I am using kalanchoe plants for my science fair project. The ones with the acid solution grew mold on the soil. Does anyone know why this happened?

Probably we'd need a bit more info than that - maybe a brief outline of the project? And, what kind of Kalanchoe, what kind of soil, lighting conditions, watering habits, what do you mean by acid solution, etc...

well, the temperature was about 70 degrees f. lighting conditions- incandescant bulb, about 10 hrs a day. watering- watered every 3 days. calandiva, kalanchoes. acid solution- i used lemon juice and water, so i'm not sure if the lemon juice caused it or the acid.