Citric Acid
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40 free Citric Acid trivia questions with answers — science & nature quiz, new questions added Oct 2026.
Citric acid is one of those ingredients almost everyone eats every day and almost nobody thinks about. It gives citrus fruit its tang, sharpens fizzy drinks and sweets, and turns up on labels in every aisle of the supermarket. Its story runs from an eighteenth-century laboratory to giant modern fermentation tanks. This quiz collects the best fun facts about citric acid. Expect questions on how it was first isolated and how it is manufactured now, the jobs it does in the kitchen, the bathroom and the medicine cabinet, and the famous cycle of reactions that carries its name in every biology textbook. It is pitched at curious general players, students revising chemistry or biology, and teachers who want a ready-made round. Every answer comes with a short explanation, so you leave knowing why each fact is true.
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Q 01Most of the world's citric acid is not squeezed from fruit but made by feeding sugar to which organism?
A black mould
A black mould, Aspergillus niger, is the workhorse of the citric acid industry. The same species causes the sooty rot sometimes found on onions and grapes.
Q 02Which American drug company, later known for a COVID-19 vaccine, owed its early growth to making citric acid?
Pfizer
Pfizer grew on citric acid from the 1880s. Its later skill at fermenting the acid in deep tanks was the same know-how it used to mass-produce penicillin in the 1940s.
Q 03The citric acid cycle, which releases energy from food inside cells, is also named after which Nobel laureate?
Hans Krebs
Hans Krebs identified the cycle in 1937 and won the Nobel Prize in Physiology or Medicine in 1953, sharing it with Fritz Lipmann.
Q 04Which toiletry, invented by Lush co-founder Mo Constantine, relies on citric acid reacting with baking soda?
Bath bombs
Bath bombs were invented in 1989 in a shed in Dorset. Dropped in water, the acid and the soda react and release a stream of gas bubbles.
Q 05Which famous impotence drug, approved in 1998, contains its active ingredient sildenafil as a salt of citric acid?
Viagra
Viagra is sildenafil citrate. The compound was first studied as a treatment for high blood pressure and angina before its more famous effect showed up in trials.
Q 06Which raw seafood dish relies on the citric acid in citrus juice to turn fish firm and opaque without heat?
Ceviche
Ceviche is the national dish of Peru. The acid denatures the proteins in the fish, changing their structure much as cooking would.
Q 07Which remedy, launched in 1931, combines citric acid with aspirin and baking soda in a single tablet?
Alka-Seltzer
Alka-Seltzer fizzes because its citric acid and baking soda react as soon as they hit water, releasing a rush of gas bubbles.
Q 08Sodium citrate, a salt of citric acid, is added to donated blood to stop it from doing what?
Clotting
Clotting needs calcium, and citrate binds the calcium ions in blood so the process cannot begin. That lets donated blood be kept in storage.
Q 09Which lemon-lime soda was once named a 'Lithiated Lemon Soda' because it claimed to contain lithium citrate?
7UP
7UP was marketed as a hangover cure, and lithium citrate is a real mood-stabilising drug. The US government made the maker drop its health claims in 1936.
Q 10Citric acid was first used to separate the rare-earth metals by ion exchange in which secret 1940s wartime programme?
Manhattan Project
Manhattan Project chemists used citric acid to wash the lanthanides apart on ion-exchange columns. The far more efficient EDTA replaced it in the 1950s.
Q 11Which nutrient, found alongside citric acid in oranges and lemons, is a separate compound known as ascorbic acid?
Vitamin C
Vitamin C and citric acid together give citrus fruit its sharp taste. Only the vitamin protects against scurvy, the disease oranges and lemons cured in a 1747 naval trial.
Q 12Citric acid descales kettles by dissolving the chalky crust left behind by which type of water?
Hard
Hard water picks up calcium and magnesium as it seeps through limestone and chalk. Heating leaves the minerals behind as limescale, which the acid then dissolves.
Q 13Sipping drinks rich in citric acid day after day for years can wear away which part of the body?
Tooth enamel
Q 21Before fermentation took over, the citric acid industry relied on citrus fruit from which Mediterranean country?
