Atoms, Molecules & Their Properties
The building blocks of matter — and the properties that tell substances apart
A diamond and the "lead" in your pencil are both pure carbon — nothing else. Yet one is the hardest natural substance on Earth and the other is so soft it smears onto paper when you write.
The difference isn't what they're made of. It's how the atoms are arranged. That is the first big idea of this chapter: a substance's properties come from its structure, not just its ingredients.
Atoms, Molecules and Compounds
Tell apart an element, a compound, and a molecule — and why a compound behaves nothing like the atoms inside it.
You already know that elements are pure substances made of one type of atom. But atoms don't always float around alone:
- Some elements exist as single atoms: sodium (), copper (), iron ()
- Some exist as diatomic molecules: hydrogen (), oxygen (), nitrogen ()
- Compounds form when atoms of different elements combine in a fixed, definite ratio
A compound behaves nothing like the elements it's made from. burns. supports combustion. Combine them in a 2:1 ratio and you get — water, which puts out fires.
Classification of Matter — Pure Substances and Mixtures
Sort any sample into element, compound, homogeneous or heterogeneous mixture, and tell them apart with the sampling test.
Every sample of matter is either a pure substance or a mixture, and that one split is the map for the whole subject.
A pure substance has a fixed composition and fixed properties. It comes in two kinds. An element cannot be broken into anything simpler by a chemical reaction — copper (), gold (), oxygen (). A compound has two or more elements chemically bonded in a fixed ratio — water (), common salt (), glucose (). You met both in the last section.
A mixture is two or more substances physically together, each keeping its own identity, in no fixed ratio — so it can be pulled apart by physical means. Mixtures split again by whether their composition is uniform:
- Homogeneous mixture — the same throughout; you cannot pick out the parts. Air and a salt solution are homogeneous.
- Heterogeneous mixture — the composition changes from place to place and the parts are distinguishable. Sand stirred into salt, or gravel, is heterogeneous.
How do you tell homogeneous from heterogeneous? Take small samples from different points and compare them.
Dissolve salt in water and you get a brine solution: scoop a sample from the top, the middle, the bottom — every sample is equally salty. Same composition everywhere, so it is homogeneous.
Stir sand or dirt into water instead. The top sample is nearly clear while the lower ones carry more grit, and it settles on standing. The samples differ, so it is heterogeneous.
Stoichiometric vs Non-Stoichiometric Compounds
Most compounds obey the law of definite proportions — their elements sit in one fixed, whole-number ratio by mass. Water is always , never . A compound like this is stoichiometric.
A few solids quietly break the rule. In a non-stoichiometric compound the ratio drifts slightly from the ideal formula because of defects in the crystal — missing atoms or extra ones. Iron(II) oxide is the classic case: written , real samples are closer to because some sites are vacant and a few ions keep the charge balanced. Cuprous oxide () and many high-temperature superconductors behave the same way. Their composition is genuinely variable, so a single clean formula only approximates them.
Physical vs Chemical Properties
Decide, for any change, whether a brand-new substance formed — the real line between physical and chemical.
Physical Properties
A physical property is something you can observe about a substance without changing what it is chemically. Colour, melting point, boiling point, density, solubility, pressure, volume — these are all physical properties.
When you observe these, the substance doesn’t become something else. Hold a copper wire under different lights and you see the same orange-brown gleam each time. Heat water in a kettle until it boils, then cool it all the way back to ice — at every point along the journey, it is still . The physical state changes from solid to liquid to gas, but the chemical identity stays put.
Melting an ice cube is the classic example. The cube goes from a rigid crystal lattice (solid) to a flowing mass of molecules (liquid), but every single molecule in the puddle on your table is the same it was inside the freezer. The change is physical — the chemistry is unchanged.
Chemical Properties
A chemical property describes a chemical change — a reaction — that a substance can undergo. The instant you observe a chemical property, the substance has become something different.
Take acidity. To check whether a substance is acidic, you have to see whether it reacts with a base, releases ions in water, or accepts a pair of electrons. Each of those tests changes the chemical composition of the substance.
