A good scientific theory should enable scientists to make predictions. According to VSEPR theory the electron domain geometry around the central oxygen atom should be tetrahedral made up of three bonding pairs and one non-bonding pair of electrons so that the actual shape of the ion is trigonal pyramidal. Apart from the overall positive charge this is the same as ammonia.  The prediction therefore is that the bond angle will be less than 109.5^o and about the same as ammonia which is 107^o. However information from infrared studies have shown that the bond angle in the gaseous phase is actually 113.6^o  which brings into question the validity of VSEPR theory.

A continuous variable is a variable whose value is obtained by measuring whereas a discrete variable is a variable whose value is obtained by counting. Continuous variables (which are used most often in chemistry) can have an infinite number of possible values between two intervals, for example the volume of a gas evolved during a reaction. Discrete variables have separate indivisible categories so that no value can exist between neighbouring categories. This means that you cannot interpolate between the intervals so a best fit line is meaningless. Note that the x-axis is actually linear if atomic number (rather than group) is used as the value increases by one for each interval.

The first synthetic thermoplastic polymer was a polyamide called nylon discovered in 1935 by Wallace Carothers. It was made by reacting hexanedioic acid, HOOC-(CH₂)₄-COOH with hexane1,6-diamine, H₂N-(CH₂)₆)-NH₂. Carothers suffered from severe depression and died in 1937 just before nylon became commercially available as a substitute for silk.

Wallace Carothers (1896-1937)

By the late 18^th century the bottom of some wooden merchant ships were lined with copper. It helped to prevent attack by worms and barnacles as the copper acted both as a barrier and as a poison. Before this practice valuable cargo was sometimes lost through ships sinking. These ships were often funded by a group of investors. Placing their money in ships that had copper bottoms was seen as a much more reliable investment.

Many chemicals that have been known for a long time have common names. CuSO₄.5H₂O is blue vitriol, Na₂CO₃.10H₂O is washing soda, NaNO₃ is Chile saltpetre and MgSO₄.7H₂O is Epsom salts. Now the IUPAC systematic way of naming compounds is used by scientists (e.g. magnesium sulfate heptahydrate) but their common names are still in use elsewhere. Epsom salts is named after a town in the UK where it occurs in the mineral water. It can be added to bath water to aid relaxation but taken internally it can cause diarrhoea. “If the bottom is falling out of your world, take Epsom salts and the world will fall out of your bottom”.

In 1865 Jules Verne described aluminium in a similar fashion (an early example of almost plagiarism?) as he chose it as the material for his rocket in his novel “De la terre á la lune” (From the Earth to the Moon).

"This valuable metal possesses the whiteness of silver, the indestructibility of gold, the tenacity of iron, the fusibility of copper, the lightness of glass. It is easily wrought, is very widely distributed, forming the base of most of the rocks, is three times lighter than iron, and seems to have been created for the express purpose of furnishing us with the material for our projectile."

Helium was discovered in 1886 in the spectral lines from the sun by the French astronomer Pierre Janssen and independently by the British astronomer Norman Lockyer who named it helium after Helios, the Greek god of the sun. It was confirmed as being present on Earth in 1895 when it was found emanating from an ore of uranium.

The reaction must be able to release a large amount of energy in a short space of time and the exhaust velocity must be high to provide the thrust which means the molar mass of the products should be as low as possible.

PCBs are persistent organic pollutants as they are very stable chemically with a long half-life so they persist in the environment. PCBs cause skin damage, liver disease, cancer and can affect cognitive ability in children.

This is an example of the language of chemistry. Normality measures concentration in gram equivalent weight per litre. Equivalent weight is the mass of a given substance which will combine with or displace a fixed quantity of another substance. Since sulfuric acid is a diprotic acid it contains two equivalents so a 1 mol dm⁻³ solution of H₂SO₄ has a concentration of 2N. Although still used in some industries, equivalent weights and normality ceased being used in schools in most countries in the 1970s. IUPAC does not recommend the use of normality as the definition of the equivalence factor may vary depending on the type of chemical reaction involved.