Science 9 concepts: matter and chemical change

The words, first

The idea: Physical versus chemical is the first sorting question in this unit, and almost everything else depends on it.

Word	What it means
Pure substance	One kind of particle: an element (one kind of atom, like aluminium) or a compound (elements chemically joined, like water).
Mixture	Substances together but not joined, so they can be separated physically: a solution (salt water) or a mechanical mixture (granola).
Physical change / property	A change that leaves the same substance in a new form: melting, dissolving. A physical property can be observed without making a new substance: colour, melting point.
Chemical change / property	A change that makes new substances: burning, rusting. Also called a chemical reaction. A chemical property says how a substance reacts: whether it burns, for example.
WHMIS 2015	The labels and pictograms that mark hazardous products in Canadian workplaces and school labs.
Atom	The smallest particle of an element: a tiny nucleus of protons (positive) and neutrons (no charge), with electrons (negative) around it.
Atomic number	The number of protons. It decides which element an atom is.
Group / period	A column / a row of the periodic table. Elements in a group behave alike.
Ion	An atom that has gained or lost electrons, so it has a charge. Metals lose electrons (positive ions); non-metals gain them (negative ions).
Ionic / molecular compound	Ionic: a metal and a non-metal held together by opposite charges (table salt). Molecular: non-metals sharing electrons, forming molecules (water, sugar).
Conservation of mass	In a reaction, the total mass before equals the total mass after. Atoms are rearranged, never created or destroyed.
Exothermic / endothermic	Releases energy, so the surroundings warm / absorbs energy, so they cool.
Catalyst	Speeds up a reaction without being used up.

Physical or chemical, and staying safe

The idea: A chemical change makes a new substance. You cannot watch that happen, so you look for its evidence — and avoid the one trap.

Evidence of a chemical change: a new colour; a gas given off when nothing is boiling; a solid forming when two liquids are mixed (a precipitate); heat or light released or absorbed; a change that is hard to reverse.

Worked: a burning candle. Two changes happen at once. Wax melting near the flame is physical — it is still wax, and it hardens again. Wax burning at the wick is chemical — it makes carbon dioxide and water vapour and gives off heat and light. A candle question is usually checking that you name both.

The trap. Boiling water bubbles, and the bubbles are water vapour — still water. Bubbles count as evidence only when a new gas forms, as when baking soda meets vinegar and gives off carbon dioxide.

WHMIS 2015 pictograms are black symbols in a red-bordered diamond; the biohazard one is in a circle.

Pictogram	Means
Flame	Flammable: catches fire easily
Flame over circle	Oxidizer: makes other things burn more fiercely
Exploding bomb	Explosive, or can react violently
Gas cylinder	Gas under pressure
Corrosion	Burns skin and eyes, eats metal
Skull and crossbones	Small amounts can kill or seriously harm
Health hazard	Serious long-term harm, such as cancer
Exclamation mark	Less severe harm, such as skin irritation
Biohazardous infectious materials	Organisms or toxins that cause disease

Atoms and the periodic table

The idea: Four models of the atom in about a hundred years. Each kept what the evidence supported and changed what it did not.

Model	The picture	What led to it
Dalton, early 1800s	Tiny solid spheres, one kind per element	Compounds always form in fixed proportions
Thomson, 1897	Electrons stuck in a positive sphere, like raisins in a bun	He discovered the electron
Rutherford, 1911	A tiny, dense, positive nucleus in mostly empty space	Most particles fired at gold foil went straight through; a few bounced back
Bohr, 1913	Electrons in fixed energy levels, or shells	Each element gives off only certain colours of light

protons = atomic number · electrons = protons (neutral atom) · neutrons = atomic mass, rounded − atomic number

Worked. Sodium has atomic number 11 and atomic mass 22.99, so a sodium atom has 11 protons, 11 electrons and 23 − 11 = 12 neutrons. The ion Na⁺ has lost one electron: still 11 protons, now 10 electrons. Chlorine gains one instead, so Cl⁻ has 17 protons and 18 electrons.

