Introduction to the particulate nature of matter

A pure substance made of **only one type of atom**; it cannot be broken down chemically.

**Two or more different atoms chemically bonded** together in a fixed ratio.

Two or more substances **physically combined but not chemically bonded**, in any ratio.

Only by **chemical** means (a reaction) — not by physical methods.

By **physical** means (e.g. filtration, distillation), because nothing is bonded.

Homogeneous = **uniform** (e.g. solution, air); heterogeneous = **not uniform**, parts visible (e.g. sand + iron).

Element = one type of atom; compound = different atoms **chemically bonded** in a fixed ratio.

A single element or compound — it has a **sharp, fixed** melting and boiling point.

A pure substance melts **sharply**; impurities **lower** it and spread it over a **range**.

A **mixture** (an alloy of copper and zinc) — the metals are not chemically bonded.

**Solid**, **liquid** and **gas** — they differ in how close the particles are and how freely they move.

**Packed** close in fixed positions; they only **vibrate**. A solid has a fixed shape and volume.

**Touching** but not fixed; they **slide** past each other. A liquid has fixed volume but takes the container's shape.

**Far apart**, moving **fast and randomly**. A gas fills its container and is easily compressed.

A model treating matter as **small particles in constant random motion**, with attractive forces between them that weaken as they spread apart.

The **average kinetic energy** of the particles — hotter means the particles move faster on average.

**Add 273.15** (≈ 273): T(K) = θ(°C) + 273.15.

**0 K** (about −273 °C) — the lowest possible temperature, where particle motion is at a minimum.

Gas particles are **far apart** with large gaps to close up; liquid particles are already **touching** with little space.

The added energy is used to **overcome the forces** between particles, not to speed them up, so the average kinetic energy (temperature) stays the same.

Its particles are **not held in fixed positions**, so they **slide** and flow to fit the container.

They **gain kinetic energy** and **vibrate more**, until they have enough energy to break free of their fixed positions and the solid melts.

Their components are **not chemically bonded**, so they keep their own properties and can be separated by **physical** methods.

An **insoluble solid** from a liquid — the solid is too large to pass through the **filter paper** (uses particle size).

A **dissolved (soluble) solid** from its solution — the **solvent boils off**, leaving the solid behind.

Liquids (or a liquid from a dissolved solid) by their difference in **boiling point**.

The dissolved components of a mixture by their difference in **solubility / attraction** to the paper.

R_{f} = distance moved by **spot** ÷ distance moved by **solvent** — a ratio with **no units**, between 0 and 1.

The component is **more soluble** in the solvent, so it was carried **further** up the paper.

Add water to dissolve the salt, then **filter** — the sand stays as the residue.

Use a **magnet** — iron is **magnetic**, sand and salt are not.

**Crystallisation** — dissolve in hot solvent, cool to form pure crystals, then filter them off.

**No** — the spot cannot move further than the solvent front, so 0 < R_{f} < 1.

Size → **filtration**; boiling point → **distillation**; solubility → **crystallisation / chromatography**.