A child drops a silvery chunk of pure sodium into a flask of water. Almost immediately, the liquid starts to bubble. Sparks, flames, and clouds of gas fill the air.

No need to react with alarm, however. It all takes place on the screen of an iPad.

Lamenting the rarity of chemistry sets with serious, eyebrow-singeing chemicals these days, officials at the Chemical Heritage Foundation in Philadelphia commissioned a virtual equivalent. And it is smoking.

ChemCrafter has been downloaded more than 224,000 times from Apple's iTunes store since it went live April 6, most of them overseas, said Neil Gussman, spokesman for the foundation, based on Chestnut Street. The free app is designed as a game, with players able to accumulate points and acquire more supplies as they complete experiments.

Use your finger to select a test tube from a handsome wooden cabinet, dump the virtual contents into a beaker full of water or acid, and watch what happens. As in real life, sometimes nothing. Or something impressive.

Karl Meyer, 14, of Wynnewood, demonstrated recently by adding some zinc to a flask of concentrated nitric acid, yielding zinc nitrate, water, and a brownish cloud of nitrogen dioxide gas on the screen.

"Really cool," said Meyer, who helped test the game with his 16-year-old sister, Anna, before the launch. The two agreed to help because their mother is a friend of project leader Shelley Wilks Geehr.

Ask scientists of a certain age, and chances are good that they have fond remembrances of a childhood chemistry set. More than a dozen Nobel Prize winners have credited such sets with sparking an early interest in science.

The foundation, which has a library, research center, and museum devoted to the gospel of chemistry and related sciences, owns more than 100 vintage sets, some of which originally contained such goodies as uranium and lead acetate.

A foundation podcast on the history of chemistry sets describes one from the early 20th century, made by A.C. Gilbert, which contained the toxic substance sodium cyanide.

"When inhaled, ingested, or touched, the compound inhibits the body's ability to use oxygen," the podcast narrator says, matter of factly.

With the advent of safety regulations and concerns over litigation, chemistry sets have become increasingly tame since the 1970s, more likely to contain ingredients for soaps and lotions than anything that would scare the neighbors.

The idea for the ChemCrafter app came from Roy T. Eddleman, a longtime benefactor of the foundation and the founder of California-based Spectrum Laboratories Inc. Eddleman, for whom the foundation's outreach institute is named, provided start-up funds to explore making an app.

After the foundation decided to proceed, it later got a grant from the Alfred P. Sloan Foundation, said project leader Geehr, director of the foundation's Roy Eddleman Institute.

The total costs, including preliminary research, development, marketing, and foundation staff time, came to just under $500,000, Geehr said.

Jeremy Wolf, a chemistry teacher at Palisades High School in Kintnersville, Bucks County, was enlisted to come up with reactions to include in the game. Additional input came from Bryn Mawr College chemistry professor Michelle Francl and Joseph Rucker, cofounder of Integral Molecular, a biotech company in West Philadelphia.

The app was created by Bluecadet, a Philadelphia firm that makes software, websites, and interactive exhibits for museums and other nonprofits.

It became clear early on that the game could not accommodate unlimited combinations of substances, Geehr said. The app, aimed at ages 12 to 16, would have become unmanageable in its complexity.

Still, players have access to 19 elements, water, and three acids: nitric, hydrochloric, and sulfuric. In addition to the animated images and bubbling sounds, the app includes text about the compounds produced, and tallies the amounts of energy released.

"It's not going to help anybody pass a chemistry exam," Geehr said. "But it does teach you things you might find interesting, and you might be intrigued enough to pay attention when you get to chemistry class."

Or you might be tempted to venture into the garage.

Craig Meyer, father of Karl and Anna, said he had shown his children how to melt aluminum with a blowtorch. Karl Meyer also has taken apart computer disk drives, his father said.

After all, this is a family that owns a copy of a book called 50 Dangerous Things (you should let your children do).

"It shouldn't all be virtual," said Craig Meyer, a software engineer.

At least one company still makes a chemistry set with heavy-duty ingredients, though it is not cheap. A top-of-the-line set from Thames & Kosmos of Rhode Island goes for close to $250.

And last month, Stanford University engineers won a competition to reimagine the chemistry set for the 21st century, making a handheld device that allows users to combine small amounts of chemicals.

But for those interested in getting started on an iPad, there is ChemCrafter. Whether a teenager uses test tubes or touch screens, the key is to explore, said Wolf, the Bucks County teacher.

"Let their curiosity drive them to figure it out," Wolf said. "That's the most powerful tool that they can have."