Italy
Italy supplied the juice, which was treated with lime to make calcium citrate. Sulfuric acid then turned that salt back into pure citric acid.
Q 22Fermentation-made citric acid became big business only after which conflict disrupted Europe's citrus exports?
World War I
World War I cut manufacturers off from their supply of citrus-derived raw material, and a sugar-based fermentation process filled the gap within a few years.
Q 23Powdered citric acid, a kitchen stand-in for vinegar or lemon juice, is sold in groceries under which nickname?
Sour salt
Tooth enamel erodes under repeated acid attack, and how often a person sips matters more than the total amount drunk. Fruit juice counts, however healthy it seems.
Q 14In human cells, the citric acid cycle runs inside which structures?
Mitochondria
Mitochondria, often nicknamed the powerhouses of the cell, host the cycle in their innermost compartment, the matrix. Bacteria lack these structures and run the same reactions in their cytosol.
Q 15Which waste gas is released as the citric acid cycle breaks down food molecules?
Carbon dioxide
Carbon dioxide leaves the cycle twice on every turn, as each two-carbon acetyl group fed in is stripped down for its energy.
Q 16The citric acid cycle helps cells build which molecule, often described as the cell's energy currency?
ATP
ATP carries the energy the cycle releases. In animals and other higher organisms, the cycle is the source of about two-thirds of the energy drawn from food.
Q 17The biochemist who discovered the citric acid cycle fled which country in 1933 after losing his job for being Jewish?
Germany
Germany barred him from his post under a new Nazi civil service law. Within months he was working in Cambridge on a Rockefeller Foundation grant.
Q 18Which word do chemists use for acids like citric acid that only partly split into ions in water?
Weak
Weak describes how completely an acid ionises, not how much of it is in the bottle. Vinegar's acetic acid belongs to the same class.
Q 19By 2018, which country was producing more than half of the world's manufactured citric acid?
China
China made more than half of a global output above two million tons. Over half of all citric acid ends up in drinks as an acidity regulator.
Q 20Citrate medicines such as potassium citrate are prescribed to prevent which painful mineral build-up in the body?
Kidney stones
Kidney stones form less readily when urine carries more citrate, which also makes it less acidic. Calcium, uric acid and cystine stones are all targeted this way.
Sour salt earned its name because the white crystals look so much like table salt. Cooks use it when a dish needs pure acidity.
Q 24In the classic lemon battery experiment, the fruit's citric acid juice plays which role between the two metals?
Electrolyte
Electrolyte is the juice's job, sitting between a zinc nail and a copper coin that act as the electrodes. Lemons wired in a chain give enough voltage to light an LED.
Q 25In 1911, two Swiss inventors melted Emmental with sodium citrate to create which long-lasting dairy product?
Processed cheese
Processed cheese was born when Walter Gerber and Fritz Stettler found the citrate let melted cheese cool back into a smooth solid. Fondue was their inspiration.
Q 26Magnesium citrate, a salt of citric acid, is drunk before a colonoscopy because it acts as which kind of medicine?
Laxative
Laxative action comes from the salt drawing water into the intestine by osmosis. The extra water gets the bowel moving within half an hour to three hours.
Q 27Caffeine citrate, a medicine made with citric acid, is given to premature babies to help with which vital function?
Breathing
Breathing can stall in babies born too early, and the drug works by stimulating the brain's respiratory centres. It is on the WHO's List of Essential Medicines.
Q 28US food-safety guidance tells home canners to add citric acid or lemon juice to jars of which borderline-acid crop?
Tomatoes
Tomatoes sit on the border between acid and low-acid foods, so half a teaspoon of citric acid per quart is added to make sure the jar is safely acidic.
Q 29Each citric acid molecule can give up three of which particle when dissolved, the behaviour that defines an acid?
Protons
Protons leave the molecule one at a time, each less readily than the one before. Chemists call such an acid triprotic, and phosphoric acid is another example.
Q 30Industry uses citric acid as a safer alternative to nitric acid for passivating which common alloy?
Stainless steel
Stainless steel is bathed in citric acid to strip stray iron and rust from its surface while sparing the chromium. The acid is also far less toxic to handle.