This is a physical process not a chemical one, but can happen as our parcels are subject to the cooler temperatures during postage in the autumn/winter/spring months.
If you have received your flea treatment and it appears to have become solid, replace the cap back on the pipette and place it somewhere at room temperature (away from pets and children) for at least an hour and anything up to 24 hours, every so often giving the pipette a gentle shake (do this carefully in a way that will prevent the cap from coming off). The treatment will eventually melt/liquefy again and be ready for use.
There is a substance in the some flea spot-on treatments that is used to dissolve and carry the active ingredients across the skin barrier. This chemical on it's own has a freezing point of, and so solidifies at, about 18 degrees Centigrade, but with the ingredients this lowers that freezing point to just under 14 degrees Centigrade. Essentially what this means is that prolonged exposure to temperatures below 14 degrees Centigrade cause it to solidify.