Elements, Compounds & Mixtures
Key idea: Elements vs compounds vs mixtures: mark-scheme definitions, fixed ratio vs physical mixing, and common examples like alloys and separation methods.
Before you start: Describe atomic structure and the relative charges and masses of subatomic particles
By the end, you can
- Distinguish elements, compounds, mixtures and alloys from composition and structure
Classification questions are definition questions. If you cannot say chemically combined vs physically mixed, you lose marks.
1. Definition
| Term | Mark-scheme definition |
|---|---|
| Element | A pure substance made of one type of atom only, which cannot be broken down into simpler substances by chemical methods. |
| Compound | A pure substance made of two or more different elements chemically combined in a fixed ratio, which can be broken down by chemical methods (e.g., electrolysis). |
| Mixture | Two or more substances (elements and/or compounds) physically mixed together, not chemically combined, with no fixed composition; components can be separated by physical methods. |
| Alloy | A mixture containing a metal and one or more other elements (e.g., brass is copper + zinc). |
Physical methods do not change a substance’s chemical identity (e.g., filtration, distillation, chromatography). See Separation Techniques.
2. Key Ideas
- Element vs compound: element = one type of atom; compound = different elements chemically combined.
- Compound vs mixture: compound has a fixed ratio; mixture has a variable ratio.
- Separation: mixtures → physical methods; compounds → chemical methods.
- Melting/boiling point: pure substance → sharp; mixture → usually a range.
- Alloys are mixtures, not compounds.
3. Detailed Explanations
- Compound: elements chemically combined in a fixed ratio.
- Mixture: substances physically mixed, no fixed composition.
- Mixtures → physical separation. Compounds → chemical decomposition.
A. Elements (Atoms and Molecules)
An atom is the smallest particle of an element that still has the chemical properties of that element.
Some elements exist as molecules (two or more atoms bonded together), e.g. O₂, N₂, Cl₂.
B. Compounds (Molecules vs Formula Units)
A compound is chemically combined. This means new bonds form and you get a new substance with new properties.
- Covalent compounds exist as molecules (e.g., water molecules are H₂O).
- Ionic compounds exist as a giant ionic lattice. There are no “molecules of NaCl” — the simplest ratio is called a formula unit.
C. Compounds vs Mixtures (Exam Comparison Table)
| Compounds | Mixtures |
|---|---|
| Pure substances, with two or more elements chemically joined together. | Not pure substances; two or more substances mixed together with no new chemical bonds. |
| Fixed composition (fixed ratio). | Variable composition (ratio can change). |
| Separated into elements only by chemical methods (e.g., electrolysis, thermal decomposition). | Separated by physical methods (e.g., filtration, distillation, chromatography, magnet). |
| Usually have sharp melting/boiling points. | Usually melt/boil over a range. |
| Properties are different from the elements that form them. | Components largely keep their own properties. |
| Example: water (H₂O), carbon dioxide (CO₂). | Example: air, brass, salt solution. |
D. Alloys (Metallic Mixtures)
An alloy is a mixture because the components are not chemically combined in a fixed ratio.
Alloys are often harder than pure metals because different-sized atoms disrupt the regular layers and prevent them from sliding easily. (See Metallic Bonding.)
Example: the composition of an alloy can vary (still “brass”, but different ratios):
Data table
| Component | Brass sample A | Brass sample B |
|---|---|---|
| Copper | 70 | 60 |
| Zinc | 30 | 40 |
4. Common Mistakes
- Saying “element cannot be broken down by physical methods”. Physical methods are irrelevant for elements; the mark is cannot be broken down by chemical methods.
- Calling ionic compounds “molecules” (e.g., “a molecule of NaCl”). Use formula unit.
- Calling alloys “compounds” because they contain two elements. Brass is a mixture.
- Using “mixture = two or more atoms mixed together”. Mixtures can contain elements and compounds (and in solutions, ions).
