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Test for water

The essential about test for water

How to identify water in a substance ?

This question worth asking because a liquid doesn't necessarily contain water, and a solid substance may contains water wich is not observable with the naked eye.
A Test for water must be used identify water.
Different tests exist but the most commonly used requires anhydrous copper sulphate.

Anhydrous copper sulphate

The test for water requires a chemical compound: Copper sulfate anhydrous.
This is a white powder.
Comment: The term anhydrous is formed of "an" (privative prefix) and "hydrous" meaning "water": "anhydrous" is therefore almost synonymous with dehydrated.
                                      Photo of anhydrous copper sulphate
Photo: anhydrous copper sulphate      

Copper sulphate formula

The complete name is copper II sulphate which indicates that it contains copper II ions ( whose formula is Cu2+ ) and Sulphate ions ( whose formula is SO42-).
Copper sulphate formula is therefore CuSO4.

Hydrated copper sulphate

Diagram: water and anhydrous sulfate
Addition of water on anhydrous copper sulfat

White anydrous copper sulphate that has been in contact with water becomes hydrated copper sulphate and turns blue.
A copper sulfate hydrate still contains some CuSO4 cristals but also five water molecules ( H2O )
Copper sulfate hydrate can therefore also be called copper sulphate pentahydrate ( the penta prefix means "five" ).
This change of color is exploited to test for water.
                               Blue hydrated copper sulphate
 Photo: blue hydrated copper sulphate and white anhydrous copper  sulphate

Chemical test for water with anhydrous copper sulphate

Diagram: test for water in a liquid
test for water in liquid

Diagram: test for water in a solid
test for water in solid substances

In each case if the copper sulphate turns blue it can be concluded that the substance contains water.

Some results obtained with the test for water

Substance Apple juice Lemonade Cola Washing up liquid
Result Positive Positive Positive Negative
Is there water in the substance ? Yes Yes Yes No

Substance Oil Vinegar Petroleum Milk
Result Negative Positive Negative Positive
Is there water in the substance ? No Yes No Yes

Substance Apple Potato Tomato Paper
Result Positive Positive Positive Negative
Is there water in the substance ? Yes Yes Yes No

Most of the foods (liquid or solid) contain water but in different percentages:
Meat: about 60%
A potato: about 80%
An apple: 85%
A Potato: 93%
A salad: about 95%
A tomato: 98%

Note: fats (such as oil or butter) are exceptions because they do not contain water

Learn more about test for water

- Investigating the action of heat on copper sulphate: method to obtain anhydrous copper sulphate with hydrated copper sulphate.

- Hydrating anhydrous copper sulphate: video showing drops of water cast upon anhydrous copper sulphate.

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Water on Earth
Changes of state in the nature: the water cycle
Water in human body
Test for water
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