Sweet Chemistry

Table sugar, or sucrose, C₁₂H₂₂O₁₁, is a disaccharide of glucose and fructose.
When a pure substance melts, it melts at a consistent, repeatable temperature, and retains its chemical identity when moving from the solid state to the liquid state.
The melting point of sucrose is usually given as a range, such as 160^oC to 180^oC, not as a well-defined melting point. This is because sucrose decomposes as it is heated. University of Illinois scientists have shown that glucose and fructose also decompose on heating.

If no oxygen were present, then sucrose would decompose into carbon and water. Caramelized sugar is due to the process of sucrose decomposing, and the brown colour is due to the presence of carbon in the mixture.

Carbon and water are also the products of the exothermic reaction between sucrose and sulfuric acid. This is an example of a dehydration reaction in which water is eliminated from the organic molecule during the reaction. The sulfuric acid then oxidizes the carbon produced as a result of the dehydration reaction, producing both carbon dioxide and toxic sulfur dioxide gases.

If potassium nitrate is used to oxidize sucrose, the products are potassium carbonate, water, nitrogen gas and carbon dioxide gas.

48 KNO₃ + 5 C₁₂H₂₂O₁₁ → 24 K₂CO₃ + 24 N₂ + 55 H₂O + 36 CO₂

This reaction can be used as the basis for a propellant for model rockets, such as in the Sugar Shot to Space program.

Reference
Joo Won Lee, Leonard C. Thomas, Shelly J. Schmidt. Can the Thermodynamic Melting Temperature of Sucrose, Glucose, and Fructose Be Measured Using Rapid-Scanning Differential Scanning Calorimetry (DSC)? Journal of Agricultural and Food Chemistry, 2011; 59 (7): 3306 DOI: 10.1021/jf104852u

Further Reading
Carbohydrates
Pure Substances and Mixtures

Study Questions

1. Draw a structural formula for both glucose and fructose.
2. Give the molecular formula for both glucose and fructose.
   
3. Explain why glucose and fructose are considered to be monosaccharides.
4. Explain what is meant by the term disaccharide.
5. Draw the structural formula for a disaccharide composed of:
   • glucose
   • fructose
   • glucose and fructose
6. Draw a sketch of the temperature vs time graph you expect to see if a sample of sucrose was:
   • a pure sample
   • a mixture of different sugar
7. Write a balanced chemical equation to represent the decomposition of sucrose into carbon and water in the absence of oxygen.
   
8. Write a balanced chemical equation for the dehydration of sucrose using sulfuric acid.
9. What is the difference between the decomposition of sucrose reaction and the dehydration of sucrose sucrose? Why is the reaction with sulfuric acid not considered to be a decomposition reaction?