Sulfuric acid as a dehydrating agent
These two demonstrations illustrate the phenomenon of dehydration as distinct from drying. Concentrated sulfuric acid removes water of crystallisation from hydrated copper(II) sulfate and ‘the elements of water’ from sucrose. An extension of the discussion to the oxidising action of concentrated sulfuric acid is possible.
Each demonstration takes only a few minutes and can be used in a variety of contexts.
Sulfuric acid is well known for its ability to act in three distinct ways: as an acid, as an oxidising agent, and as a dehydrating agent; these demonstrations support the third of these. For a suitable audience you can also use the reaction with sucrose to illustrate the oxidising action of concentrated sulfuric acid.
These experiments are suitable for including in a set of ‘fun’ demonstrations.
Sucrose (table sugar), 50 g
Concentrated sulfuric acid (CORROSIVE), approximately 20 cm3
Hydrated copper(II) sulfate (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 3 g
Cobalt chloride paper (optional) (TOXIC)
Potassium dichromate(VI) paper (optional) (VERY TOXIC)
Refer to Health & Safety and Technical notes section below for additional information.
For one demonstration of each reaction the teacher will need:
Access to a fume cupboard with good all-round visibility
Beaker (100 cm3)
Large watch glass or white tile
Measuring cylinder (25 cm3)
Measuring cylinder (10 cm3 or 5 cm3)
Health & Safety and Technical notes
The demonstration must be carried out in a fume cupboard. Eye protection must be worn throughout these demonstrations by teachers and students.
Concentrated sulfuric acid, H2SO4(aq), (CORROSIVE) - see CLEAPSS Hazcard. A great deal of care is necessary in dispensing concentrated sulfuric acid and in disposing of the remains left by the demonstrations afterwards.
Hydrated copper(II) sulfate, CuSO4.5H2O(s), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Cobalt chloride paper (TOXIC) - see CLEAPSS Hazcard. Cobalt chloride paper may be purchased or made. See Standard procedure: Make your own cobalt chloride paper.
Potassium dichromate(VI) paper (VERY TOXIC) - see CLEAPSS Hazcard. Potassium dichromate(VI) paper may be made in the prep room. See Standard procedure: Preparing and using potassium dichromater(VI) indicator papers.
1 The carbon ‘sponge’ can pose a disposal problem. Place the beaker and contents in a large bowl of water and leave for some time to dilute any remaining acid. Small quantities can be broken up with a gloved hand and flushed down the sink. Larger amounts can be placed inside several sealed plastic bags before putting it in the waste bin.
a Weigh about 50 g of sucrose (ordinary table sugar) into the 100 cm3 beaker. Stand the beaker on a large watch glass (or white tile) in a fume cupboard. Clamp the beaker.
b Pour onto the sugar about 20 cm3 of concentrated sulfuric acid (CORROSIVE).
The sugar will turn yellow, then brown. After about a minute, the sugar will start to blacken. A spongy mass of carbon will begin to rise up the beaker, and steam will be evolved. The carbon will eventually rise to two or three times the height of the beaker.
c The steam can be tested with cobalt chloride paper. This will turn from blue to pink.
d Sulfur dioxide (TOXIC) is also given off. This is why a fume cupboard is needed. See CLEAPSS Hazcard 97. Sulfur dioxide will turn the potassium dichromate paper from orange to blue-green.
The beaker itself becomes very hot. One drop of water from a wash bottle squirted on to the outside of the beaker will steam.
e Place about 3 g of blue, hydrated copper(II) sulfate on a watch glass and pour onto it about 2 cm3 of concentrated sulfuric acid.
Over a period of about three minutes, the colour will change from blue to white, as the concentrated sulfuric acid removes the water of crystallisation from the salt. Heat is evolved. The change can be reversed by adding water. This needs to be done very carefully – adding water to acid is not normally good practice!
These demonstrations are a spectacular warning of the danger of handling concentrated sulfuric acid. It is worth pointing out that body tissue contains a high proportion of water – eye tissue in particular!
The reaction is usually written as
C12H22O11 → 12C + 11H2O
but this is an oversimplification. Some of the carbon is oxidised to carbon monoxide and carbon dioxide and some of the sulfur is reduced from an oxidation state of +6 in sulfuric acid to +4 in sulfur dioxide.
Hydrated copper(II) sulfate has the formula CuSO4. 5H2O. The addition of concentrated sulfuric acid removes the water of crystallisation to produce anhydrous copper(II) sulfate. The reaction is reversible.
CuSO4.5H2O(blue)(s) ⇔ CuSO4(white)(s) + 5H2O(l) or (g)
Health and Safety checked November 2006
The action of concentrated sulphuric acid on sucrose’, E. G. Meeks, School Science Review 1979, 61, (215), 281
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Page last updated on 07 December 2011