- Using examples without stating what they are made of (e.g., “air” with no mention that it’s a mixture of gases).
5. Exam Tips
Compound: “chemically combined” + “fixed ratio”.
Mixture: “physically mixed” + “can be separated by physical methods”.
Sharp melting point/boiling point → likely a pure substance (element or compound).
Range → likely a mixture.
If the question mentions “brass”, “steel”, or “stainless steel”, write mixture (alloy) unless a fixed ratio is explicitly given (it usually is not).
6. Worked Examples
Example 1: Quick Classification (Must Give a Reason)Core
Question: Classify each as an element, compound, or mixture: (a) iron, (b) distilled water, (c) air, (d) brass.
Show Answer
- (a) Iron: element (one type of atom only).
- (b) Distilled water: compound (hydrogen and oxygen chemically combined in a fixed ratio, H₂O).
- (c) Air: mixture (different gases physically mixed, not chemically combined).
- (d) Brass: mixture (alloy) (copper and zinc mixed, not chemically combined in a fixed ratio).
Example 2: Separating a Mixture (Method Selection)Core
Question: Describe how to separate a mixture of sand and sodium chloride solution to obtain dry sand and dry salt.
Show Answer
- Filtration: sand is insoluble and remains as residue; salt solution passes through as filtrate.
- Evaporation/crystallisation: heat the filtrate to evaporate water and leave sodium chloride crystals.
Final: Use filtration, then evaporate the filtrate to get the salt.
Example 3: Using Melting Point DataCore
Question: Substance A melts sharply at 801circC. Sample B melts between 750circC and 790circC. Which is more likely to be a pure substance? Explain.
Show Answer
Pure substances melt at a sharp temperature. Mixtures usually melt over a range.
Final: Substance A is more likely to be a pure substance.
Example 4: Iron + Sulfur (Mixture vs Compound)Core
Question: A student mixes iron filings and sulfur powder.
(a) Is the mixture a compound or a mixture?
(b) State one physical method to separate it.
The student then heats the mixture strongly until it glows and a black solid forms.
(c) Is the black solid a compound or a mixture?
(d) Give one piece of evidence to support your answer.
Show Answer
(a) Mixture: iron and sulfur are physically mixed, not chemically combined.
(b) Use a magnet to remove the iron (iron is magnetic; sulfur is not).
(c) Compound: heating causes a chemical reaction, forming iron(II) sulfide:
Fe + S → FeS
(d) Evidence: the product is a new substance with different properties and cannot be separated by a magnet (the iron is no longer present as iron filings).
Example 5: Error Analysis (Fix the Bad Statement)Core
Question: A student says: “Brass is a compound because it contains copper and zinc.” Correct the statement using mark-scheme words.
Show Answer
Brass is a mixture (alloy) because copper and zinc are physically mixed and not chemically combined in a fixed ratio.
7. Mind Stretchers
Mind stretcher 1: Identify X From EvidenceExtension
Question: Substance X has a sharp melting point and cannot be separated by filtration, distillation, or a magnet. When X is electrolysed, two different gases are produced. Is X an element, compound, or mixture? Explain.
Show Answer
Electrolysis producing different substances shows X can be broken down by a chemical method. That means X is a compound, not an element. It is also not a mixture because it has a sharp melting point and cannot be separated by physical methods.
Final: X is a compound.
Mind stretcher 2: Fixed Ratio TrapExtension
Question: A sample contains copper and oxygen. A student claims it must be a compound. Explain why this conclusion is unsafe, and what extra information would make it safe.
Show Answer
Copper and oxygen could be:
- a compound (e.g., copper(II) oxide) if they are chemically combined in a fixed ratio, or
- a mixture (e.g., copper powder mixed with copper(II) oxide) if physically mixed.
To make the conclusion safe, you need evidence of a fixed composition (or a chemical test showing it can be decomposed into elements by a chemical method).
8. Quiz
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