Other chemical properties include basicity, flammability, toxicity, heat of combustion, pH value, rate of radioactive decay, and chemical stability. Every one of them tells you what a substance turns into under specific conditions.
Rusting of iron is the textbook example. When iron is left exposed to moist air, it reacts with oxygen and water to form a hydrated iron oxide — what we call rust (). The substance you started with (shiny, malleable iron) is now genuinely a different substance (brittle, orange-brown oxide). That’s chemistry, not physics.
Understanding physical and chemical properties
Listen to the audio explanation
You leave a steel knife outside. After a week it is coated in reddish-brown flakes and weighs a little MORE than before. Physical change or chemical change — and how can you tell?
Intensive vs Extensive Properties
Know which properties depend on how much you have and which don’t — and why that fingerprints a substance.
There’s another way to classify properties — and this one matters every time you do a calculation in a lab.
A property is either intensive or extensive, depending on whether it changes when you change the amount of substance.
Intensive properties don’t depend on how much you have. Take a 1-gram gold ring and a 100-gram gold biscuit. They have wildly different masses and volumes, but both gleam the same characteristic yellow, both melt at exactly , and both boil at exactly . Colour, melting point, boiling point, density, refractive index, viscosity, pressure, temperature — all intensive. Sample size doesn’t move them.
Extensive properties scale with the amount. Double the sample, double the value. Mass, volume, energy, total moles, heat capacity, length — all extensive.
Why this matters: identification. Because intensive properties stay fixed no matter the sample size, they are exactly what we use to identify a substance. Gold miners used this principle to separate real gold from fool’s gold — the mineral pyrite ().
The two look almost identical: same yellow metallic shine, similar density to the eye. But hold either one in a flame and the difference shows at once. Gold sits there, unchanged. Pyrite sputters, smokes, and releases foul-smelling sulphur dioxide () as the iron sulphide reacts with atmospheric oxygen. The chemical property — reactivity with at flame temperature — is intensive. It doesn’t matter whether you tested a gram or a kilogram; the answer is the same.
So intensive properties are what we use to identify a substance: they stay fixed no matter the sample size, while extensive properties only tell you how much you have.
Two bars of pure gold: bar A is 10 g, bar B is 200 g. Which property is the SAME for both, and which is DIFFERENT?
You can now tell substances apart by their properties — what's physical, what's chemical, and what depends on amount. But spotting a property is only half the job: science demands that we measure it. So before we move on, let's nail down what a measurement actually is.
Measurement of Physical and Chemical Properties
Tell qualitative from quantitative observations, and see what every measurement is built from.
A key step in the scientific method is observation, and observations come in two kinds.
Qualitative observations carry no numbers — the colour of a chemical, its odour, or the fact that a flask turns hot when a reaction starts. Quantitative observations are measurements: they hand you a number. Weight, area, length, volume, pressure, amount of substance, acidity (pH) and temperature are all quantitative.
Chemistry leans heavily on the quantitative kind — which raises a simple question: what is a measurement actually made of?
AI Generation Prompt
Annotated equation diagram of a single measurement: large centred expression 'p = 1.00 × 10⁵ Pa'. Three rounded callout bubbles with leader lines point to its parts — a blue bubble 'Symbol for pressure' → points to 'p'; a green bubble 'Number' → points to '1.00 × 10⁵'; a red bubble 'Unit of pressure' → points to 'Pa'. Friendly, clean, textbook style. Dark background (#0a0a0a or near-black), orange accent leader lines, clean technical illustration style.
Two things are always true about a measurement.
First, a measurement is a comparison. Saying a person is six feet tall really means they are six times as tall as a reference length of one foot. The 'foot' is the unit — the agreed yardstick we compare against. Every measurement is a number of some unit; the number alone is meaningless.
Second, a measurement always carries some uncertainty. No instrument is perfect, so a little doubt clings to the last digit of any reading. Careful technique can shrink that uncertainty, but it can never be removed entirely.
On the next page we put this to work — the units, the conversions, and the tools that turn matter into reliable numbers.
Q1.Hydrogen gas (H₂) is highly flammable and oxygen gas (O₂) makes fires burn fiercely. What is water (H₂O) — the compound made from both — actually like?
