C. Chemistry: The Nature of Matter · Atoms, Elements and CompoundsPhysical and Chemical Properties and Changes
C2.6 A1.2 A1.5About 75 minutes
Learning goals- I can classify matter as an element, compound, mechanical mixture or solution.
- I can tell the difference between a physical property and a chemical property, and between qualitative and quantitative properties.
- I can calculate density and use it to help identify a substance.
- I can use evidence to decide whether a change is physical or chemical, and explain it with particles.
Words to know
physical property
A property you can observe or measure without changing the substance into something new, like colour, melting point or density.
chemical property
A property that describes how a substance reacts to form a new substance, like whether it burns or reacts with acid.
qualitative property
A property described in words, with no number, like 'shiny' or 'smells of vinegar'.
quantitative property
A property that is measured and has a number and a unit, like a boiling point of 100 °C.
density
The mass of a substance per unit of volume: density = mass ÷ volume. Often measured in g/mL or g/cm³.
physical change
A change in form or state where no new substance is made, like melting or dissolving.
chemical change
A change that produces one or more new substances with new properties; also called a chemical reaction.
precipitate
A solid that forms when two solutions are mixed and react.
Remember from earlier grades
In Grade 5 you sorted changes into physical and chemical changes. In Grade 7 you used the particle theory of matter, sorted matter into pure substances and mixtures, and met elements and compounds, with symbols like O and formulas like H₂O. In Grade 8 you measured density. This unit puts all of that together and goes inside the atom to explain why substances behave the way they do.
A mystery at the cottage
You find an old metal cube in a drawer at the cottage. Is it aluminium? Steel? Could it be silver? You can't just look it up. Chemists solve mysteries like this by studying properties: the features that describe a substance and help tell it apart from every other substance.
By the end of this lesson, you will be able to identify a mystery metal from its measurements, and tell whether a change you see is physical or chemical.
Sorting matter
All matter is either a pure substance (only one kind of particle) or a mixture (two or more kinds of particles mixed together, not chemically joined).
- An element is a pure substance made of only one kind of atom. It cannot be broken down into simpler substances by chemical means. There are 118 known elements.
- A compound is a pure substance made of two or more kinds of atoms chemically joined in a fixed ratio. Water is always H₂O: two hydrogen atoms for every oxygen atom.
- A mechanical mixture (heterogeneous mixture) has parts you can see, like trail mix.
- A solution (homogeneous mixture) looks like one substance all the way through, like salt water or air.
Physical and chemical properties
A physical property can be observed or measured without turning the substance into something new. Examples:
- colour, lustre (shininess), texture, odour
- state at room temperature (solid, liquid or gas)
- hardness, malleability (can be hammered into sheets), ductility (can be pulled into wire)
- solubility (how much dissolves in a solvent such as water)
- melting point, boiling point, density
- conductivity (whether it lets heat or electric current pass through)
A chemical property describes how a substance reacts to form a new substance. You can only observe it by trying the reaction. Examples:
- combustibility: does it burn? (paper does; glass does not)
- reactivity with acid: does it fizz with vinegar? (baking soda does; table salt does not)
- reactivity with water or oxygen: does it rust or tarnish? (iron rusts; gold does not)
- stability: does it break down when heated?
Properties can also be qualitative (words only: "shiny", "brittle") or quantitative (a number with a unit: "melts at 660 °C").
| Property of copper | Physical or chemical? | Qualitative or quantitative? |
|---|
| Reddish-brown and shiny | Physical | Qualitative |
| Melting point 1085 °C | Physical | Quantitative |
| Density 8.96 g/cm³ | Physical | Quantitative |
| Can be pulled into thin wire (ductile) | Physical | Qualitative |
| Slowly forms a green coating in moist air | Chemical | Qualitative |
Density: a property that identifies
The density of a pure substance is the same for any size of sample (at the same temperature). That makes it a great clue for identifying a substance.
density = mass ÷ volume, written D = m / V
For solids, density is often given in g/cm³; for liquids, in g/mL. Since 1 mL = 1 cm³, these are the same size of unit.
| Substance | Density (g/cm³) |
|---|
| Water (liquid, about 4 °C) | 1.00 |
| Aluminium | 2.70 |
| Iron | 7.87 |
| Copper | 8.96 |
| Silver | 10.5 |
| Lead | 11.3 |
| Gold | 19.3 |
Worked exampleExample 1: Identify the mystery cube
The metal cube from the cottage has a mass of 54.0 g. When it is lowered into a graduated cylinder, the water level rises from 50.0 mL to 70.0 mL. What metal could it be?
