Concentrated sulphuric acid as a dehydrating agent
A DEHYDRATING AGENT is a substance that removes
water from another substance. Sulphuric acid is also
a powerful drying agent.
time, the volume of very concentrated sulphuric acid
exposed to air will increase as it absorbs water.
To demonstrate this property, two bottles, one
half filled with water and the other containing an
equal volume of concentrated sulphuric acid (yellow
dye was added to identify the acid), are placed inside
an airtight container (햲). Over the following weeks,
the concentrated sulphuric acid takes the water out of
(dries) the air and becomes more dilute. Evaporation
occurs from the bottle containing the water, and this
water vapour is also absorbed by
the concentrated acid. As a
result, the level of liquid in the
acid bottle rises and the acid
becomes more dilute, while the
level in the water bottle falls (햳).
Demonstration 1: drying properties of
concentrated sulphuric acid
Concentrated sulphuric acid has a strong affinity for
water and will absorb it from the air to make a more
dilute solution of the acid, becoming progressively
more HYDRATED. This can be used to show the
presence of water vapour in the air because, over
햲
Airtight
container
햳
Bottles left
open
Demonstration 2:
dehydration of copper(II)
sulphate using concentrated
sulphuric acid
Distilled water
Concentrated sulphuric
acid (with yellow dye)
A disc of dark blue, hydrated
copper(II) sulphate crystals is
prepared by allowing some
copper(II) sulphate solution to
evaporate in a Petri dish.
Hydrated copper(II) sulphate is
more accurately named
Evaporation of water
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Hydration of
sulphuric acid
copper(II) sulphate pentahydrate because of the water held in the
crystal structure – WATER OF CRYSTALLISATION.
When concentrated sulphuric acid is added to part of the
hydrated copper(II) sulphate (햴) using a pipette, the copper(II)
sulphate begins to change colour (햵 & 햶). This indicates
dehydration as the copper(II) sulphate pentahydrate is changed
to white copper(II) sulphate or anhydrous copper sulphate (햷).
In the process, the acid is diluted.
Dark blue hydrated
copper(II) sulphate
crystals, (CuSO4•5H2O)
햴
Some concentrated
sulphuric acid is
added using a pipette.
햷
햵
햶
White anhydrous copper(II)
sulphate (CuSO4)
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