A diamond and the "lead" in your pencil are both pure carbon — nothing else. Yet one is the hardest natural substance on Earth and the other is so soft it smears onto paper when you write.
The difference isn't what they're made of. It's how the atoms are arranged. That is the first big idea of this chapter: a substance's properties come from its structure, not just its ingredients.
Atoms, Molecules and Compounds
Tell apart an element, a compound, and a molecule — and why a compound behaves nothing like the atoms inside it.
You already know that elements are pure substances made of one type of atom. But atoms don't always float around alone:
- Some elements exist as single atoms: sodium (), copper (), iron ()
- Some exist as diatomic molecules: hydrogen (), oxygen (), nitrogen ()
- Compounds form when atoms of different elements combine in a fixed, definite ratio
A compound behaves nothing like the elements it's made from. burns. supports combustion. Combine them in a 2:1 ratio and you get — water, which puts out fires.
Classification of Matter — Pure Substances and Mixtures
Sort any sample into element, compound, homogeneous or heterogeneous mixture, and tell them apart with the sampling test.
Every sample of matter is either a pure substance or a mixture, and that one split is the map for the whole subject.
A pure substance has a fixed composition and fixed properties. It comes in two kinds. An element cannot be broken into anything simpler by a chemical reaction — copper (), gold (), oxygen (). A compound has two or more elements chemically bonded in a fixed ratio — water (), common salt (), glucose (). You met both in the last section.
A mixture is two or more substances physically together, each keeping its own identity, in no fixed ratio — so it can be pulled apart by physical means. Mixtures split again by whether their composition is uniform:
- Homogeneous mixture — the same throughout; you cannot pick out the parts. Air and a salt solution are homogeneous.
- Heterogeneous mixture — the composition changes from place to place and the parts are distinguishable. Sand stirred into salt, or gravel, is heterogeneous.
How do you tell homogeneous from heterogeneous? Take small samples from different points and compare them.
Dissolve salt in water and you get a brine solution: scoop a sample from the top, the middle, the bottom — every sample is equally salty. Same composition everywhere, so it is homogeneous.
Stir sand or dirt into water instead. The top sample is nearly clear while the lower ones carry more grit, and it settles on standing. The samples differ, so it is heterogeneous.
Stoichiometric vs Non-Stoichiometric Compounds
Most compounds obey the law of definite proportions — their elements sit in one fixed, whole-number ratio by mass. Water is always , never . A compound like this is stoichiometric.
A few solids quietly break the rule. In a non-stoichiometric compound the ratio drifts slightly from the ideal formula because of defects in the crystal — missing atoms or extra ones. Iron(II) oxide is the classic case: written , real samples are closer to because some sites are vacant and a few ions keep the charge balanced. Cuprous oxide () and many high-temperature superconductors behave the same way. Their composition is genuinely variable, so a single clean formula only approximates them.
Physical vs Chemical Properties
Decide, for any change, whether a brand-new substance formed — the real line between physical and chemical.
Physical Properties
A physical property is something you can observe about a substance without changing what it is chemically. Colour, melting point, boiling point, density, solubility, pressure, volume — these are all physical properties.
When you observe these, the substance doesn’t become something else. Hold a copper wire under different lights and you see the same orange-brown gleam each time. Heat water in a kettle until it boils, then cool it all the way back to ice — at every point along the journey, it is still . The physical state changes from solid to liquid to gas, but the chemical identity stays put.
Melting an ice cube is the classic example. The cube goes from a rigid crystal lattice (solid) to a flowing mass of molecules (liquid), but every single molecule in the puddle on your table is the same it was inside the freezer. The change is physical — the chemistry is unchanged.
Chemical Properties
A chemical property describes a chemical change — a reaction — that a substance can undergo. The instant you observe a chemical property, the substance has become something different.
Take acidity. To check whether a substance is acidic, you have to see whether it reacts with a base, releases ions in water, or accepts a pair of electrons. Each of those tests changes the chemical composition of the substance.
Other chemical properties include basicity, flammability, toxicity, heat of combustion, pH value, rate of radioactive decay, and chemical stability. Every one of them tells you what a substance turns into under specific conditions.
