What are the physical and chemical changes that occur in fireworks?

Paul Nicholas Worsey, professor of mining engineering at the University of Missouri at Rolla, teaches a course in pyrotechnics. He provides the following answer:

Fireworks, or pyrotechnics, contain burning compounds. The most common is the aerial shell, which is fired from a mortar tube. It has four components: a lift charge, a timedelay fuse, a breaking charge and a light-effect generator.

The lift charge is a compound that burns rapidly and propels the shell from its tube. When the lift charge fires, it also lights the delay fuse, which in turn ignites the break charge when the shell reaches the appropriate height. The break charge explodes, scattering the shell's now fiery contents. The shell's payload usually comprises small pyrotechnic pellets designed to generate light. These capsules burn from the outside inward, and color changes result from layers of different compositions.

The metal within the pyrotechnic mixture and the burning temperature dictate the color and intensity of the display. When certain metals are heated, electrons jump between so-called electron shells, or energy levels, within the atoms. When they fall back to a lower state they emit a photon whose wavelength determines the color. The easiest hues to create are red (from strontium), green (barium), yellow (sodium) and white (titanium). Blue is more difficult because the reaction temperature has to be just right. Sparks, in contrast, come from slowerburning mixtures. Shapes such as rings, hearts and smiley faces require precise placement of the pellets inside the shell.

All pyrotechnic mixtures contain a fuel and an oxidizer. The mixture usually consists of a metal nitrate and a carbonbased fuel. When the solid blend ignites, it turns into predominantly gases and gives out a lot of heat—in excess of 2,000 degrees Celsius for brilliant colors. A number of factors control the speed of a firework reaction: the composition of the shell and other physical characteristics, such as the grain size (smaller means faster), the presence of accelerators (sulfur and sugars, for example) or retarders (salt, for instance), high pressure or confinement (which increases the reaction rate), lower packing density (which reduces the rate), and moisture content.

How do antibiotics kill bacterial cells but not human cells?

Harry Mobley, chair, department of microbiology and immunology at the University of Michigan Medical School, replies:

Antibiotics can selectively target bacteria for eradication, leaving human cells unmolested, in several ways.

Most bacterial cell walls contain a macromolecule called peptidoglycan, which human cells do not make or need. Penicillin, for instance, prevents the final cross-linking step, or transpeptidation, in the macromolecule's assembly. The result is a fragile cell wall that bursts, killing the bacterium.

Certain drugs target bacterial metabolic pathways. Sulfonamide drugs are structurally similar to para-aminobenzoic acid, a compound needed for folic acid synthesis. All cells require folic acid. Whereas the vitamin diffuses easily into human cells, it cannot enter bacterial cells, and so bacteria must make their own. Sulfa drugs inhibit a critical enzyme in this process, and the bacteria can no longer grow.

Another antibiotic—tetracycline—interferes with bacterial growth by stopping protein synthesis. Both bacterial and human cells carry out protein synthesis on structures called ribosomes. Tetracycline binds to a site on the ribosome and blocks a key RNA interaction, shutting off the lengthening protein chain. In human cells, tetracycline does not accumulate enough to hinder protein synthesis.

Similarly, DNA replication must occur in both bacteria and human cells. Antibiotics such as ciprofloxacin can specifically target an essential enzyme called DNA gyrase in bacteria. But this antibiotic does not affect human DNA gyrases.