper sulfate example is blue.
- Anhydrous means no water in the substance's composition; the example is white.
- The appropriate hydration/dehydration change alters combined water, not merely surface moisture.
Hydrated: combined water, blue CuSO₄ example. Anhydrous: no water, white CuSO₄ example.
Look for these in your work
- Both compositional definitions are present.
- Colours are assigned to the correct forms.
Explain why damp anhydrous material and a surface-dry hydrated crystal cannot be classified solely by visible wetness.
Give me a hint
Distinguish an observation from the composition it establishes.
Compare my reasoning
- Liquid on a surface is external moisture, not by itself evidence for a hydrate composition.
- A surface-dry hydrate can retain chemically combined water.
- Actual contact with water may cause hydration for a suitable substance, but appearance alone does not establish the final composition.
Visible wetness alone does not distinguish combined water from surface moisture; composition must be established.
Look for these in your work
- The two water roles remain separate.
- Possible hydration is not ruled out merely from the starting label.
A plan makes hydrated salt crystals, then heats them strongly to ensure they are anhydrous and calls the result the same purified hydrated product. Assess the claim.
Give me a hint
The intended composition and the drying treatment must agree.
Compare my reasoning
- The intended product contains chemically combined water.
- Removing that water changes it to an anhydrous form, if the stated dehydration occurs.
- Use suitable drying conditions that preserve the intended hydrate rather than claiming a changed composition is the identical hydrated product.
Dehydrating changes the intended hydrate; suitable drying should remove adhering liquid while preserving its required form.
Look for these in your work
- The intended product is kept explicit.
- Purification is not confused with changing composition.
Common mistakes
- Defining hydrated as simply wet to the touch.
- Defining anhydrous as lacking every hydrogen or oxygen atom.
- Assuming drying recovered crystals must make them anhydrous.
- Swapping copper sulfate's and cobalt chloride's colour pairs.
What can you explain now?
Can a crystal with a dry surface still be hydrated? Explain.
Compare with the explanation
Yes; it can retain chemically combined water.
Surface moisture is different from water forming part of the hydrate's composition.
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Make it stick
Define both terms without using wet or dry as the whole definition. Explain how a crystal can be surface-dry yet hydrated.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.