So Hard Trivia Quiz

Mohs' Scale of Hardness for Minerals for reference:
Talc - 1
Gypsum - 2
Calcite - 3
Fluorite - 4
Apatite - 5
Feldspar - 6
Quartz - 7
Topaz - 8
Corundum - 9
Diamond - 10

Iron is one of the strongest metals but its hardness is only about five, sometimes a bit lower depending on its process of manufacture. Making iron into steel (by adding carbon) certainly makes the finished metal harder. A knife blade has a hardness rating of around 5.5 and is used to scratch substances to determine how hard it is. A knife blade will scratch anything less than 5.5 (e.g. apatite down) but will not mark anything harder (feldspar upwards).

Galvanising steel (adding zinc during manufacture) will harden steel to about 6.5 and adding chromium (itself 9 on the Mohs' Scale) to make stainless steel, will yield a hardness of up to 6.3. Case hardening, a complex process to make steel files, for example, can give this type of steel a hardness of up to 8.

The photo depicts five iron objects hinting that the hardness of iron on Mohs' Scale is five.

Gold and silver are both precious metals because they are malleable (pliable) and as such, they can be easily moulded into different forms as seen in jewellery. As such, both metals are quite soft with Mohs hardness of 2-2.4. Adding copper, palladium or nickel to form alloys will give different colours of gold as well as increase hardness. Sterling silver is 92.5% silver and 7.5% other metals, usually copper.

The photo depicts two coins indicating that these two metals have a Mohs hardness rating of 2

Fingernails are a biological substance comprised mainly of a protein called keratin, Whilst one of the hardest human substances, it is soft and has a Mohs hardness rating of 2.5. Fingernails are used in the 'scratch' test to determine the hardness of minerals (eg fingernails will scratch gypsum but not fluorite).

It was difficult to depict 2.5 fingernails so artistic license was invoked and rounded up to three fingernails.

Copper is harder than gold and silver but not by much. It has a Mohs hardness of three. However, it is malleable, meaning it can be drawn easily into a wire. Because of this property, and because copper is such a good conductor of electricity, it is used for electrical wiring in electrical generators and motors. It is also used in electronic goods, such as televisions, computers and radios. Copper also conducts heat well, so it is frequently used in radiators, air-conditioners and home heating devices.

The photo depicts three Canadian pennies indicating a Mohs hardness rating of 3. Copper pennies are also used in the "scratch test". However one needs to be careful here. The first Canadian and US pennies were 100% copper but in the 21st century, they were mainly steel or nickel in composition with a small amount of copper coating.

Sapphires and rubies are essentially the same mineral. They are both made of corundum and therefore have a hardness rating of 9. Rubies are red-coloured mainly because of the presence of traces of chromium. Sapphires are blue because they contain traces of iron and titanium.
Emeralds are beryl-based and, as such are a bit softer with a hardness rating of 8-8.5. The other precious stone, the diamond, is the hardest substance on the scale with a hardness rating of 10.

The photo depicts four sapphires and five rubies making nine precious stones.

There is a widely held view that glass is not hard as it breaks easily but this 'flaw' is because glass is brittle and has nothing to do with hardness. 'Ordinary glass' has a hardness rating of 5-5.5. "Hard-glass" is an acronym for borosilicate glass, like Pyrex (and laboratory glassware) and has a hardness rating of about 5.5. "Soft-glass" is an acronym for soda-lime glass and has a hardness rating of 5. Bottles and glass homeware are generally made from soda-lime glass.

Heat-strengthened glass is a bit harder and toughened glass is incrementally harder again. Laminated glass is the hardest glass of all. These latter two types of glass are used when safety is paramount, e.g. windscreens in automobiles.

Five glasses were shown in the photo to provide a hint.

Tungsten is a greyish-white lustrous metal, which is a solid at room temperature. Tungsten has the highest melting point and, with temperatures over 1650C, is the metal with the highest tensile strength. It has unparalleled corrosion resistance.

Tungsten carbide is a ceramic material composed of tungsten and carbon atoms. It is hard due to its unique atomic structure and covalent bonding between tungsten and carbon atoms. This results in an extremely hard, durable material. Only industrial diamonds can scratch tungsten carbide as it has a Mohs hardness scale of 9-9.5.

Pictured are nine tungsten carbide-tipped tools used for shaping and cutting other metals.

Candlewax is soft and pliable and is easily scratched with a fingernail. Its hardness is less than 1. Liquids have a hardness of zero. Candlewax is actually a generic term but, usually, the majority of candles are made from paraffin wax which melts at 46-68C , reflecting its variable composition. Soy wax will melt from around 49-82C, and beeswax melts between 62-65C.

Interestingly, when a candle is lit, many people believe that the wax is burned as it disappears rather than just melts into a puddle. However, candle wax does not burn directly - it needs a wick. As such a candle has two components: wax and wick. The wax is the candle's fuel. Wax cannot burn on its own, even when it is directly exposed to a flame. It needs a lit wick to begin the combustion (burning) process. When the wick is lit, the flame's high temperature melts the wax at the base of the wick, reducing it into a liquid. The wick acts like a straw, drawing the liquid wax up into the flame by capillary action. The heat of flame causes the melted wax in the wick to vaporise, turning it into flammable gas vapour which in turn then combusts into a flame, releasing energy, in the form of light and heat, traces of soot and smoke, carbon dioxide and water particles. The overall reaction is the long-chain hydrocarbons of candlewax react with O2 to give CO2 + H2O.

The depiction of a single candle in the photo indicates a Mohs hardness rating of 1 for candlewax.

Number 5 on the Mohs Hardness Scale is apatite. It is a heterogeneous group of phosphate minerals. The main use of apatite is as a source of phosphate in the manufacture of fertiliser and similar industrial uses. It is occasionally used as a gemstone though, with a hardness of only five, there are difficulties in the gemstone manufacturing process. Apatite is one of a only few minerals produced and used by biological systems. Hydroxyapatite is the major component of tooth enamel, the outer coating of the teeth, and bone mineral. It is the hardest substance in the human body and, with a hardness rating of 5, is as hard as steel!

Just because it is hard does not mean it cannot break: Enamel can chip, often more easily than expected. In many, but not all of these cases, this is because the enamel had been demineralised or eroded making it prone to chopping. Enamel, once lost, unusually for the human body, cannot be regenerated.

The photo depicts five teeth hinting at its rating on Mohs Hardness Scale.

Porcelain is a ceramic material made by heating kaolinite, a specific clay, to temperatures between 1,200 and 1,400 C (2,200 and 2,600 F). The greater strength of porcelain, compared with other types of pottery, arises mainly from vitrification. In this process, the proportion of glassy bonds increases and the lattice-like structure of the raw mineral is lost and the apparent porosity becomes progressively lower and hardness increases. This is the basis of all pottery being able to hold liquids. However, comparing three materials that undergo vitrification upon heavy heating (glass, ceramic and porcelain), porcelain had the lowest porosity and does not need a glaze to make it impervious to water. It is the hardest of the three and rated at 7 on the Mohs Hardness Scale.

Porcelain tiles, because of their hardness and smoothness make an ideal floor covering. They are made by clay powder being pressed under high pressure (e.g. 500 kg/cm2) into metal moulds and then fired at high temperatures. The thin tiles are usually then glazed making them even harder.

The photo depicts seven porcelain tiles reflecting their hardness rating.