- Find the volume by displacement: 70.0 mL − 50.0 mL = 20.0 mL, which is 20.0 cm³.
- Use the formula: D = m / V = 54.0 g ÷ 20.0 cm³ = 2.70 g/cm³.
- Compare with the table: 2.70 g/cm³ matches aluminium.
- Check that the answer makes sense: aluminium is a light metal, and the cube is fairly light for its size.
The cube is most likely aluminium (D = 2.70 g/cm³). To be more sure, a chemist would check a second property, such as its melting point or whether it is attracted to a magnet (aluminium is not).
Physical and chemical changes
In a physical change, the substance may change shape, size or state, but it is still the same substance. Melting ice, dissolving sugar, crushing a can and boiling water are all physical changes. The particles stay the same; only their arrangement or spacing changes.
In a chemical change (a chemical reaction), atoms are rearranged to make one or more new substances with new properties. Burning wood, baking a cake, rusting iron and an egg cooking are chemical changes.
Evidence of a chemical change
You cannot see atoms, so you look for evidence. Any one clue can be misleading, so look for more than one.
- Gas bubbles form (and the liquid is not boiling).
- A new colour appears.
- A precipitate (a solid) forms when two liquids are mixed.
- Energy is given off or taken in: the mixture gets hot or cold, or gives off light.
- A new odour appears.
- The change is difficult to reverse.
Careful: bubbles also appear when water boils, and colour changes when you mix paints. Those are physical changes. Ask yourself: is there a new substance?
Worked exampleExample 2: Physical or chemical?
Classify each change and give your evidence: (a) butter melts in a hot pan; (b) the same butter turns brown and smells nutty if you keep heating it; (c) an antacid tablet fizzes in water.
- (a) Melting is a change of state. The butter is still butter and becomes solid again in the fridge. Physical change.
- (b) A new colour and a new odour appear, and you can't turn browned butter back into plain butter. Chemical change.
- (c) Gas bubbles form in cold water, which is not boiling. The tablet's ingredients are reacting. Chemical change.
(a) physical; (b) chemical; (c) chemical. Each decision is backed by evidence, not a guess.
Safety first
Lab safety starts now (A1.5). In any investigation: wear eye protection (safety glasses or goggles), tie back long hair, never taste anything, smell only by wafting (waving the air toward your nose), label every container, and clean up spills right away. Ask a grown-up before using any product from under the sink.
Hands-on activityKitchen chemistry: properties of four white powders
You need: a grown-up helper, safety glasses or sunglasses, 1/4 teaspoon each of table salt, sugar, baking soda and flour, 4 small clear cups or glasses, a spoon, water, white vinegar, a dark piece of paper, a magnifying glass if you have one, and masking tape and a pen for labels.
Safety: wear eye protection, do not taste anything, and wipe up spills. Vinegar can sting your eyes.
- Label four spots on the dark paper and put a small pinch of each powder on its label. Record physical properties: colour, texture, crystal shape (use the magnifier).
- Put 1/4 teaspoon of each powder in its own labelled cup. Add 2 tablespoons of water and stir 20 times. Record solubility: does it disappear, partly dissolve or make the water cloudy?
- Rinse and dry the cups. Put a fresh 1/4 teaspoon of each powder in its cup and add 1 tablespoon of vinegar. Record any evidence of a chemical change.
- Make a data table with columns: powder, colour, texture, dissolves in water?, reaction with vinegar?
- Conclusion: which test could tell baking soda apart from the other three? Was that test checking a physical or a chemical property?
What to look for: salt and sugar both dissolve well (sugar crystals look like small blocks too, so look closely); flour makes the water cloudy and does not dissolve; baking soda dissolves too, but more slowly, and the water may look a little cloudy at first. Only baking soda fizzes with vinegar: that is a chemical property, and the bubbles are a new substance (carbon dioxide gas).
Just for funA chemist walks into a bakery and asks, "Is your bread made by a physical change or a chemical change?" The baker says, "Both. I knead it, then I need the oven." Mixing dough is physical; baking it is chemical.
Your turn
1Classify each as an element, compound, mechanical mixture or solution: (a) helium, (b) carbon dioxide, (c) apple juice (filtered, clear), (d) a bowl of cereal and milk.
(a) element (one kind of atom); (b) compound (carbon and oxygen joined); (c) solution (looks the same throughout); (d) mechanical mixture (you can see the parts).
2Is "aluminium melts at 660 °C" a physical or chemical property? Is it qualitative or quantitative?
Physical (melting does not make a new substance) and quantitative (it has a number and a unit).
3Is "wood burns in air" a physical or chemical property?
