Nouns ending in -ance are feminine.
The suffix forms abstract nouns, often from verbs.
Is substance masculine or feminine?
lasubstance
The rule behind it
100% of -ance nouns are feminine
In context
Ces substances ne sont pas vénéneuses en elles-mêmes.
These substances are not poisonous in themselves.
Une substance peut être solide, gazeuse ou liquide.
A substance can be solid, gaseous, or liquid.
Tu fumes du crack ou quelque autre substance ?
Are you smoking crack or something?