Iridium
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40 free Iridium trivia questions with answers — science & nature quiz, new questions added Oct 2026.
Iridium is one of the rarest metals on Earth, and one of the strangest. It is astonishingly heavy, almost impossible to damage, and so hard to melt that for decades after its discovery nobody could do much with it. Most people have never seen a piece, yet it has quietly shaped the way we measure the world and the way we read the planet's past. This quiz gathers the best fun facts about iridium into multiple-choice questions. They cover how the element was discovered and named, where it sits among its neighbours, where on Earth it is found, and the surprising jobs it does in engines, hospitals, factories and spacecraft. There are also questions on the phone network that borrowed its name and on the thin layer of clay that turned geologists into detectives. The questions run from easy to tricky, so they suit curious beginners, chemistry students and quiz hosts looking for a science round with stories attached. Every answer comes with a short explanation.
30 of 40 questions with answers and explanations. Play the quiz
Q 01A thin layer of iridium-rich clay found in rocks around the world helped explain the disappearance of which animals?
Dinosaurs
Dinosaurs died out 66 million years ago, just when that clay was laid down. Asteroids carry far more iridium than Earth's crust, so the layer pointed to a giant impact.
Q 02Iridium is named after Iris, a winged messenger of the Olympian gods and the Greek goddess of what?
The rainbow
The rainbow suited an element whose compounds turned a striking variety of colours as its discoverer dissolved them. The pure metal itself is a plain silvery-white.
Q 03A golf-ball-sized cylinder of platinum and iridium, locked in a vault near Paris, defined which unit until 2019?
The kilogram
The kilogram was simply whatever that one cylinder weighed, for 130 years. Three separate keys were needed to open its vault, and a definition built on constants of physics replaced it.
Q 04Many long-life versions of which small car engine part are sold with a fine tip of iridium?
Spark plugs
Spark plugs lose a little metal with every spark, and iridium stands up to that erosion far better than most metals. The same toughness makes iridium plugs a favourite in aircraft engines.
Q 05A hard iridium tip, first fitted to gold in 1834, became the traditional wearing point of which writing instrument?
Fountain pen
Fountain pen makers still call the tipping material 'iridium', though a modern nib seldom contains any. Cheaper hard metals such as tungsten have taken its place.
Q 06The Iridium satellite phone network was named because its planned 77 satellites matched which property of the element?
Atomic number
Atomic number 77 means 77 electrons circling a nucleus, and the planners pictured their satellites circling Earth the same way. Only 66 turned out to be needed, but the name stayed.
Q 07The asteroid that spread iridium worldwide left a buried crater under the Yucatán Peninsula of which country?
Mexico
Mexico's crater takes its name from the nearby town of Chicxulub. Geophysicists hunting for oil first spotted it in the late 1970s, before anyone knew an impact site was missing.
Q 08Doctors place tiny sealed sources of radioactive iridium-192 inside the body to treat which disease?
Cancer
Cancer cells are killed by the source's gamma rays, and putting it inside or right beside a tumour cuts the dose that reaches healthy tissue. The technique is called brachytherapy.
Q 09Which American company, maker of the first handheld mobile phone, bankrolled the Iridium satellite network?
Motorola
Motorola turned out a new satellite every 4.3 days at the project's peak. The business still filed for bankruptcy in 1999, nine months after it opened, for want of subscribers.
Q 10Iridium is about twice as dense as which heavy metal, long used for fishing weights and roofing?
Lead
Lead feels heavy, yet a block of iridium the same size weighs about twice as much. Only one natural element is denser still, and by barely a tenth of one percent.
Q 11Iridium is the second-densest natural element, narrowly behind which metal found in the very same laboratory residue?
Osmium
Osmium edges ahead by about a tenth of one percent, a gap so slim that chemists long argued over which of the pair was denser. X-ray measurements of their crystals settled it.
Q 12Which English chemist discovered iridium in 1803, after helping to prove diamond and charcoal are one substance?
Smithson Tennant
Smithson Tennant had far more of the mysterious black residue to work on than the French chemists who had also noticed it. That let him pull two new elements out of it.
Q 13Iridium was found in the black sludge left when crude platinum dissolved in which gold-dissolving acid mixture?
Aqua regia
Q 21Iridium compounds give off the light in which display technology, found in premium smartphones and televisions?
OLED
OLED pixels shine when electricity excites their organic molecules. Molecules built around a heavy metal such as iridium can reach an internal efficiency of nearly 100 percent.
Q 22Which country owned the dead satellite that destroyed an Iridium craft in 2009, the first crash between two satellites?
