The rainbow flame experiment that caused the Oct. 30 fire at W.T. Woodson High School and injured six people is such a popular high school chemistry demonstration that it appeared in AMC's 'Breaking Bad.' (Monica Akhtar/The Washington Post)

A high school chemistry class fire that injured five Virginia students and a teacher last week during a popular chemistry demonstration has prompted a wave of safety alerts across the country, with warnings to teachers about the dangers of using flammable liquids on open desks.

The Oct. 30 fire, at W.T. Woodson High School in Fairfax County, seriously burned two students — one of whom was still in the hospital Thursday — and sent three others to the hospital. Students who said they were in the class said the blaze started when a chemistry demonstration went awry.

The students said the teacher poured out a flammable liquid, set it on fire and introduced different metals to show how the flame changed colors, a demonstration widely known as a “rainbow flame.” When the flame appeared to die down, students said, the teacher poured out more liquid, causing a “splash of fire” that burned students. One student in the class said neither the teacher nor the students had on protective gear.

Officials have declined to release key details about the fire, including whether the demonstration was performed under a fume hood — as district safety guidelines advise — or what kind of liquid the teacher used.

Given the potential dangers involved, the National Science Teachers Association this week sent out a safety advisory to 68,000 science teachers telling them to “halt the use of methanol-based flame tests on an open laboratory desk,” referring to the liquid commonly used in such demonstrations.

“When carried out on open laboratory desks (outside of a chemical hood) these demonstrations present a high risk level for flash fires and deflagrations that can cause serious injuries to students and teachers,” the association wrote. “Teachers who conduct these types of demonstrations outside of a fume hood put themselves and their students at unnecessary and serious risk during this demonstration.”

The U.S. Chemical Safety Board, an independent federal agency responsible for investigating chemical accidents, said it was not investigating the Woodson fire but noted that it issued a safety bulletin last year advising teachers to use a fume hood — which ventilates flammable vapors — if they are going to perform demonstrations using flammable liquids because of “an unacceptable risk of flash fire.”

The Chemical Safety Board also released a safety video in late 2013 titled “After the Rainbow” featuring Calais Weber, who was badly burned after a rainbow flame demonstration exploded at her Ohio boarding school in 2006.

There have been at least three serious accidents in U.S. high schools involving open-flame experiments since 2002, each of them burning students.

The rainbow flame demonstration teaches a concept central to the curriculum of high school chemistry: that certain metals, when burned, produce a flame color that is characteristic of its chemical makeup. But safety experts and educators say there are far safer ways to perform rainbow flame demonstrations that do not involve flammable liquids.

Experts and veteran teachers recommend dipping wooden sticks into metal salts and then passing them through the controlled flame of a Bunsen burner, allowing students to observe the change in the flame’s color without using a flammable liquid.

Some school districts have long steered science teachers toward the safer method and, in recent days, have reminded them of safety guidelines.

In Florida’s Hillsborough County, one of the nation’s largest districts, a districtwide chemistry curriculum outlines the safer version of rainbow flame, said Daniel McFarland, science supervisor for the district. The curriculum includes laboratory activities that have been vetted by the district for safety.

In Prince William County, spokesman Phil Kavits said that the rainbow flame demonstration is permitted but that teachers are encouraged to use ethanol, a safer, less-flammable cousin of methanol. The Woodson fire prompted Jason Calhoun, Prince William’s supervisor of science and family life education, to send out reminders to staff about the importance of using protective gear and safety shields, devices that separate a teacher from student observers during demonstrations with risks of explosion.

Others, including Loudoun County, have banned rainbow flame demonstrations. There, schools officials sent out messages recently reminding science teachers of the school district’s safety procedures, spokesman Wayde Byard said.

The Woodson accident led Fairfax Superintendent Karen Garza to suspend all open-flame experiments on Monday. Garza’s response was praised by county school board members, who called it an appropriate move after a frightening incident. Outside the district, science educators said that experiments involving open flame are an important part of the curriculum and that suspending the use of flame could hamper science instruction and derail student research.

Vikram Gupta, a senior at Thomas Jefferson High School for Science and Technology, is among many students who are working on advanced research projects during their final year at the top-ranked magnet school. The 17-year-old was planning on doing research related to cystic fibrosis, but it requires that he sterilize a piece of lab equipment using ethanol and a Bunsen burner.

“I have completed [National Institutes of Health] and other safety training, so I see no reason that I should not be able to carry out the procedure,” Gupta said. “There is a good chance that I would have to research and implement entirely new methodology if the policy does not change.”

District spokesman John Torre said Wednesday that each Fairfax County high school will be allowed to use open flames in laboratories once all of its science teachers receive additional safety training. He said the training should be complete by the end of December, meaning some students could be without the use of open flames in class for two months.