Questions by topic

Fiat currency is legal tender issued by a government, which has no intrinsic value and is not backed by a commodity of value, such as gold or silver. Its value is essentially underpinned by the stability and creditworthiness of the issuing government. Most major currencies are now classed as fiat money, including the US dollar, pound sterling and the euro, Read more...

Run flat tires have additional structural rigidity built-in, either through stiffer sidewalls or an inner support ring, enabling the tire to bear vehicle weight even when not fully inflated, unlike a conventional tire. A standard tire relies almost entirely on internal air pressure to give it sufficient rigidity to support a vehicle’s weight. In the event of a puncture, it Read more...

The definition of an oil is any nonpolar substance that exists as a viscous liquid at ambient temperatures and has the characteristics of being both hydrophobic (meaning it does not mix with water) and lipophilic (and so mixes with other oils). Different chemical compounds that share these characteristics may be classed as oils, although they may have no other similarities Read more...

When an apple is sliced open, the flesh exposed to air turns brown due to a series of biochemical reactions called enzymatic browning. The flesh inside an apple is made up of tiny cells containing, among other things, naturally-occurring polyphenol molecules and an enzyme called polyphenol oxidase. When an apple is cut or bruised, some of the cells are broken Read more...

Sunscreen products such as sun tan lotion contain chemical components that reduce the amount of the sun’s ultraviolet (or UV) radiation that reaches the skin. The active ingredients in sunscreen work by blocking the UV rays from the skin by reflecting them away or chemically absorbing them and converting the light energy into heat energy. The physical ultraviolet-reflecting sunblock components Read more...

AM radio signals can often be received from extremely distant transmitters at night due to changes that occur in the earth’s ionosphere after dark. The ionosphere is an atmospheric layer of electrically-charged atoms, molecules and free electrons that mainly arise due the ionizing effect of the sun’s ultraviolet radiation. The ionosphere extends closer to the earth’s surface during daylight, and Read more...

It is possible to get a tan through an ordinary glass window but much more slowly than when exposed to sunlight directly in the open air. Sunlight reaching us on earth contains two types of ultraviolet (or UV) rays, called UVA and UVB. Glass filters sunlight passing through a window and removes most of the UVB rays, the type that Read more...

Caffeine is a nitrogen-containing organic compound that is classed as an alkaloid and known by the IUPAC name 1,3,7-trimethylpurine-2,6-dione. In common with many other alkaloids, it is a naturally occurring compound of plant origin that has strong biological effects on animal and human organisms. One of the most common sources of human caffeine consumption is coffee drinks, and its stimulant Read more...

A frog is an omnivore, eating both plants and animals, although more accurately described as a life-history omnivore as it feeds on different types of food through its life stages. Tadpoles, the aquatic larval stage of a frog, are herbivores, feeding only on plant material before becoming mostly carnivorous once they have developed into frogs. Read more...

Each nitrogen molecule is two nitrogen atoms bound together by a very strong triple covalent bond. Its strongly bonded symmetrical structure makes nitrogen very stable and unreactive, requiring a lot of energy to break the bond within the molecule, significantly more than is available at room temperature. The attractive forces between nitrogen molecules themselves are dispersion forces (or London forces). Read more...

A Brocken spectre is a visual effect in cloud or mist where a person sees their shadow with colored concentric rings centered around the head. It occurs at high elevations like mountain peaks when sun directly behind an observer casts their shadow on cloud or mist below. The colored rings, called a glory, arise from backscattering of sunlight that has Read more...

Orbitals, the regions around an atom’s nucleus likely to be occupied by electrons, are grouped into subshells identified by energy level (1, 2, 3, 4 and so on) and orbital shape (s, p, d, and f). The 3d subshell contains five orbitals, each able to hold up to 2 electrons, a total of 10 electrons if all filled. Transition metals Read more...

Most liquids contract when cooled, getting denser until they freeze into a solid. Their moving molecules lose kinetic energy, slowing them down, so they become prone to the attractive forces of nearby molecules drawing them closer together. Water exhibits this normal behavior down to 4 degrees Celsius (or 39.2 degrees Fahrenheit) where its density reaches a peak, but cooling actually Read more...

Glow sticks contain hydrogen peroxide solution and a separate sealed glass tube of diphenyl oxalate ester solution mixed with a fluorescent dye. Bending the glowstick breaks the inner glass tube, mixing the two solutions, and starts a chemical reaction. The reaction makes 1,2-dioxetanedione, a cyclic dimer of carbon dioxide, which rapidly decomposes to carbon dioxide, releasing energy that excites electrons Read more...

Sound travels almost 3 times as fast in helium gas than in normal air because helium is less dense. Contrary to common perception, helium does not make the voice high-pitched, because voice pitch is determined by the vibration frequency of the vocal chords generating the sound. So, while the fundamental pitch of the voice is actually unchanged, what helium does Read more...

An ideal gas is a scientific concept underpinning the Ideal Gas Law, of a theoretical gas with strictly defined properties for the purpose of understanding, predicting, and explaining the behavior of real gases under different conditions of temperature, pressure and volume. In an ideal gas: Molecules are spheres which collide elastically with each other and the container walls, so the Read more...

A temperature of minus 40 degrees is the same in both Celsius and Fahrenheit. It is the only temperature point on the F and C scales where this occurs. Read more...

The light we see from the sun is made up of a rainbow spectrum of colors from red through blue to violet. As light passes through the Earth’s atmosphere, it hits small gas molecules and particles in the air and gets scattered in different directions. This is known as Rayleigh scattering. As blue light has more energy than, say, red Read more...

An ion is an atom or molecule carrying an electrical charge because it has an unequal number of protons and electrons. By convention, protons are considered positively charged, and electrons negative and a net electrical charge results whenever there is more of one than the other. An ion with more protons than electrons carries a net positive charge and is Read more...

Most commercial aircraft cabin windows are made of three plastic panes, each separated by a small air gap. The tiny hole at the bottom of the window is a bleed hole through the center pane to equalize the pressure inside the window unit with the cabin air pressure. By design, this ensures it is the outermost window pane bearing the Read more...