Changes in Everyday Life

Introduction

You do not need a laboratory to see physical and chemical changes: your kitchen, garden and street are full of them. Ice melts in a drink, bread rises and browns in the oven, an apple turns brown when cut, a stone wall slowly crumbles. This page applies everything in the unit — the signs of chemical change, reversibility and the particle model — to changes you meet every day.

Core explanation

The key question. For any change, ask: has a new substance been formed? If yes, it is a chemical change; if no, it is physical. Signs such as colour change, gas production, a precipitate, energy change or a new smell are clues, but each must be checked, because some physical changes show them too.

In the kitchen.

Change Type Evidence --- --- --- Melting butter Physical Same substance, change of state; solidifies again on cooling Dissolving sugar in tea Physical Sugar can be recovered by evaporating the water Boiling water for pasta Physical Steam condenses back to water Frying an egg Chemical Proteins change permanently; clear white becomes opaque Toasting bread Chemical Browning, new flavours and smells; cannot be untoasted Baking a cake Chemical Baking powder releases carbon dioxide; browning; texture changes Cutting an apple that goes brown Chemical Substances react with oxygen in air to form brown compounds Souring milk Chemical Bacteria make lactic acid; sour smell and curdling

Baking powder. Baking powder contains sodium hydrogencarbonate and a solid acid. When wet and heated, they react to release carbon dioxide , which forms bubbles that make cakes rise. Heat also causes thermal decomposition of the hydrogencarbonate, releasing more gas.

Yeast and fermentation. In bread dough, yeast ferments sugar to form ethanol and carbon dioxide. The gas makes the dough rise; the ethanol evaporates during baking.

Outdoors.

- Freeze–thaw weathering is physical: water in cracks expands by about 9% as it freezes, gradually splitting rocks. No new substance forms. - Chemical weathering occurs when slightly acidic rainwater reacts with limestone, forming soluble compounds that are washed away. - Rusting of iron railings and tarnishing of silver are chemical changes involving oxygen or other substances in the air. - Leaves changing colour in autumn is chemical: green chlorophyll breaks down, revealing other pigments.

Changes working together. Many processes combine both types. In cooking, water evaporates (physical) while proteins and sugars react (chemical). In a burning candle, wax melts and vaporises before it burns.

Step-by-step reasoning

To classify an everyday change:

1. Identify the substances before and after. 2. Ask whether a new substance has formed — check colour, smell, gas, energy and texture. 3. Check whether the change is easily reversed. 4. Decide: new substance → chemical; same substance in a new form → physical. Note if both occur.

Visual explanation

Imagine a kitchen split down the middle. On the left, labelled "physical", are an ice cube tray, a melting chocolate bar and a glass of salty water. On the right, labelled "chemical", are a slice of toast, a fried egg, a rising loaf and a browned apple. A cake in the oven sits on the dividing line, with arrows to both sides.

Real-world analogy

Classifying changes is like sorting a wardrobe. A physical change is like folding a shirt a new way — it is still the same shirt. A chemical change is like unpicking the shirt and sewing the fabric into a completely different garment.

Real-world example

Lemon juice is often squeezed onto sliced apples and avocados to stop them browning. The browning is a chemical change caused by substances in the fruit reacting with oxygen, helped by enzymes. The acid and antioxidants in lemon juice slow this reaction, keeping the fruit looking fresh for longer.

Why?

Why can melted chocolate be set again but a baked cake cannot turn back into batter? Melting only moves chocolate particles further apart and lets them flow; the substances remain the same. Baking forms new substances through irreversible reactions, so the original ingredients no longer exist.

Common misconception

"Any change that cannot be reversed must be chemical." Some physical changes are hard to reverse in practice, such as breaking a glass or grinding coffee beans, yet no new substance forms. Irreversibility is a clue, not a proof.

Worked example

Question: When sugar is heated gently it melts; when heated more strongly it turns brown and smells of caramel. Classify each stage.

Reasoning: Melting leaves sugar as sugar in liquid form. Browning and a new smell show new substances forming.

Answer: Melting is a physical change; caramelisation is a chemical change.

Quick check

1. Is dissolving salt in water a physical or chemical change? Answer: Physical, because the salt can be recovered by evaporating the water.

Exam focus

Exam questions often give a list of everyday changes to classify. Always give a reason linked to whether a new substance is formed, not just "it changes colour". Watch for tricky cases such as dissolving, breaking glass and freeze–thaw weathering, which are physical.

Advanced insight

The browning of toast and roast food is due to the Maillard reaction , a complex set of reactions between sugars and amino acids above about 140 °C that produces hundreds of flavour and colour compounds. It is different from caramelisation, which involves sugars alone. Chefs control temperature carefully to manage these chemical changes.

Summary

Everyday life is full of physical changes (melting, dissolving, boiling, freeze–thaw weathering) and chemical changes (cooking, baking, browning, souring, rusting, chemical weathering). Classify each by asking whether a new substance forms, using signs of change as evidence. Many familiar processes involve both types at once.

Practice questions

1. Classify frying an egg and give a reason. Answer: Chemical; the proteins change permanently into new substances and the white turns from clear to opaque. 2. Which gas makes bread and cakes rise? Answer: Carbon dioxide. 3. Explain why freeze–thaw weathering is a physical change. Answer: Water expands on freezing and splits the rock, but no new substance is formed. 4. Why is a cut apple turning brown a chemical change? Answer: Substances in the apple react with oxygen from the air to form new brown compounds. 5. Name one everyday process that involves both physical and chemical changes. Answer: Baking a cake (water evaporates while ingredients react), or a burning candle.