92021 · Chemical reactions in context

• A chemical reaction rearranges particles

  In a chemical reaction the atoms you start with are the atoms you finish with - they are just bonded together differently. When magnesium burns, magnesium atoms and oxygen atoms stop existing as a shiny metal and a colourless gas and start existing as a white solid, magnesium oxide. Nothing is added and nothing vanishes. This is why a chemical reaction always produces a new substance with new properties, and why a physical change like melting or dissolving is not a reaction: melting ice rearranges how water molecules are packed, not what they are.

• Atoms, ions and molecules are different particles

  An atom is the smallest particle of an element. A molecule is two or more atoms joined by covalent bonds and behaving as one particle - O₂, H₂O, CO₂. An ion is an atom (or group of atoms) that has gained or lost electrons and therefore carries a charge: Na⁺, Mg²⁺, Cl⁻, NO₃⁻. Ionic compounds such as sodium chloride are built from ions in a repeating lattice rather than from separate molecules, which is why we write NaCl as a formula unit - the simplest whole-number ratio - not as a molecule.

• Elements, compounds and mixtures

  An element contains only one kind of atom (iron, oxygen, carbon). A compound contains two or more different elements chemically joined in a fixed ratio, so it has properties of its own - sodium is a reactive metal and chlorine a poisonous gas, yet sodium chloride is table salt. A mixture is substances physically together but not chemically joined, so the ratio can vary and the parts keep their own properties. Air is a mixture of N₂, O₂ and other gases; carbon dioxide within it is a compound.

• A formula carries information: subscripts and charges

  Subscripts in a formula tell you how many atoms of each element are in one unit of the substance. H₂O has two hydrogen atoms and one oxygen atom. Brackets multiply everything inside them, so Ca(NO₃)₂ has one calcium, two nitrogen and six oxygen atoms - nine atoms in all. For an ionic compound the formula comes from balancing the charges: magnesium ions are Mg²⁺ and chloride ions are Cl⁻, so it takes two chloride ions to cancel one magnesium ion, giving MgCl₂.

• Word equations first, symbol equations second

  Always start with the word equation: reactants on the left, an arrow meaning "turns into", products on the right. "Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide" says what happens without needing a single formula. Once the chemistry is right in words, replace each name with its formula and add state symbols - (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Doing it in this order stops you inventing formulae to make an equation balance.

• Balancing: change the coefficients, never the subscripts

  A balanced equation has the same number of atoms of every element on both sides. You achieve that by putting numbers (coefficients) in front of whole formulae, which means "this many of that particle". You may never change a subscript, because that changes what the substance is: 2H₂O is two water molecules, but H₄O₂ is not water at all. Work through one element at a time, leave oxygen and hydrogen until last where you can, and always re-count every element at the end.

• Conservation of mass

  The law of conservation of mass says the total mass of the products equals the total mass of the reactants, because the atoms are the same atoms. In a sealed container the balance reading never changes. In an open container it can appear to: heat limestone in an open kiln and the mass falls, because carbon dioxide gas escapes into the air; leave a nail to rust in the open and the mass rises, because oxygen from the air has joined on. Neither breaks the law - you simply have not weighed everything that took part.

• The main types of reaction

  Most reactions you meet belong to a family. Combustion: a fuel burns in oxygen, releasing energy - complete combustion of a hydrocarbon gives carbon dioxide and water. Neutralisation: an acid reacts with a base, giving a salt and water; with a carbonate you also get carbon dioxide. Metal with acid: gives a salt and hydrogen gas, never water. Precipitation: two solutions are mixed and an insoluble solid drops out. Thermal decomposition: heat alone breaks one compound into simpler ones. Corrosion: a slow oxidation, such as iron reacting with oxygen and water to form rust. Naming the family tells you what to expect the products to be.

• Observations are your evidence

  You cannot see atoms, so you argue from what you can see. Bubbles of a gas, a solid appearing from two clear solutions, a permanent colour change, a temperature rise or fall, a smell, or a mass change all suggest a new substance has formed. The strongest evidence names the new substance and says why the observation fits it - "the white solid is silver chloride, which is insoluble, so it settles out" is far better than "it went cloudy so a reaction happened". Be careful: bubbles can also come from a gas simply escaping from solution, and a colour change can be dilution, so one observation on its own is rarely conclusive.

• Rate is about collisions

  Particles must collide, and collide hard enough, before they can react. Anything that makes collisions more frequent or more energetic speeds the reaction up. Raising the temperature makes particles move faster, so collisions are both more frequent and more energetic. Increasing concentration (or gas pressure) packs more reacting particles into the same space, so collisions are more frequent. Breaking a solid into smaller pieces increases the surface area exposed, so more particles are available to be hit. A catalyst provides an easier route that needs less energy, and is not used up - it speeds the reaction up without changing how much product you finish with.

Internal · marked per part.

This is an internal achievement standard. Whetū does not offer a sit-down Exam paper for it. Learn and Practise stay available.

4 authored Learn units for this standard.

• Particles, atoms and ions

  Table salt on a chip, garden lime on a paddock, and the chlorine added to a school pool are all built from charged particles. This page is about those particles: how an atom of a metal or a non-metal becomes an ion, and how you write a formula by showing that the charges cancel. Every later page in this standard writes equations with those formulae. If the formula is guessed from the look of the letters, the equation is already wrong.

• Equations and conservation of mass

  A chemical change can be written three ways, and this standard expects you to move between them. A generic word equation names the classes: acid + base → salt + water. A chemical word equation names the actual substances: hydrochloric acid + magnesium hydroxide → magnesium chloride + water. A balanced chemical equation counts the atoms: 2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O. The three forms are the same event at three levels of precision.

• Types of chemical reaction

  This standard assesses five reaction types and no others: neutralisation, combustion, precipitation, combination and decomposition. The type is not a heading you copy from the question. It is a pattern you justify from what you see and from the products the equation is allowed to make. A sixth family — displacement, redox-as-a-type, metal-plus-acid as its own class — is outside the assessment, even when the chemistry is real.

• Reactions in real contexts

  A reaction written on a bench is not yet this standard’s object. Context here means a situation and environment beyond the laboratory, natural or human-made: a kiln making lime, a paddock being limed, a burner under a pot, a waterway carrying phosphate, a tool rusting in a shed. The chemistry has to be part of that situation, not a place-name stuck on a conical flask.

34 Practise questions in “Chemical reactions”. Feedback here is formative and is not an official NCEA grade.

• Particles and change

  Tell a chemical reaction from a physical change, using particles.

• Atoms, ions and compounds

  Name the particles and build a correct formula from charges.

• Word equations

  Write what goes in and what comes out, in the right order.

• Balancing equations

  Make every element match on both sides using coefficients only.

• Conservation of mass

  Explain an apparent mass change without losing any atoms.

• Types of reaction

  Recognise the reaction family from its reactants and products.

• Acids, bases and metals

  Predict the products when an acid meets a base, carbonate or metal.

• Rate of reaction

  Explain speed through the frequency and energy of collisions.

• Evidence and observations

  Judge which observation really shows a reaction has happened.

• Chemistry in context

  Apply the whole toolkit to a real New Zealand situation.

Sample questions

1. What happens to the atoms during a chemical reaction?
2. Which of these is a chemical change rather than a physical change?
3. Match each state symbol to what it tells you about the particles.
4. Which of these substances is a compound?
5. Match each particle term to its meaning.
6. What is the formula of calcium nitrate, and how many atoms are in one formula unit of it?
7. In this word equation, which substances are the reactants?
8. Write the word equation for this reaction, and name the gas being given off.

Practise 92021 in Whetū