depending on your and your target’s degree of synaesthesia, you could try associating shapes and/or colours with smells. won’t work for everyone though, and some might look at you strangely.

for instance, chlorine, smells a bit concave and brownish-white, whereas ammonia smells like a highlighter-green (the darker kind) arrowhead.

then there’s the question of whether we all smell things in the same way (even without the synaesthesia)