Some Basic Concepts of Chemistry · Lesson 2 of 11
States and Classification of Matter
“Use particle models and composition to distinguish states, mixtures, elements, and compounds.”
• Explain the bulk properties of solids, liquids, and gases using particles. • Classify matter as a pure substance or a mixture, then into appropriate subgroups. • Distinguish atoms, molecules, elements, and compounds. • Choose physical separation or chemical change appropriately.
Matter at the visible and particle scales
A drop of water, a stone, and the air inside a bottle seem very different, but each has mass and occupies space. These are the two features used here to identify matter. A particle model helps us explain why materials can share these features while behaving differently.
Anything that has mass and occupies space.
The macroscopic scale is the bulk scale we observe directly. The microscopic description considers constituent particles such as atoms, molecules, and ions. A drawing of particles is a model: the circles are symbols, and the actual particles are too small to be seen individually in an ordinary sample.
In a solid, constituent particles remain close to fixed relative positions and vibrate about them. This supports a definite shape and volume. In a liquid, particles remain close but can move past one another. A liquid therefore keeps an approximately fixed volume while taking the shape of its container.
Gas particles are widely separated compared with their own size and move freely through the available space. A gas has neither a fixed shape nor a fixed volume: it spreads throughout its container. The large spaces between gas particles also explain why a gas can be compressed much more easily than a liquid or solid.
| State | Shape | Volume | Particle picture |
|---|---|---|---|
| Solid | Definite | Definite | Close particles with limited movement about relative positions |
| Liquid | Takes container shape | Definite for a given amount and conditions | Close particles that move past one another |
| Gas | Takes container shape | Occupies available container volume | Widely separated, freely moving particles |
Changing temperature or pressure can change the state of a substance. On heating, ice melts to liquid water and the liquid can become water vapour. On cooling, the reverse changes can occur. The molecules remain water molecules during these state changes; their arrangement, separation, and movement change.
Problem
Water vapour condenses on a cold lid. Has the substance necessarily become a different compound?
- 1.The observation is a change from gas to liquid.
- 2.Condensation brings water molecules closer together; it does not require changing their chemical identity.
- 3.The material is still water. A change of state alone is not evidence of a new compound.
Classifying by composition
State and composition answer different questions. State tells us how particles are arranged and move under the conditions; composition tells us what kinds of substances or elements are present. For example, both a pure sample of water and a sugar solution can be liquids, although their compositions differ.
A substance with a characteristic, fixed chemical composition. Its constituent chemical entities have the same chemical nature.
A combination of two or more substances whose relative amounts can vary. The substances present are its components.
Pure water has a fixed chemical composition. A sugar solution contains water and sugar, and another solution can contain more sugar without becoming an entirely new compound. Its composition is variable. This distinction concerns chemical composition, so everyday words such as clean or natural do not establish that a sample is a pure substance.
A homogeneous mixture has a uniform composition throughout the sample on the scale being considered. Well-mixed sugar solution and ordinary air are examples. A heterogeneous mixture is non-uniform: a mixture of grains and small stones, or salt and sugar crystals, can have visibly different portions. Uniform appearance does not by itself make something a pure substance.
Problem
One beaker contains pure water and another contains a well-mixed sugar solution. Can clarity distinguish a pure substance from a mixture?
- 1.Both can look clear and uniform.
- 2.The sugar solution still contains two components and its composition can vary.
- 3.Clarity alone is insufficient. Classification requires information about composition or separation, not just appearance.
Mixture components can be separated using suitable physical methods because mixing does not require them to become a single new compound. Hand-picking can remove stones from grain; filtration separates an insoluble solid from a liquid; crystallisation can recover a dissolved solid; distillation uses differences in volatility. The method must fit the properties of the components.
Elements, compounds, atoms, and molecules
Pure substances are classified further into elements and compounds. The distinction is the types of atoms present, not simply the number of atoms in a particle. A molecule can contain several atoms and still belong to an element if every atom is of that one element.
A substance consisting of atoms of only one element. Different elements contain different kinds of atoms.
A discrete group of bonded atoms that forms an individual entity. A molecule may contain atoms of one element or of different elements.
A substance containing atoms of two or more elements chemically combined in a characteristic fixed proportion.
Hydrogen and oxygen gases contain molecules represented by and . The subscript 2 tells us that each molecule has two atoms. Each gas is an element because its molecules contain only one kind of atom. Sodium, copper, and silver are also elements, but a metal sample should not be pictured as a collection of separate two-atom molecules.
A water molecule, , contains two hydrogen atoms and one oxygen atom. A carbon dioxide molecule, , contains one carbon atom and two oxygen atoms. These are compounds because different elements are combined in fixed proportions. Some compounds, such as solid sodium chloride, form extended ionic structures rather than discrete molecules; their formulas describe a ratio of ions.
| Sample | Classification | Reason |
|---|---|---|
| Element | Two atoms per molecule, but only one kind of atom | |
| Compound | Hydrogen and oxygen combined in a fixed ratio | |
| Glucose | Compound | Carbon, hydrogen, and oxygen chemically combined in fixed proportions |
| Air | Homogeneous mixture | Several gases mixed in variable proportions |
| Grains and stones | Heterogeneous mixture | Different portions have different composition |
A compound can have properties very different from those of its constituent elements. Hydrogen is combustible and oxygen supports combustion, while water is commonly used to extinguish suitable fires. Combining elements chemically produces a substance with its own properties; it does not simply preserve the separate properties side by side.
Evaporating pure water does not split it into hydrogen and oxygen. It changes state. Separating a compound into its constituent elements requires chemical change; ordinary physical separation methods do not do this.
Problem
Classify and , and explain the difference.
- 1.Each molecule contains two nitrogen atoms. Only one element is present, so nitrogen gas is an element.
- 2.Each molecule contains nitrogen and hydrogen atoms in a fixed ratio, so ammonia is a compound.
- 3.Both samples can contain molecules. Molecularity alone does not determine whether a substance is an element or compound.
Quiz
Why does a liquid take the shape of its container?
Which sample is a homogeneous mixture?
Why is an element?
Which change must involve more than physical separation?
What changes when liquid water becomes water vapour?
Practice Problems
- Explain why air counts as matter even though it is usually invisible.
- Use particle movement to compare a solid and a liquid.
- Classify copper, carbon dioxide, air, and salt mixed with sand.
- Explain why changing the amount of sugar in a solution does not produce a new compound each time.
- Why can a pure substance consist of molecules containing more than one atom?
- Choose a suitable physical method for separating an insoluble powder from water and explain why it works.
Key Takeaways
State depends on particle arrangement and movement; composition determines the type of substance. Mixtures have variable composition; pure substances have characteristic fixed composition. Elements contain one kind of atom; compounds contain different elements chemically combined in fixed proportions. A molecule can belong to either an element or a compound. Physical separation of mixtures and chemical decomposition of compounds are different processes.