Matter in Motion: The Science of Chemical Reactions

Think about these two situations. First: you crush an ice cube into smaller pieces. Second: you leave a iron nail outside in the rain and it turns orange-brown with rust. Both situations involve change — but are they the same kind of change? In one, the substance is still water; it just looks different. In the other, something entirely new has been created. Your job in this lesson is to figure out exactly what makes those two situations so different at a molecular level.

Matter is made of atoms — tiny particles that are the building blocks of everything around you. In everyday life, atoms are bonded together into groups called molecules. A chemical reaction is a process in which the bonds between atoms break apart and re-form in a completely new arrangement, producing one or more new substances. The substances you start with are called reactants. The new substances produced are called products. Here is the key point: products have different properties from the reactants. They may look different, behave differently, or have a different chemical formula altogether. How do you know a chemical reaction has happened? Scientists look for evidence. There are four main signs: 1. Colour change — a new colour appears that cannot be explained by simply mixing colours (e.g. silver nitrate solution turns black when touched by skin). 2. Gas production — bubbles or a new gas are released (e.g. vinegar and bicarbonate of soda fizz vigorously). 3. Temperature change — the surroundings get notably hotter (exothermic reaction) or colder (endothermic reaction). Burning wood releases heat; dissolving ammonium nitrate in water absorbs heat, making the solution feel cold. 4. Formation of a precipitate — a solid appears inside a liquid where there was none before (e.g. mixing two clear solutions can produce a cloudy solid). A fifth clue is that the change is usually irreversible — you cannot easily get the reactants back. Physical changes vs chemical changes: In a physical change, no new substance is formed. The atoms stay bonded in the same molecules; only the shape, size, or state changes. Melting chocolate, tearing paper, and dissolving salt in water are all physical changes — the chocolate, paper, and salt still exist as chocolate, paper, and salt at the molecular level. In a chemical change, new substances with new properties are formed. Burning toast, digesting food, and photosynthesis are chemical changes. A real-world example you can observe: When you burn a piece of toast, the bread (made largely of starch molecules) reacts with oxygen in the air. New substances — carbon compounds and water vapour — are produced. The toast looks, smells, and tastes completely different. You cannot un-burn it. That irreversibility, combined with the colour change and heat release, is strong evidence of a chemical reaction.