Chemistry

Free online chemistry lesson for elementary school, middle school and high school.

 

Water

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: 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 SO₄^2-).
Copper sulphate formula is therefore CuSO4.

Hydrated copper sulphate

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.

Photo: blue hydrated copper sulphate and white anhydrous copper  sulphate

Chemical test for water with anhydrous copper sulphate



 

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

Conclusion:
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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