Chemistry · Fundamentals of chemistry · Grade 7
Physical and chemical changes
Melting ice and rusting a nail: in which one is a new substance formed?
The situation
Look at what happens in a kitchen in one day: ice from the freezer melts, water boils in the kettle, sugar put into tea disappears from view. At the same time a cut apple turns brown, milk left on the table turns sour, a nail left in a damp place rusts, and sugar in a pan turns into brown caramel. In which of these does the substance stay the same, and in which does a completely new substance form? How can we tell?
Topic
Physical and chemical changes. Chemistry in the kitchen: ice melts and water boils (physical), sugar caramelises, an apple browns, milk sours, an iron nail rusts (chemical)
Key ideas
- Physical change
- Signs of a chemical reaction
- States of matter
Formulas
Physical change: ice → water → steam. The substance is the same — H₂O — only its state of matter changes.
Chemical change: the rusting of iron — a new substance forms from iron, oxygen and water.
Sign of a reaction — a precipitate: pale blue copper(II) hydroxide, which does not dissolve in water.
Sign of a reaction — heat: when 1 g of quicklime reacts with water, ≈ 1.15 kJ of heat is released.
Quantity of heat: to heat water by , ( = 4.18 J/(g·°C)).
Theory
Physical and chemical changes
In a physical change the substance stays the same — only its state, shape or size changes: ice melting, water boiling, sugar dissolving, glass breaking, butter melting. Often the process is easy to reverse: freeze water and you get ice again; evaporate sugar solution and the sugar comes back. In a chemical change (chemical reaction) new substances form from the original ones, with a different composition and different properties: iron rusts, sugar turns into caramel, an apple turns brown, milk turns sour, gas burns. In the “Physical or chemical?” mode, sort the eight kitchen changes yourself.
Signs of a chemical reaction
We know that a new substance has formed from these signs: a colour change (starch + iodine → dark blue; an apple turning brown), gas given off (baking soda + vinegar → bubbles of CO₂), a precipitate forming (CuSO₄ + NaOH → pale blue Cu(OH)₂), heat given out or absorbed (slaking lime), light given out (burning magnesium, gas, a candle), a smell appearing (sour milk, caramel). Be careful: a single sign does not always mean a reaction — bubbles in boiling water also look like gas, but they are water vapour, so that is a physical change.
States of matter
A substance can be solid, liquid or gas. In a solid (ice) the molecules vibrate at the lattice points; in a liquid they slide over one another; in a gas they fly about freely. When we heat ice and the temperature reaches 0 °C, it stays constant until all the ice has melted: the heat goes into breaking the bonds between the molecules (334 J to melt 1 g of ice). Water boils at 100 °C (at normal pressure); turning 1 g of water into steam takes 2257 J. A change of state is a physical change. For more detail, see “Melting, boiling and evaporation” and “Phase transitions” in the physics section.
Chemical processes in everyday life and the home
Rusting: iron rusts only when water and oxygen are present together; salt (road salt in winter, sea water) speeds it up. That is why iron objects are painted, oiled or coated with zinc. Dough rising: yeast fungi ferment sugar, forming carbon dioxide and alcohol — ; the CO₂ bubbles make the dough swell. Yeast is most active at about 30–35 °C, slows down in the refrigerator and dies at ≈ 55–60 °C. Similar processes: dairy products (yoghurt, cheese), compost rotting, baking flatbread (non) in a tandoor, fuel burning.
Practical: observing and describing
Observation plan: 1) what happened (how did the colour, state, volume, smell and temperature change)? 2) did a new substance form? 3) can the process be reversed? 4) conclusion — physical or chemical. In the “Everyday” mode, observe the rusting experiment for 7 days and the dough rising for 3 hours, and write the result in a log. Safety: do not taste substances; if alkali (NaOH) gets into your eyes, rinse at once with plenty of water; slaking lime can spit — wear safety goggles; do not look straight at burning magnesium; do not point the mouth of a vessel being heated at people.
About the model
The kitchen processes are shown sped up (time-lapse): the real duration is written on each change card; the temperatures of caramel and butter are approximate. For slaking lime, the heat released is assumed to warm only 50 g of water (the heating of the vessel and the lime, and evaporation, are ignored). In the heating curve the useful power of the hotplate is constant and no heat is lost; this mode runs 20 times faster than real time. The rates of rusting and dough rising are a qualitative model of the results of school experiments, not exact measurements. The box of molecules is a diagram.
Did you know?
- Rust is not a single substance: it is mainly a mixture of hydrated oxides and hydroxides of iron (Fe₂O₃·nH₂O, FeO(OH)). Because rust is porous and brittle, it does not protect the iron, whereas the thin oxide film on the surface of aluminium is dense and protects the metal from further oxidation.
- A cut apple turns brown because of the enzyme polyphenol oxidase: in the presence of oxygen from the air it oxidises phenols. Ascorbic acid (vitamin C) in lemon juice slows this reaction down — which is why lemon juice is sprinkled on sliced apples.
Inside the experiment
The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.
More experiments in this section
- The chemistry lab and safety — Grade 7
- Pure substances and mixtures — Grade 7
- Chemical formula and valency — Grade 7
- Mole, molar mass and volume — Grade 7
- From alchemy to the periodic law — Year 1