Sulfuric acid as a dehydrating agent

In this practical, students observe the phenomenon of dehydration as distinct from drying, as concentrated sulfuric acid removes water of crystallisation from hydrated copper(II) sulfate and ‘the elements of water’ from sucrose. You may also extend the discussion to the oxidising action of concentrated sulfuric acid.

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.

Equipment

Apparatus

• Eye protection
• 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

Chemicals

• 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)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Teachers and students should wear eye protection throughout.
• The demonstration must be carried out in a fume cupboard.
• Concentrated sulfuric acid, H₂SO₄(aq), (CORROSIVE) – see CLEAPSS Hazcard HC098a. 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 HC027c. 
• Cobalt chloride paper (TOXIC) – see CLEAPSS Hazcard HC025. Cobalt chloride paper may be purchased or made. See this standard procedure for making your own cobalt chloride paper.
• Potassium dichromate(VI) paper (VERY TOXIC) – see CLEAPSS Hazcard HC078c. Potassium dichromate(VI) paper may be made in the prep room. See Standard procedure: Preparing and using potassium dichromate(VI) indicator papers.
• 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.

Procedure

1. 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.
2. 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.
3. The steam can be tested with cobalt chloride paper. This will turn from blue to pink.
4. Sulfur dioxide (TOXIC) is also given off. This is why a fume cupboard is needed (see CLEAPSS Hazcard HC097). 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.
5. 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.

Teaching notes

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

However, 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)