Reading the table. Metals, on the left and middle, are shiny good conductors; non-metals, on the upper right, are dull and brittle. Group 1, the alkali metals, form 1+ ions; group 17, the halogens, form 1− ions; group 18, the noble gases, barely react. Science 10 picks this up and explains why the table has this shape.

Ionic and molecular compounds, and their names

The idea: Metal plus non-metal is ionic and named without prefixes. Non-metals only is molecular and named with them.

	Ionic	Molecular
Made from	A metal and a non-metal	Non-metals only
At room temperature	Solid, high melting point	Solid, liquid or gas; low melting point
Solution conducts?	Yes	No

Ionic names and formulas. Metal first, then the non-metal ending in -ide: NaCl is sodium chloride. The charges in the formula must add to zero:

aluminium oxide: Al³⁺ and O²⁻ → 2 × (+3) = +6 and 3 × (−2) = −6 → Al₂O₃

Some metals, such as iron, form more than one ion, and a Roman numeral says which: iron(III) oxide, Fe₂O₃, is the main substance in rust.

Molecular names. Prefixes count the atoms — mono- 1, di- 2, tri- 3, tetra- 4 — and the first element never takes mono-. CO₂ is carbon dioxide, CO is carbon monoxide, CCl₄ is carbon tetrachloride. Water (H₂O), ammonia (NH₃) and methane (CH₄) keep their common names.

On the test: identify the unknown. A white solid with a very high melting point dissolves, and the solution conducts. It is ionic: its ions are free to move in the water, and moving charges are a current. A sugar solution does not conduct, because sugar dissolves as whole molecules with no charge.

Chemical reactions: mass, energy and speed

The idea: Atoms are rearranged, never lost. What changes is which atoms are joined, whether energy comes out or goes in, and how fast it all happens.

methane + oxygen → carbon dioxide + water

That is combustion: a fast reaction with oxygen that releases heat and light. Corrosion is a slow reaction of a metal with its surroundings; rusting is iron + oxygen, with water present → iron(III) oxide.

Worked: the open-beaker trap. Baking soda and vinegar are mixed in an open beaker on a balance, and the reading falls from 150.0 g to 147.8 g. Nothing was destroyed: 150.0 − 147.8 = 2.2 g of carbon dioxide escaped into the room, and in a sealed bag the reading does not change. When mass seems to change, look for a gas getting in or out. Counting atoms says the same: 2H₂ + O₂ → 2H₂O has 4 hydrogen and 2 oxygen atoms on each side.

Exothermic and endothermic. Burning, rusting and iron-powder hand warmers release energy and feel warm. Baking soda and vinegar is endothermic: the mixture gets slightly colder as it takes in energy. A thermometer that rises during a reaction means exothermic; one that falls means endothermic.

Reaction rate. Particles must collide to react. Warmer particles move faster and collide more often and harder, which is why food keeps in a fridge. A higher concentration puts more particles in the same space. More surface area exposes more of a solid: a log burns slowly, but flour dust in the air can explode. A catalyst speeds things up without being used up — enzymes in your body, platinum in a car's catalytic converter.

An antacid tablet in water at five temperatures, three trials each. The fizzing is carbon dioxide from a reaction inside the tablet, so a shorter time means a faster reaction.

On the test. A graph like this brings questions about the investigation as well as the chemistry. Temperature is the manipulated variable and time the responding one, and every tablet must be whole: a broken one has more surface area and would react faster for a reason that has nothing to do with temperature. The toolkit Designing a fair test works through this data in full.

What costs marks

The idea: Six, and three of them are one word swapped for another.

Calling melting, dissolving or boiling a chemical change.
Prefixes on an ionic compound. CaCl₂ is calcium chloride, never calcium dichloride.
Saying mass was lost when a gas escaped.
Using the atomic mass as the number of protons. Protons come from the atomic number.
Reading flame over circle as “very flammable.” It means oxidizer.
Saying a catalyst gets used up.