Questions by topic

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 in chemical structure, properties, and uses. Oils may be animal, vegetable, or petrochemical in origin.

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 open, allowing the oxygen in air, and the polyphenols to come into contact, triggering an oxidation reaction driven by the enzyme. The initial reaction produces compounds called o-quinones, which then further react with amino acids in the apple to form brown-colored melanins. While it may look unappealing, the brown coloration is not harmful.

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 tend to be inorganic compounds such as zinc oxide or titanium dioxide, and those that absorb UV rays are commonly organic compounds such as aminobenzoic acid, avobenzone, oxybenzone, octisalate, and octocrylene. Ultraviolet radiation in sunlight is known to cause sunburn, skin cancers and premature ageing of the skin, and good sunscreen products are an effective way to reduce exposure.

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 retreats at night when the ionizing UV light from the sun fades. Lower energy radio waves, such as AM signals, are less able to penetrate the ionosphere, and during the daytime, the denser, low levels of the ionosphere absorb much of the medium wave AM signals. At night, as those lower ionosphere levels disappear, AM radio waves can reach the upper layers of the ionosphere where rather than being absorbed, they tend to get reflected back towards the ground. This process, called skywave propagation, results in AM radio waves being bounced back to earth over a much wider geographical area at night.

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 causes sunburn and skin cancers, but a significant amount of the UVA rays still pass through and can reach the skin. UVA rays cause tanning of the skin but are also linked to premature skin ageing, wrinkles and cancer. A double-glazed window offers more protection as two panes of glass will absorb more of the UV rays. Certain special glass coatings are designed to offer more protection by filtering out substantially all UV rays.

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 effect on the central nervous system is considered to promote alertness and reduce feelings of drowsiness.

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.

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). The kinetic energy of nitrogen molecules at room temperature is sufficient to overcome these weak forces of attraction between molecules, enabling them freely move apart and exist in a gaseous state. The key distinction is the strong attractive forces between atoms within the molecule and the weak attractive forces between the molecules themselves.

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 undergone multiple internal reflections within tiny water droplets in the cloud. It was called a spectre because optical illusions often make the shadow appear oversized. The name Brocken comes from the highest peak in Germany’s Harz mountains, one of the places where the phenomenon has previously been observed.

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 have stable ions with only a partially filled 3d subshell, and the five d-orbitals are degenerate, meaning they all have the same energy. Crystal Field Theory describes how when a transition metal ion bonds with a ligand to form a transition metal complex, electron repulsion causes the d-orbitals to become non-degenerate, separating into two different energy levels. As the subshell is not full, electrons from the lower energy level 3d orbitals can jump to the higher energy 3d orbitals by absorbing visible light. The light absorption during these d-d energy transitions accounts for the characteristic vivid colours of transition metal complexes. As the frequency of light absorbed corresponds to the energy difference between levels, what we see is distinct color corresponding to the remaining visible light frequencies not absorbed.

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 sees it start to expand again. From 4°C, whether you cool or heat it, water expands. This anomalous behavior occurs because as water molecules come closer together with cooling, they are increasingly inclined to form hydrogen bonds between the oxygen atom of one molecule and the hydrogen atom of a neighboring molecule. This starts to form structured hydrogen-bonded clusters of water molecules which actually begins to increase the distance between them, reversing the increasing density trend observed up to that point. Further cooling to freezing ultimately forms an orderly crystalline lattice of solid ice which is actually less dense than its liquid water state. 4°C is the point above which the normal effects of thermal kinetic expansion prevail over water’s tendency to form hydrogen bonds.

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 in the dye molecules. As those electrons fall back to a stable state, the dye molecules release energy as photons of light. This is called chemiluminescence and the dye molecule used governs the light wavelength emitted, and hence the glow stick color.

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 is change the timbre of the vocal sound compared to speech produced in normal air. The vocal tract, nasal cavities and mouth create acoustic resonances, known as formants, from the sound generated by the vocal cords. Because the sound waves move faster in helium-laden breath, the stronger resonances shift towards higher frequencies and drastically change the overall tone quality of the emitted voice.

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 total kinetic energy after a collision remains the same as before. There are no long-range forces of attraction or repulsion between molecules. Individual molecules occupy no volume, so a molecule’s size is ignored relative to the volume occupied by the gas as a whole. In reality no real gas exhibits such properties without deviation, but most real gases at room temperature and normal atmospheric pressures behave sufficiently closely that the theoretical model is a useful approximation for scientific study.

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.

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 light, the blue light component of sunlight is scattered more strongly in the atmosphere. So, what we see when looking up is an excess of the higher energy blue light being scattered compared to other lower energy colors, and hence the sky appears blue.

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 called a cation. Conversely, an anion is a negatively charged ion because it has more electrons than protons.

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 load of the internal cabin air pressure pushing outwards on the window during high altitude flight. Venting through the hole also prevents condensation fogging up the windows.