Chemical. Burning makes new substances (carbon dioxide, water vapour, ash).
4A rock has a mass of 39.0 g and a volume of 15.0 cm³. What is its density?
D = m / V = 39.0 ÷ 15.0 = 2.60 g/cm³.
5Name four pieces of evidence that a chemical change may have happened.
Any four of: gas bubbles (not from boiling), a new colour, a precipitate forms, heat or light given off or taken in, a new odour, hard to reverse.
6Sugar dissolving in tea: physical or chemical change? Why?
Physical. The sugar molecules spread out among the water molecules but are still sugar; if you evaporate the water, the sugar is left behind.
7Why is it safer to waft than to sniff a substance directly?
Wafting brings only a small amount of vapour to your nose, so you can detect an odour without breathing in a big dose that could irritate or harm you.
More practice: warm-up
1Is colour a physical or chemical property?
2What is the formula for density?
D = m / V (density equals mass divided by volume).
3Is an egg frying a physical or a chemical change?
Chemical. The egg changes colour and texture and cannot change back.
4Is air a pure substance or a mixture?
A mixture (a solution of gases, mostly nitrogen and oxygen).
5Give one example of a qualitative physical property of ice.
For example: it is colourless/clear, hard, slippery, or solid at −5 °C.
More practice: on your own
1A 25.0 mL sample of a clear liquid has a mass of 19.75 g. What is its density, and could it be pure water?
D = 19.75 ÷ 25.0 = 0.790 g/mL. Water's density is 1.00 g/mL, so this liquid is not pure water. (Its density is close to that of ethanol, 0.789 g/mL, but you would need more tests to be sure.)
2A gold ring has a volume of 0.50 cm³. What mass should it have if it is pure gold (19.3 g/cm³)?
m = D × V = 19.3 × 0.50 = 9.65 g (about 9.7 g).
3When you mix two clear solutions, the mixture turns cloudy white and a solid settles to the bottom. What is the solid called, and what kind of change happened?
The solid is a precipitate. A new substance formed, so it is a chemical change.
4Explain using particles why melting chocolate is a physical change.
The particles in the chocolate are the same before and after; heating only lets them move more freely and slide past one another. No new particles form, and the chocolate becomes solid again when it cools.
5Why do scientists usually test more than one property to identify a substance?
Different substances can share one property (many metals are shiny and grey; some liquids have similar densities). Testing several properties narrows it down to only one substance that matches all of them.
Challenge
1A student says, "Bubbles formed, so it must be a chemical change." Describe a situation where she would be wrong, and one where she would be right.
Wrong: water boiling makes bubbles of water vapour, but the water is still water (physical). Also wrong: opening a pop bottle releases dissolved carbon dioxide that was already there. Right: baking soda and vinegar make bubbles of new carbon dioxide from a reaction (chemical). The key question is whether a new substance formed.
2Two cubes look identical. Cube A: 8.0 cm³, 21.6 g. Cube B: 8.0 cm³, 63.0 g. Identify each metal using the density table.
A: 21.6 ÷ 8.0 = 2.70 g/cm³ → aluminium. B: 63.0 ÷ 8.0 = 7.875, about 7.9 g/cm³ → iron (7.87 g/cm³). Same size, but very different masses, because their densities differ.
3A copper statue slowly turns green, while a stainless-steel sculpture beside it stays shiny. What chemical property do the two metals differ in, and why might an artist care?
They differ in reactivity with air and moisture (how easily they corrode). Copper reacts to form a green coating; stainless steel resists this. An artist would choose based on the look they want over time and how much upkeep the sculpture will need.
Quiz
1. Which of these is a chemical property?
2. Which is a quantitative physical property?
3. A 10.0 cm³ block has a mass of 113 g. What is its density?
4. Which change is physical?
6. Which observation is the strongest evidence of a chemical change?
Big idea: expectation C2.6 asks students to investigate and describe the physical and chemical properties of elements and compounds. This lesson builds the vocabulary for the whole unit and practises density calculations, which Grade 9 students are expected to do confidently.
Common difficulties:
- Thinking every colour change or bubble means a chemical change. Ask, "Is there a new substance?"
- Mixing up a chemical property with a chemical change. A property describes what a substance can do ("wood can burn"); a change is what is happening ("the log is burning").
- Dividing the wrong way for density. Mass goes on top: D = m ÷ V. Check units: g/cm³ means grams per cubic centimetre.
How to help: when cooking, ask "physical or chemical?" (melting butter, toasting bread, dissolving salt). Supervise the vinegar activity; vinegar is safe but stings eyes. Model safe habits: no tasting, wafting, labels.