Don’t go breakin’ (down) my (love) heart

Kit

• tin lid or ceramic evaporating basin
• sand
• heat resistant mat
• 2 cm³ ethanol or IDA (industrial denatured alcohol)*
• mixture of 4 g sucrose and 1 g sodium hydrogen carbonate or 5 ‘fizzer’ tablet sugar sweets (approximately 5 g). Examples include Love Hearts, Giant Fizzers, Refreshers – look for sodium hydrogen carbonate or sodium bicarbonate in the ingredients

* Note: this is highly flammable.

Preparation

Place a pile of sand on a tin lid or evaporating basin a few centimetres in depth in order to create a well for the sugar/sodium hydrogen carbonate mixture or the sweets. Place the mixture or sweets into the well.

In front of the class

Wear eye protection. Add 2 cm³ of ethanol or IDA to the mixture and light it. The ethanol burns off initially with a colourless flame. Soon after, the sugar begins to caramelise and decompose (producing steam and carbon), while the sodium hydrogen carbonate decomposes (producing carbon dioxide). Over the next minute, a black ‘snake’ of carbon climbs out from the mixture or the sweets.

Teaching goal

Beyond showing a quick and simple example of a chemical change (as evidenced by a colour change) and the production of gas, heat and light, this reaction can also be used to explore the thermal stability of carbonates.

As the mixture heats up, the sodium hydrogen carbonate begins to thermally decompose to produce steam and carbon dioxide (Equation 1). At the edges of the mixture in contact with the flame, the temperature rapidly passes the 160°C required to initiate caramelisation and the sucrose in the mixture decomposes (Equation 2) and combusts (Equation 3) to produce mostly carbon and steam.

Equation 1: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)

Equation 2: C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(g)

Equation 3: C₁₂H₂₂O₁₁(s) + 12O₂(g) → 12CO₂(g) + 11H₂O(g)

The rapid production of carbon dioxide inhibits the combustion reaction, leaving a significant proportion of the reagents behind as carbon. The demonstration is an example of an intumescent reaction – one in which a substance expands significantly through chemical means upon exposure to heat. Intumescent substances are commonly applied as passive fire suppressants as the expanding foam resists thermal conduction through to a combusting material.

If you would like to explore sweets further with older students, it’s worth demonstrating that the reaction does not work in the same way with other carbonates. Other sweets that use magnesium carbonate instead as an acidity regulator (eg Double Lollies or Swizzle Sticks) will melt, caramelise and burn – but not produce the black foam snake.

Using the thermodynamic data from the table below, older students will be able to use the Gibbs equation to predict the temperatures at which the thermal decomposition of NaHCO₃ and MgCO₃ become thermodynamically feasible.

Both decompositions have similar enthalpy changes. In fact, decomposition of NaHCO₃ has a slightly more endothermic profile than MgCO₃: +130 kJ mol^-1 and +115 kJ mol^-1 respectively. However, the greater entropy change associated with the production of two moles of gas from the decomposition of NaHCO₃ instead of just one from the decomposition of MgCO₃ increases the temperature at which decomposition is predicted to become feasible from approximately 116°C to 383°C.