Story highlights The teacher was using chemicals to change the colors of a flame she had lit when the mixture suddenly erupted

Twelve students were injured, most with minor burns, and seven were rushed to the hospital

(CNN) A teacher in Houston was trying to show a preschool class how flames change color when her science experiment went terribly wrong.

The teacher was using chemicals to change the colors of a flame she had lit when the mixture suddenly erupted, a fire official said.

Twelve students were injured, most with minor burns, and seven were rushed to Children's Memorial Hermann Hospital, according to CNN affiliate reports.

"Fire was changing colors and the last one wasn't working, so we put in a little bit more, and then it exploded," 5-year-old Kate Earnest t old CNN affiliate KTRK . "That's how the other kids got burned, and they were crying."

The incident occurred outdoors Tuesday morning at the Yellow School, a preschool run by Memorial Drive Presbyterian Church.

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