A hot dinner from a can

Learners create their own hot dinner from a can.

This experiment should take between one and two hours.

Equipment

• Fresh calcium oxide (approximately 50–100 g per group). 
• Items from the junk list, including types of insulation
• Glass stirring rods
• Glass beakers
• Boiling tubes
• Test-tubes
• Thermometers
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Always wear eye protection.
• Wear clothing protection, if desired.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• The reaction can be violent, potentially breaking even Pyrex equipment, great care should be taken. 
• Care should be taken in disposing of unreacted CaO as a delayed exothermic reaction could occur in the waste pipe if it is accidentally washed down the sink.

Commentary 

It is suggested that the students use the exothermic reaction between calcium oxide and water to produce a steady supply of heat.

CaO(s) + H₂O(l) → Ca(OH)₂ (s)

The students will need to calculate the quantities of heat involved before designing the system. The enthalpy change for the reaction above is approximately–65 kJ mol^–1.

This information may be provided, or they may calculate it by using standard molar enthalpy changes obtained from a data book. They should then find the heat transferred when 1 g of calcium oxide reacts with water. During trials, the students were encouraged to do small-scale trial runs, and this worked particularly well. The specific heat capacity of the food may be approximated to that of water for the purposes of this activity.

An important criterion in assessing the final design is that the system should be as compact and light as possible. ‘Hotcans’ that work on this principle can be bought in camping shops, and the best student designs could be compared with the commercial product.

Extension

The students could be asked to consider how the proposed device would perform under adverse weather conditions – i.e. would it work below freezing point?

Notes

• This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.
• The calcium oxide must not be slaked and fresh commercial CaO is recommended; alternatively, CaO can be obtained by roasting CaCO₃ in a kiln (some institutions may have a Muffle furnace). The temperature required is over 1000 °C, so a Bunsen burner is not hot enough. The CaO should be tested before the session to avoid disappointment.