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Chemistry
Reactions & Everyday Chemistry
What happens when substances meet, and why.
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What is the chemical formula for water?
Everyday Chemistry Explained
You do chemistry every time you cook, clean or open a packet. These ten questions are about the reactions that turn up in an ordinary day.
About this topic
Household chemistry has a habit of being more interesting than it looks. Ice floating seems unremarkable until you realise almost every other substance sinks in its own liquid, and that ponds freezing from the top down is the only reason things survive winter underneath them. Bread going hard is not the bag failing to keep the moisture in, it is starch quietly rearranging itself. None of the ten answers here need a laboratory. They need a kettle, a fruit bowl and enough curiosity to ask why the thing in front of you is behaving the way it is.
Questions & answers
Everyday Chemistry Explained: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1Baking soda makes a cake rise by releasing which gas?
- A Oxygen
- B Hydrogen
- C Steam alone
- D Carbon dioxide Correct
Answer: D. Carbon dioxide
Why: Sodium bicarbonate reacts with an acid in the batter, such as buttermilk or lemon juice, and the bubbles it produces get trapped as the cake sets. Baking powder is the same thing with a dry acid already mixed in, so it works without one being added.
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2Why does a cut apple turn brown?
- A Bacteria immediately colonise the cut surface
- B Water evaporates and stains the surface
- C An enzyme in the flesh reacts with oxygen in the air Correct
- D The sugars in the fruit caramelise
Answer: C. An enzyme in the flesh reacts with oxygen in the air
Why: Cutting the apple exposes an enzyme that reacts with oxygen to make brown pigments, a process called enzymatic browning. Lemon juice slows it down because the acidity and the vitamin C both interfere with the enzyme.
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3Soap lifts grease off a plate because each soap molecule has what?
- A Microscopic abrasive crystals
- B A water-loving end and an oil-loving end Correct
- C A strong positive charge along its whole length
- D An acidic core that dissolves fat
Answer: B. A water-loving end and an oil-loving end
Why: The oil-loving ends bury themselves in the grease while the water-loving ends face outwards, so the droplet can be carried away in the rinse water. The little spheres this forms are called micelles.
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4Which dissolved substances cause the limescale that builds up in a kettle?
- A Calcium and magnesium compounds Correct
- B Sodium and potassium salts
- C Iron and copper
- D Chlorine and fluoride
Answer: A. Calcium and magnesium compounds
Why: Water picks them up as it filters through chalk and limestone, which is why hard water areas map onto the underlying rock. Heating drives the calcium carbonate out of solution, so it settles on the hottest surface it can find.
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5Why does ice float on water?
- A Trapped air bubbles hold the ice up
- B Cold substances always rise above warmer ones
- C Surface tension supports it
- D Water expands as it freezes, so ice is less dense Correct
Answer: D. Water expands as it freezes, so ice is less dense
Why: Hydrogen bonds lock water molecules into an open hexagonal structure as they freeze, leaving ice about nine percent less dense than the liquid. Almost every other substance is denser as a solid, and ponds freezing from the surface down is a direct consequence.
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6Which compound makes chilli peppers taste hot?
- A Allicin
- B Menthol
- C Capsaicin Correct
- D Piperine
Answer: C. Capsaicin
Why: Capsaicin triggers the same nerve receptor that responds to actual heat, so the brain reports a burn that is not there. Piperine does a similar job in black pepper, and allicin is what gives garlic its bite.
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7Crisp packets are filled with which gas to keep the contents fresh?
- A Argon
- B Nitrogen Correct
- C Carbon dioxide
- D Oxygen
Answer: B. Nitrogen
Why: Nitrogen is cheap and unreactive, so displacing the air stops the fat in the crisps turning rancid. The inflated bag also cushions the contents on the way to the shop.
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8How does an antacid tablet work?
- A It neutralises excess acid in the stomach Correct
- B It coats the stomach lining in oil
- C It kills the bacteria that produce acid
- D It soaks up water from the stomach
Answer: A. It neutralises excess acid in the stomach
Why: Antacids are mild bases, often carbonates, that react with stomach acid and reduce how strong it is. The fizzing you get from some of them is carbon dioxide coming off in the reaction.
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9Why does bread go stale even in a sealed bag?
- A All the water escapes through the packaging
- B The yeast carries on fermenting
- C Oxygen slowly oxidises the flour
- D Its starch molecules gradually recrystallise Correct
Answer: D. Its starch molecules gradually recrystallise
Why: The process is called retrogradation: starch that was softened by baking reorganises into a firmer structure as it sits. Warming stale bread reverses part of it, and keeping bread in the fridge makes it go stale faster, not slower.
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10What makes your eyes sting when you cut an onion?
- A Pollen released from the bulb
- B The fibres irritating the eyelid
- C A sulphur compound released as a gas Correct
- D Fine droplets of acidic juice spraying out
Answer: C. A sulphur compound released as a gas
Why: Cutting ruptures cells and mixes an enzyme with sulphur compounds, producing a volatile gas that turns mildly acidic on contact with the moisture in your eyes. Chilling the onion first slows the enzyme down and makes the effect noticeably weaker.
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