Nitrogen is the most abundant gas in our atmosphere and is luckily quite inert. It generally just sits there doing nothing while the more chemically relevant gasses like oxygen and carbon dioxide interact with our bodies. Almost all plants, however, require some nitrate (NO3–) in order to grow. There is a natural cycle of nitrogen where animals who die release the nitrogen from what they’ve eaten back into the soil and also an atmospheric cycle where occurrences like lightening can convert nitrogen in the atmosphere into nitrogen dioxide (NO2) which is then dissolved by the rain and ends up in the soil as a nitrate. These natural methods can sometimes be too slow for industrialised agriculture and so ammonia (NH3) can be added which is changed into nitrates by bacteria in the soil.
Although there is always talk about how Earth may not have enough habitable space for whatever the future population may be, I have never been particularly worried about this. Over the last fifty years places on this planet that were only habitable to the occasional hunter-gatherer group have managed to support massive growing urbanisation. For instance the United Arab Emirates, despite being next to the Persian Gulf, is a desert biome in its natural state. Drinking water is obtained through massive desalination plants which use reverse osmosis to remove the salt from the sea water. Normally the filter which the water is forced through is thin film composite (a film made of multiple layers) which normally contains a polyamide (as an active component) and polysulphone (as a support) layer.
Perovskite is the name of a mineral of calcium titanium oxide (CaTiO3). The structure of the mineral is shown on the right and any material that shares this structures is also called a perovskite (it should be noted that this is just one of the more common structures but others exist with symmetry below that of the cubic).
Perovskites have found a definite purpose in the world of physics. They demonstrate high efficiency energy conversions along with the ability for artificial versions to be manufactured using thin film production methods. The solar cell efficiencies when constructed with perovskites has improved from about 4% in 2009 to 22% (although the average is still 15%) in 2016 making it the fastest developing solar technology at the current time. One of the main problems presented by perovskite cells, however, is their instability and willingness to decay when exposed to moisture or ultraviolet light (that second one is a pretty big concern for a solar cell).
In 1839 the first ever hydrogen fuel cell was created. It was already well known that when enough energy was put into water it could be broken into the hydrogen and oxygen that form it. One William Robert Grove had the idea that if hydrogen and oxygen could be recombined with an electrolyte present to reproduce the water but also gain a flow of charge. Now vehicles can be fuelled with hydrogen but there is a catch. How is the least dense element going to be contained in a reasonable volume. It could be cooled into a liquid but this doesn’t seem practical for a car or perhaps out under high pressure but there are quite a lot of safety concerns about putting high pressure tanks in road vehicles. One of the suggestions that is being looked into is that of using metal hydrides as a way of chemically storing hydrogen by reacting it with a metal from which it can be extracted when used in the car.
If you were to ask any child for a method of cleaning water then filtering is probably the answer they would give. A step up from a piece of filter paper is a membrane which only allows water or water sized particles through while resisting others. Separation by membrane is a very useful process as it is nontoxic, requires little energy input and can be easily up sized for different water quantities. Unfortunately, like with all filters, the impurities and contaminants will remain on the membrane if not treated and severely limits the practical applications of such membranes. What is required is a membrane that has some way of cleaning itself, hopefully during the process.
The octet rules from chemistry is the idea that chemicals, when they react, will always try to gain eight electrons in their outer shell, as this is similar the noble gases and is the most stable configuration (in most cases). You can get free radicals floating around which are atoms or molecules that have unpaired electron but these radicals are very reactive and will quickly react to stabilise.
Carbon is known to form many different allotropes such as the nice variety shown on the right (a-diamond, b-graphite, c-Lonsdaleite, d-buckminsterfullerene, e-Fullerite, f-C70, g-amorphous carbon and h-carbon nanotube). Due to the ability to form four strong covalent bonds structures like diamond are very tough due to the tetrahedral formation. The other common arrangement, which can be seen in b, d, e, f and h, is the hexagonal ring with occasional help from the pentagonal ring. An interesting fact of 3D geometry is that any plane of regular hexagons (that are, of course, tessellating) can be bent into three dimensions and form a closed surface by adding just 12 pentagons to the structure.