Learn about boron by the laboratory
Boron is the fifth element in the periodic table, and this number is destined to prevent boron from filling our world like oxygen and hydrogen.
We know that the core of a star is a super nuclear fusion factory. The No. 1 element hydrogen fuses into helium, and after the No. 2 element is ignited, it flushes the No. 26 element carbon and the No. 8 element oxygen and rushes all the way, together with the even-numbered element No. 4 beryllium They were all left alone, not to mention the embarrassing odd number 5. Therefore, the formation of boron basically depends on supernova explosions and cosmic ray spallation, which is why boron is not a common element in the earth’s crust. However, in stark contrast to its rarity, in daily life, almost no one thinks that borax or boric acid is a rare substance. This is because boron minerals are mostly soluble in water, and nature’s porters will take boron that is not much Enriched in the salt lake, it is easy for people to come to a pot.
Today, it is very simple to prepare boron in the laboratory. It is to cut an active element such as lithium into small pieces, mix it with any compound of boron such as boron oxide, put it in a metal crucible, and then heat it. There is a puffing sound and white smoke (6li+B2O3→3Li2O+2B), wait until there is no movement inside, open the cover, at this time the boron oxide is melted to form a glass-like thing, and the sea is inlaid with a group of For the black substance, pour a dilute alkaline solution into it to clean it, and after sucking it out with a dropper, you can see brown-black powder distributed inside, which is the displaced amorphous boron. The appearance of this boron is ugly, and because of the limitation of the preparation process, its purity can only reach 98%. When the equivalent is relatively large, the amorphous boron powder presents a tan powder appearance. If you observe carefully, there are shining crystals doped in it. As for high-purity crystalline boron, it looks much more dazzling than ordinary boron. As for the purity, it can be infinitely close to 100%, provided that you are willing to spend enough cost for the production.
A major feature of boron crystals is its ultra-high hardness. Among many simple substances, it is second only to diamonds, reaching a Mohs hardness of 9. Even gemstones such as emeralds that are known for their high hardness (aquamarine Mohs hardness is 8.0- 8.5), scratches can also be easily erased by it. However, you need to be very careful when doing this, because the particles of boron crystals are usually relatively small, and they are easy to break if excessive force is applied. Crystalline boron is not only expensive, but also chemically stable, and only reacts with very few substances.
Boron is located in the third main group of the periodic table of elements. Compared with alkali metals or halogens, which have very similar chemical properties, several strangers in this family were forced into a photo. No matter how you look at it, it is awkward. In addition to boron, this family also has aluminum, which is known as the king of light metals, gallium, which is not even clear whether it is solid or liquid at room temperature, and the behind-the-scenes boss hidden behind the touch screen of the smartphone Indium, as well as thallium, the ace poison that kills countless people in novels and stories, and nihonium, which is an extravagant hope for surviving for more than 10 seconds
Because there are only three valence electrons in the outermost layer, the elements in this family are all metals from aluminum to the bottom, but because boron has only two electron layers, the nucleus has a strong control over the outer electrons, so it becomes this group The only non-metallic element in . Also because of this control, boron is not active at room temperature. For example, if boron powder and dilute hydrochloric acid are soaked together, boiled and heated for a long time, even if it is boiled for several hours, no visible reaction will be seen. , compared with it, aluminum, which has one more electron than it, reacts with hydrochloric acid much more violently. But when encountering a strong oxidizing acid such as nitric acid, the situation is just the opposite. Aluminum is safe because it is passivated, but when boron comes into contact with nitric acid with a concentration of about 68%, it does not even need heating, and a violent reaction will occur immediately. . You must use a dropper for this experiment, and you can only add one drop at a time, otherwise you may spray the entire laboratory everywhere. When the reaction releases a huge amount of reddish-brown nitrogen dioxide gas, a layer of transparent crystals can be formed on the inner wall of the test tube, which is the famous boric acid.
Because nitric acid is so reactive, we prefer to use a milder oxidizing agent, potassium pyrosulfate.
Pour a small amount of potassium pyrosulfate into the quartz crucible, then add a little amorphous boron powder, increase the heat to melt the potassium pyrosulfate, and the reactant will soon start bubbling to produce boric acid and sulfur dioxide. This reaction seems to be calm but quite easy to use, even if it is an inactive substance like crystal boron, it can react.
Compared with acid, amorphous boron can also react with alkali under heating, but requires a high concentration of alkali solution, so the more convenient operation method is to manage and mix a large amount of sodium hydroxide solid and a small amount of boron powder,
(2B) 2NaOH+2H2O→2NaBO2+3H2O), then slowly inject water, with the dissolution of sodium hydroxide, fine hydrogen bubbles begin to appear in the solution.
Boron will become extremely active at high temperatures. For example, it can be used as a reducing agent to produce a boron thermal reaction similar to the aluminothermic reaction, especially the reaction with copper oxide. Even if the reactant amount is about two grams, the exothermic reaction will be generated. It is still catastrophic. It will not only melt the glass test tube, but also sputter copper vapor everywhere, so almost no one dares to do this in industrial production. A violent reaction will bring terrible consequences if you are not careful.
Boron can react with almost half of the elements of the periodic table to produce various borides, and the composition of these borides is a nightmare for chemical theoretical research. Taking chromium as an example, the known borides of chromium have Cr4B, Cr2B, Cr3B2, CrB, Cr5B3, Cr3B4, CrB6, CrB2, CrB4, the valence theory becomes elusive when encountering boron.