Mixtures in the Particle Model

Introduction

The particle model explains solids, liquids and gases by picturing tiny particles in constant motion. The same model explains mixtures beautifully. A mixture is simply a sample that contains more than one kind of particle, with each kind unchanged and not bonded to the others. Learning to draw and read particle diagrams of mixtures makes the rest of this unit much easier to understand.

Core explanation

Drawing conventions. In particle diagrams, each atom is a circle. Different elements are shown by different colours or sizes. Atoms that are chemically bonded are drawn touching or overlapping as a unit. Particles that are not bonded are drawn separately.

Pure element. All the circles are identical. They may be single (argon atoms) or joined in identical molecules (O₂ as two touching circles of the same colour).

Pure compound. Every particle is the same unit, containing two or more different atoms bonded together. For water, each unit is one larger circle (oxygen) touching two smaller ones (hydrogen), and every unit in the box is identical.

Mixture. The box contains two or more different kinds of particle , and the different kinds are not joined to each other. Air, for example, is drawn as a box of widely spaced N₂ pairs, O₂ pairs and single argon atoms, mixed randomly. A mixture of two elements shows two kinds of single circles side by side; a mixture of an element and a compound shows single atoms among multi-atom units.

Mixtures in each state. The spacing and motion of the particles follow the state of the mixture:

- Gas mixtures : particles far apart and moving randomly, thoroughly intermingled by diffusion. - Liquid mixtures and solutions : particles close together but randomly arranged, sliding past one another. In salt water, sodium and chloride ions are spread among water molecules. - Solid mixtures : in an alloy, atoms of a second metal sit among the regular rows of the main metal, disturbing the pattern. In a coarse mixture such as sand and iron filings, each grain is itself a huge cluster of its own particles.

Explaining mixture properties. Because each kind of particle is unchanged, each component keeps its own properties. Because nothing fixes how many particles of each type are present, the composition can vary. Because the different particles are not bonded to one another, physical processes such as evaporation or filtration can move one type away from the other.

Step-by-step reasoning

To interpret a particle diagram:

1. Count how many different kinds of particle there are. One kind means pure; more than one means a mixture. 2. For each kind, check whether it contains one type of atom (element) or more (compound). 3. Use the spacing to identify the state: touching and regular (solid), touching and random (liquid), far apart (gas).

Visual explanation

Draw a square box containing ten blue pairs of circles and three red pairs, scattered far apart with plenty of empty space. This is a gas mixture of two elements. Now draw a box packed with small grey circles in neat rows, with a few larger gold circles replacing some grey ones. This is a solid alloy: a mixture in which the pattern has been disturbed.

Real-world analogy

A school playground at break time is like a liquid or gas mixture. Pupils from different classes run around and mingle, but each pupil stays the same person and belongs to the same class. When the bell rings they can line up and separate into their classes again.

Real-world example

When you add a drop of food colouring to water, the colour slowly spreads through the glass even without stirring. Colouring particles and water molecules move randomly and intermingle by diffusion. The particle model shows why the final mixture is evenly coloured: the moving particles keep spreading until they are evenly distributed.

Why?

Why do the particles of a mixture spread evenly in liquids and gases? The particles are always moving randomly. Over time random motion carries each kind of particle into every part of the container, so any region with a high concentration gradually loses particles to regions with fewer. Solids mix only slowly, because their particles only vibrate.

Common misconception

"In a mixture the particles combine to make new particles." In a mixture no new particles form. If a diagram shows the atoms rearranged into new groups, a chemical reaction has occurred and the product is a compound, not a mixture.

Worked example

Question: A diagram shows a box with six separate grey circles and four units each made of one black circle joined to two white circles. All the particles are far apart. Describe the sample.

Reasoning: There are two kinds of particle, so it is a mixture. The grey circles are single atoms of one element. The other units contain two types of atom bonded together, so they are molecules of a compound. The wide spacing shows a gas.

Answer: A gas mixture of an element and a compound.

Quick check

1. How does a particle diagram show that a sample is a mixture? Answer: It contains two or more different kinds of particle that are not bonded to each other.

Exam focus

Particle diagram questions often mix classification with state. Always state both: for example, "a mixture of two elements in the gas state". Draw particles of the same element the same size and colour, and in solids show them touching in a regular pattern.

Advanced insight

A particle diagram of a solution can show that the solute particles are surrounded by solvent particles. Ionic solids such as sodium chloride split into separate Na⁺ and Cl⁻ ions in water, each attracting a shell of water molecules. The ions are still the same ions, so evaporating the water returns sodium chloride crystals.

Summary

In the particle model a mixture contains two or more different kinds of particle that are not bonded to one another. Each kind is unchanged, which explains why components keep their properties and can be separated physically. The spacing and motion of the particles depend on the state of the mixture, and random motion spreads particles evenly in liquids and gases.

Practice questions

1. Describe how a particle diagram of a pure compound differs from one of a mixture of two elements. Answer: The compound shows only one kind of particle, each containing different atoms bonded together; the mixture shows two kinds of particle, each made of one type of atom, not joined to each other. 2. Draw or describe a particle diagram for a mixture of helium and neon gases. Answer: Two sizes or colours of single circles, far apart and randomly arranged, with no circles joined. 3. Use the particle model to explain why a mixture of two gases does not settle into layers. Answer: The gas particles are in constant random motion and spread into all the space, so they stay evenly intermingled. 4. Why does the particle model predict that the components of a mixture keep their own properties? Answer: The particles of each component are unchanged and not bonded to other particles, so each substance still behaves as before.