An acid is a substance that can donate a hydrogen ion (H) (generally speaking, this will be a proton) to another substance. Acids have a pH less than 7.0.
A chemical can donate a proton if the hydrogen atom is attached to an electronegative atom like oxygen, nitrogen, or chlorine. Some acids are strong and others are weak. The weak acids hold on to some of their protons, while the strong acids let go of all of them. All acids will release hydrogen ions into solutions. The amount of ions that get released per molecule will determine if the acid is weak or strong. Weak acids are acids that partially release the hydrogen atoms that are attached. These acids, then, may lower pH by dissociation of hydrogen ions, but not completely. Weak acids generally have a pH value of 4-6 while strong acids have a pH value of 1 to 3.
A base is an acid's "chemical opposite." A base is a substance that will accept the acid's hydrogen atom. Bases are molecules that can split apart in water and release hydroxide ions.
Acids and bases typically exist together in equilibrium. This means that within a sample of an acid, some molecules will give up their protons and others will accept them. Even water is a mixture of an acidic ion, HO (called a hydronium ion) and a basic ion, OH (called a hydroxide ion). A hydronium ion will give up its proton to a hydroxide ion, forming two molecules of HO, which is neutral. This reaction happens continuously in a sample of water, but overall the sample is neutral because there are equal amounts of hydronium and hydroxide in the sample. For most reactions, however, the acids and bases are not present in equal amounts, and this imbalance is what allows a chemical reaction to occur.
Every acid has a conjugate base formed by removing the acid's proton. Hydrochloric acid (HCl), for example, is an acid and its conjugate base is a chlorine anion, or Cl. An acid and its conjugate base are opposite in strength. Since HCl is a strong acid, Cl is a weak base.
Acids can have different "strengths", some are more reactive than others. More reactive acids are often more dangerous.
Acids can have a lot of different properties depending on their molecular structure. Most acids have the following properties:
Acids can burn the skin, the severity of the burn depending on the type and concentration of the acid. These chemical burns require immediate medical attention.
Because acids donate hydrogen ions, all acids must have hydrogen in them.
Acids are important. Nucleic acids, such as DNA and RNA contain the genetic code. These molecules determine many characteristics of an organism, they are passed from the parents to offspring. DNA contains the plans how to build proteins which are made of amino acids.
Fatty acids and fatty acid derivatives are another group of carboxylic acids that play a significant role in biology. These contain long chains of hydrocarbons and a carboxylic acid group on one end. The cell membrane of nearly all organisms is primarily made up of a phospholipid bilayer, a micelle of hydrophobic fatty acid chains with polar, hydrophilic phosphate "head" groups.
In humans and many other animals, hydrochloric acid is a part of the gastric acid secreted within the stomach. It can help hydrolyze proteins and polysaccharides. It can also convert the inactive pro-enzyme, pepsinogen into the enzyme, pepsin. Some organisms produce acids for defense; for example, ants produce formic acid, and octopi produce a black acid called magneta.
Most acids can be found in nature. Some of these include the following:
White dwarf
A white dwarf is a compact star. Their matter is squashed together. Gravitation has pulled the atoms close together, and taken off their electrons. The mass of a white dwarf is similar to the mass of the Sun, but its volume is similar to that of the Earth.
White dwarfs are the final evolutionary state of all stars whose mass is not high enough to become a neutron star. Over 97% of the stars in the Milky Way will become white dwarf stars. After the hydrogen–fusing lifetime of a main-sequence star ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core. If a red giant does not have enough mass to fuse carbon, around 1 billion K, inactive carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind the core, which is the white dwarf.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. It is not supported by the heat of fusion against gravitational collapse.
A star like our Sun will become a white dwarf when it has run out of fuel. Near the end of its life, it will go through a red giant stage, and then lose most of its gas, until what is left contracts and becomes a young white dwarf.
White dwarfs were discovered in the 18th century. The first white dwarf star, called 40 Eridani B, was discovered on 31 January 1783 by William Herschel. It is part of a three star system called 40 Eridani.
The second white dwarf was discovered in 1862, but was at first thought to be a red dwarf. It was a small star near the star Sirius. This companion star, called Sirius B, had a surface temperature of about 25,000 kelvin, so it was thought of as a hot star. However, Sirius B was about 10,000 times fainter than the primary, Sirius A. Scientists have discovered that the mass of Sirius B is almost the same as that of the Sun. This means that once, Sirius B was a star similar to our own Sun.
In 1917, Adriaan van Maanen discovered a white dwarf which is called Van Maanen 2. It was the third white dwarf to be discovered. It is the closest white dwarf to Earth, except for Sirius B.
A white dwarf has low luminosity (total amount of light given off) but a very hot core. The core might be 10 K, while the surface is only 10 K.
A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool. This means that its radiation, which gives it a blue or white colour at the start, lessens over time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit light. Unless the white dwarf gets matter from a companion star or some other source, its radiation comes from its stored heat. This is not replaced.
White dwarfs cool slowly for two reasons. They have an extremely small surface area to radiate this heat from, so they cool gradually, remaining hot for a long time. Also, they are very opaque. The degenerate matter that makes up the bulk of a white dwarf stops light and other electromagnetic radiation, so radiation does not carry away much energy.
Eventually, all white dwarfs will cool down into black dwarfs, so called because they lack the energy to create light. No black dwarfs exist yet because it takes longer than the current age of the universe for a white dwarf to cool down. A black dwarf is what will be left of the star after all of its energy (heat and light) is used up.
White dwarfs may re-ignite and explode as supernovas if they get more material. There is a maximum mass for a white dwarf to remain stable. This is known as the Chandrasekhar limit.
A dwarf might pull in material from a companion star, for example, bringing it over the Chandrasekhar limit. The extra mass would start a carbon-fusion reaction. Astronomers think this re-igniting might be the cause of Type Ia supernovas.
Beach
A beach is a landform along the coast of an ocean, sea, lake, or river. It usually consists of loose particles, such as sand, gravel, shingle, or pebbles. The particles of a beach are sometimes biological in origin, such as mollusc shells or bits of coral and sometimes bits of igneous rock, but the most common mineral in beaches is quartz.
Beaches are natural landing and launching places for boats, and landing craft are specially made for beaches.