How does a smell travel?
You can smell a bakery before you see it, and dinner cooking in the kitchen fills the whole house with its smell. A smell has no legs, so what carries it to your nose?
Demo: an invisible delivery
Set up two small beakers. Beaker A holds distilled water with a few drops of phenolphthaleinphenolphthaleinAn acid–base indicator. Phenolphthalein solution is colorless; it turns pink-red in basic solutions (such as ammonia, limewater, or sodium hydroxide solution) and stays colorless in acidic or neutral ones. Lesson 1.8.4 covers indicators. (colorless). Beaker B holds concentrated ammonia solution. Cover both with one large upside-down beaker.
After a few minutes, the liquid in beaker A slowly turns pink, even though the two beakers never touch.
What happened: ammonia molecules are always moving. Some escape from beaker B into the air, travel to beaker A, and dissolve in the water there, which turns the phenolphthalein pink. When particles spread out on their own like this and mix into each other, it’s called diffusion扩散.
Basic features of molecules
For substances made of molecules, the molecules have these basic features:
- Molecules have tiny masses and sizes. One drop of water (about 0.05 mL) contains roughly water molecules.
- Molecules are always moving, and they move faster at higher temperatures. This is what causes diffusion.
- There are spaces between molecules. The spaces are large in gases and small in solids and liquids.
- Molecules of the same substance have the same chemical properties; different molecules have different ones. Oxygen gas and liquid oxygen are both made of oxygen molecules, and both support burning.
In the container below, red gas molecules are on the left and blue gas molecules are on the right. Pick a temperature, remove the divider, and watch them mix. Then try another temperature and compare how long it takes to mix evenly:
Pick a temperature, then remove the divider. Try another temperature and compare the times.
The higher the temperature, the faster the molecules move, and the faster diffusion happens. That’s why wet clothes dry faster in the sun and sugar dissolves faster in hot tea.
Molecules and atoms
A molecule分子 is made of atoms. For a substance made of molecules, the molecule is the smallest particle that still has the substance’s chemical properties: one water molecule still behaves chemically like water, but split it up and it isn’t water anymore.
In a chemical change, molecules break apart into atoms, and the atoms recombine into new molecules. The atom原子 is the smallest particle in a chemical change. Pick an example below to compare what happens to the particles in a physical change and in chemical changes:
Macroscopic: what you can see
Water slowly evaporates and becomes water vapor.
Particulate: what the particles do
Symbolic: how chemists write it
The water molecules themselves don't change; they just spread apart. No molecule is broken up, so this is a physical change.
Macroscopic: what you can see
Heating red mercury(II) oxide powder produces shiny liquid mercury and a gas that relights a glowing splint.
Particulate: what the particles do
Symbolic: how chemists write it
The mercury(II) oxide breaks apart into mercury atoms and oxygen atoms. Every 2 oxygen atoms join to form an oxygen molecule, and the mercury atoms gather into liquid mercury.
Macroscopic: what you can see
A burning jet of hydrogen lowered into a jar of chlorine keeps burning with a pale flame, and a white mist forms at the mouth of the jar.
Particulate: what the particles do
Symbolic: how chemists write it
Hydrogen and chlorine molecules split into atoms, which recombine into hydrogen chloride molecules. The kinds and numbers of atoms don't change.
| Molecules | Atoms | |
|---|---|---|
| Difference | Can be broken apart in a chemical change | Cannot be broken apart in a chemical change |
| How they’re related | Molecules are made of atoms | Atoms can form molecules or make up a substance directly |
| In common | Both are tiny, always moving, and have spaces between them | Same |
What substances are made of
- Made of molecules: water, oxygen, carbon dioxide, hydrogen, and many more.
- Made directly of atoms: metals (such as iron, copper, and mercury), noble gases (such as helium), diamond, and graphite.
- Some substances, such as table salt (sodium chloride), are made of ions. You’ll meet them in lesson 1.3.6.
Example 1: Smelling flowers over a wall
Use the idea of molecules to explain why you can smell flowers on the other side of a wall, and why the smell is stronger on a hot summer day.
- Identify the particles. The scent comes from molecules given off by the flowers.
- Molecules move. The scent molecules keep moving and spread through the air over the wall until some reach your nose.
- Temperature matters. On a hot day the molecules move faster and spread faster, so the smell is stronger.
Example 2: Evaporating water vs. splitting water
How is water evaporating different from water being split by electricity?
- Evaporation. The water molecules stay the same; only the spaces between them grow. No new substance forms: physical change.
- Splitting water. Water molecules break into hydrogen and oxygen atoms. Hydrogen atoms pair up into hydrogen molecules and oxygen atoms pair up into oxygen molecules, forming new substances: chemical change.
- Conclusion. At the particle level, a physical change leaves the molecules intact, while a chemical change breaks molecules apart and recombines their atoms into new molecules.
- You can smell popcorn from the other end of the movie theater.
Popcorn scent molecules keep moving and spread far.
- One drop of water contains about water molecules.
So many molecules fit into one tiny drop, so each one must be very small.
- 6000 L of oxygen gas can be squeezed into a 40 L steel cylinder.
Gas molecules are far apart; under pressure, the spaces shrink.
- Both oxygen gas and liquid oxygen support burning.
Both are made of oxygen molecules, and the same molecules have the same chemical properties.
- A spoonful of sugar sits at the bottom of a glass of water. Without stirring, the whole glass eventually tastes sweet.
Sugar molecules keep moving and spread throughout the water.
- An over-inflated bike tire is more likely to burst on a hot day.
As the temperature rises, the spaces between gas molecules grow, so the air inside expands.
- Wet clothes dry faster in the sun than in the shade.
The higher the temperature, the faster water molecules move.
- Carbon monoxide is poisonous, but carbon dioxide is not.
They are made of different molecules, so they have different chemical properties.
Common mistakes
Mistake 1: “Molecules are bigger than atoms.” You can’t compare them in general. A molecule is bigger than the atoms it’s made of, but a mercury atom is bigger than a hydrogen molecule. The correct statement is that molecules are made of atoms.
Mistake 2: “Atoms are the smallest particles and can’t be divided.” Atoms can’t be divided in a chemical change. But atoms do have smaller parts inside (a nucleus and electrons), as lesson 1.3.2 shows.
Mistake 3: “Every substance is made of molecules.” Metals such as iron and copper and the noble gases are made directly of atoms, and sodium chloride is made of ions. None of them has molecules.
Quick check
What is the smallest particle that still has the chemical properties of carbon dioxide?
Carbon dioxide is made of carbon dioxide molecules, so the smallest particle that keeps its chemical properties is one carbon dioxide molecule.
Which statement about molecules and atoms is correct?
That's the key difference between molecules and atoms. A can't be said in general, atoms are always moving too (C), and metals are made directly of atoms (D).
Summary
- Molecules are tiny, always moving (faster at higher temperatures), and have spaces between them; the same molecules have the same chemical properties.
- Diffusion happens because particles are always moving.
- For substances made of molecules, the molecule is the smallest particle that keeps the substance’s chemical properties. The atom is the smallest particle in a chemical change.
- In a chemical change, molecules break into atoms and the atoms recombine into new molecules. Substances can be made of molecules, atoms, or ions.
On to AP (for later)
AP topic 3.5, kinetic molecular theory, puts today’s ideas in numbers: the average kinetic energy of gas particles is proportional to the absolute temperature, and at higher temperatures more particles move fast (the Maxwell–Boltzmann distribution). AP also often asks you to draw particle diagrams before and after a reaction and check that the kinds and numbers of atoms are conserved.