Russia
Russia's Kosmos 2251, a military satellite out of service since 1995, hit Iridium 33 above Siberia at about 26,000 mph. The crash left more than 2,000 large fragments in orbit.
Q 23Which rocket company launched 75 second-generation Iridium NEXT satellites between 2017 and 2019?
SpaceX
Aqua regia, a blend of nitric and hydrochloric acids, eats gold and platinum yet cannot touch iridium. Earlier chemists had written off the leftover powder as graphite.
Q 14Which two-letter symbol marks iridium on the periodic table, where it sits just before platinum and gold?
Ir
Ir occupies the 77th place, in the sixth row, among the dense, unreactive metals that chemists group with platinum.
Q 15Iridium outranks every other metal for which property, the reason it is used in deep-water pipes?
Corrosion resistance
Corrosion resistance is iridium's great strength: acids do not attack it, not even the mixture that dissolves gold. It holds out at temperatures as high as 2,000 °C.
Q 16Which Nobel Prize-winning physicist led the team that in 1980 traced Earth's iridium-rich clay to an asteroid strike?
Luis Alvarez
Luis Alvarez took up the puzzle with his geologist son Walter and two nuclear chemists. His Nobel had nothing to do with rocks: it was for work on subatomic particles.
Q 17The iridium-rich clay behind the asteroid theory was discovered in a gorge outside Gubbio, a town in which country?
Italy
Italy's Bottaccione Gorge exposes an unbroken stack of ancient limestone, laid down layer by layer. An American geologist working there in the 1970s found one band unusually rich in iridium.
Q 18Iridium is 'siderophile', meaning it bonds readily with which metal and sank with it deep into the molten young Earth?
Iron
Iron dragged iridium down towards the planet's centre while Earth was still molten, which is why so little is left in the crust. Many space rocks still hold their full share.
Q 19Like gold, iridium is thought to be forged mainly in collisions between which ultra-dense stellar remnants?
Neutron stars
Neutron stars that merge fling out a storm of loose neutrons, which nuclei soak up faster than they can decay. That rapid capture builds elements far heavier than an ordinary sun can fuse.
Q 20Iridium catalysts drive the Cativa process, which makes the acid that gives which kitchen staple its sour tang?
Vinegar
Vinegar is dilute acetic acid, a chemical that industry makes by the million tonnes for glues, films and fabrics. The iridium route largely took over from an older one built on rhodium.
SpaceX won the job in 2010 with what was then the largest commercial launch deal ever signed. Every one of the new satellites rode a Falcon 9 rocket from the California coast.
Q 24The largest known reserves of iridium lie in the Bushveld Igneous Complex, a vast rock formation in which country?
South Africa
South Africa's Bushveld holds the world's largest reserves of all six platinum-group metals. Iridium is only ever a minor by-product, with just a few tonnes produced worldwide each year.
Q 25Which radioactive element, the fuel that powers space probes such as Cassini, is sealed inside iridium capsules?
Plutonium
Plutonium pellets give off steady heat, which a probe far from the Sun turns into electricity. The iridium-clad fuel modules are built to survive a launch-pad explosion or a high-speed re-entry.
Q 26Which NASA space telescope, launched from shuttle Columbia in 1999, focuses X-rays with mirrors coated in iridium?
Chandra
Chandra's mirrors are nested like barrels, because X-rays would simply be absorbed by a normal mirror. The rays reflect only when they graze a smooth surface at a very low angle.
Q 27Iridium crucibles survive the 2,000 °C melt used to grow artificial crystals of which gemstone for lasers?
Sapphire
Sapphire is grown by dipping a seed crystal into the molten oxide and drawing it slowly upward as it turns. Iridium's very high melting point lets the crucible hold that melt.
Q 28Iridium-coated electrodes endure the electrolysis of brine, the main industrial source of which yellow-green gas?
Chlorine
Chlorine is freed at an anode that has to survive both the brine and the gas itself. A thin coating of iridium oxide on titanium carries the current and speeds up the reaction.
Q 29Pipeline inspectors use gamma rays from iridium-192 like an X-ray machine to check which metal joints for hidden flaws?
Welds
Welds are checked by placing the source on one side of the joint and film on the other, so that hidden cracks and gaps show up in the image. Most industrial radiography is of this kind.
Q 30The platinum-iridium bar that defined the metre from 1889 gave way in 1960 to the light of which gas?
Krypton
Krypton gave science a metre that any laboratory could reproduce, by counting wavelengths of the gas's orange light. No one needed to compare rulers with a metal bar in France any more.