Chemistry in the kitchen
Cooking is chemistry you can eat.
- Chemical change: a new substance forms and it is hard to go back. Boiling an egg, baking bread, browning onions and toasting bread are chemical changes.
- Physical change: no new substance; it can often be reversed. Melting butter, dissolving sugar and freezing water are physical changes.
- Raising agents: baking soda gives off carbon dioxide gas when heated or mixed with acid, so cakes and dhokla rise.
- Keeping food safe: the fridge slows microbes; salt, sugar and vinegar (pickles) stop microbes growing.
- Pressure cooking: higher pressure lets water boil above 100 °C, so food cooks faster.
Chemistry for cleaning and health
How soap works
A soap particle is like a tadpole. Its head likes water. Its long tail likes oil and grease. The tails stick into the grease. The heads face the water. Rinsing carries the grease away in tiny balls called micelles.
Acids and bases at home
- Acids taste sour: lemon juice, vinegar, curd.
- Bases feel soapy: baking soda, soap, toothpaste.
- An acid and a base cancel each other: this is neutralisation. Antacids calm acidity; toothpaste protects teeth from acid made by mouth bacteria.
Other useful chemistry
- Medicines: painkillers, antibiotics and vaccines are made and tested by chemists and biologists.
- Materials: plastics, steel, glass, cement and fabrics are chosen for their properties.
- Fuels: LPG, petrol and coal burn with oxygen to give heat: combustion.
- Safety: never mix bleach with acids or ammonia cleaners; read warning symbols on labels.
Physics in sport and around us
- Force: a push or pull. A kick or a bat hit changes the speed and direction of a ball.
- Gravity pulls the ball down, so its path is a curve (a projectile path). A launch angle near 45° gives the longest flight if air resistance is small.
- Friction: spikes on running shoes give grip; ice skates have very little friction.
- Air resistance: cyclists and swimmers make their bodies streamlined to go faster.
- Spin: a spinning ball pushes air to one side and curves the other way (the swing of a cricket ball, a curling free kick).
- Energy: food energy becomes movement and heat in muscles.
Science in films, adverts and media
Films, adverts and videos often use science words. Some use them well. Some use them only to sell.
Check a science claim in 4 questions
- Who says it? A scientist in that field, or someone selling a product?
- Where is the test? Was there a fair test with many people and a control group?
- Can others repeat it? Good results are repeated by other teams.
- Does it sound too good? "Grow 10 cm in a month!" is a warning sign.
Films bend physics for fun: in space there is no sound, and real cars rarely explode when they crash. Spotting these is a good way to practise science thinking.
Try it: a science hunt at home
- Fizz test: put one spoon of baking soda in a glass. Add a few spoons of vinegar or lemon juice. Watch the bubbles (carbon dioxide).
- Soap test: put a drop of oil in water. Shake. Now add soap and shake again. What happens to the oil?
- Change hunt: list five changes in your kitchen today. Mark each as physical or chemical.
- Claim check: find one advert with a science claim. Ask the 4 questions above.
Key formulas and definitions
- Chemical change: new substance forms, hard to reverse (cooking, rusting, burning)
- Physical change: no new substance, often reversible (melting, dissolving)
- Acid + base → salt + water (neutralisation)
- Vinegar + baking soda → carbon dioxide gas + water + salt
- Soap: water-loving head + oil-loving tail → grease washed away
- Force changes speed or direction; gravity pulls things down
- Good claim = fair test + evidence + repeated by others
Worked examples
1. Is making toast a physical or chemical change? Why?
Chemical. The bread turns brown and tastes different because new substances form. You cannot turn toast back into bread.
2. Why does water alone not remove oil from a plate, but soapy water does?
Oil and water do not mix. Soap tails stick into the oil and the heads stay in water, so the oil is pulled off in small balls and rinsed away.
3. An advert says a cream makes skin "100 % germ-free for a week". What should you ask?
Who tested it? Was there a fair test with a control group? Did other labs repeat it? A full week of zero germs sounds too good to be true.
Common mistakes
- Calling every change a chemical change. Melting ice and dissolving sugar are physical changes.
- Thinking "chemical" always means harmful. Water, salt and the food we eat are all chemicals.
- Believing a claim because it uses science words or a person in a lab coat. Look for the test and evidence.
- Mixing cleaners to make them "stronger". Bleach with acid or ammonia gives off poisonous gas.