This is an AS Chemistry lesson on determining the oxidation number of an element in a compound. The starter is a very engaging potassium dichromate volcano where students describe what is happening (e.g. thermal decomposition). The demo must be done in a fume hood and CLEAPPS safety advice should be considered. Instructions for this practical are on the Royal Society for Chemistry website. The sts version of the lesson is printed 2 slides per page. The lesson has animated answers that show a careful step by step calculations so that weaker students are able to access the content. Please rate this resource and leave feedback.