Rusting of iron is the textbook example. When iron is left exposed to moist air, it reacts with oxygen and water to form a hydrated iron oxide — what we call rust (). The substance you started with (shiny, malleable iron) is now genuinely a different substance (brittle, orange-brown oxide). That’s chemistry, not physics.
Understanding physical and chemical properties
Listen to the audio explanation
You leave a steel knife outside. After a week it is coated in reddish-brown flakes and weighs a little MORE than before. Physical change or chemical change — and how can you tell?
Intensive vs Extensive Properties
Know which properties depend on how much you have and which don’t — and why that fingerprints a substance.
There’s another way to classify properties — and this one matters every time you do a calculation in a lab.
A property is either intensive or extensive, depending on whether it changes when you change the amount of substance.
Intensive properties don’t depend on how much you have. Take a 1-gram gold ring and a 100-gram gold biscuit. They have wildly different masses and volumes, but both gleam the same characteristic yellow, both melt at exactly , and both boil at exactly . Colour, melting point, boiling point, density, refractive index, viscosity, pressure, temperature — all intensive. Sample size doesn’t move them.
Extensive properties scale with the amount. Double the sample, double the value. Mass, volume, energy, total moles, heat capacity, length — all extensive.
Why this matters: identification. Because intensive properties stay fixed no matter the sample size, they are exactly what we use to identify a substance. Gold miners used this principle to separate real gold from fool’s gold — the mineral pyrite ().
The two look almost identical: same yellow metallic shine, similar density to the eye. But hold either one in a flame and the difference shows at once. Gold sits there, unchanged. Pyrite sputters, smokes, and releases foul-smelling sulphur dioxide () as the iron sulphide reacts with atmospheric oxygen. The chemical property — reactivity with at flame temperature — is intensive. It doesn’t matter whether you tested a gram or a kilogram; the answer is the same.
So intensive properties are what we use to identify a substance: they stay fixed no matter the sample size, while extensive properties only tell you how much you have.
Two bars of pure gold: bar A is 10 g, bar B is 200 g. Which property is the SAME for both, and which is DIFFERENT?
You can now tell substances apart by their properties — what's physical, what's chemical, and what depends on amount. But spotting a property is only half the job: science demands that we measure it. So before we move on, let's nail down what a measurement actually is.
Measurement of Physical and Chemical Properties
Tell qualitative from quantitative observations, and see what every measurement is built from.
A key step in the scientific method is observation, and observations come in two kinds.
Qualitative observations carry no numbers — the colour of a chemical, its odour, or the fact that a flask turns hot when a reaction starts. Quantitative observations are measurements: they hand you a number. Weight, area, length, volume, pressure, amount of substance, acidity (pH) and temperature are all quantitative.
Chemistry leans heavily on the quantitative kind — which raises a simple question: what is a measurement actually made of?
AI Generation Prompt
Annotated equation diagram of a single measurement: large centred expression 'p = 1.00 × 10⁵ Pa'. Three rounded callout bubbles with leader lines point to its parts — a blue bubble 'Symbol for pressure' → points to 'p'; a green bubble 'Number' → points to '1.00 × 10⁵'; a red bubble 'Unit of pressure' → points to 'Pa'. Friendly, clean, textbook style. Dark background (#0a0a0a or near-black), orange accent leader lines, clean technical illustration style.
Two things are always true about a measurement.
First, a measurement is a comparison. Saying a person is six feet tall really means they are six times as tall as a reference length of one foot. The 'foot' is the unit — the agreed yardstick we compare against. Every measurement is a number of some unit; the number alone is meaningless.
Second, a measurement always carries some uncertainty. No instrument is perfect, so a little doubt clings to the last digit of any reading. Careful technique can shrink that uncertainty, but it can never be removed entirely.
On the next page we put this to work — the units, the conversions, and the tools that turn matter into reliable numbers.
Q1.Hydrogen gas (H₂) is highly flammable and oxygen gas (O₂) makes fires burn fiercely. What is water (H₂O) — the compound made from both — actually like?