Elements, Compounds and Mixtures
Quick Notes:
- Elements: Pure substances made of one type of atom. Cannot be chemically broken down further.
- Compounds: Substances formed by chemically bonded atoms of two or more elements in a fixed ratio.
- Mixtures: Combinations of elements and/or compounds not chemically bonded and in variable ratios.
- Mixtures can be separated by physical methods:
- Filtration: Separates solids from liquids.
- Evaporation: Removes solvent, leaving solute behind.
- Distillation: Separates based on boiling points.
- Recrystallization: Purifies solids via solubility.
- Paper chromatography: Separates components of mixtures based on solubility and affinity to paper.
- Solvation: Process of dissolving a solute in a solvent.
- Homogeneous mixtures: Uniform composition (e.g., salt water).
- Heterogeneous mixtures: Non-uniform composition (e.g., sand and water).
Full Notes:
Elements
Elements are substances made up of only one type of atom.
- Cannot be chemically broken down into simpler substances.
- All atoms in an element have the same number of protons (same atomic number).
- Represented by chemical symbols (e.g., H for hydrogen, O for oxygen).
Examples:
Magnesium (Mg), Oxygen (O2), Carbon (C).
Compounds
A compound is a substance made when atoms of two or more different elements are chemically bonded together.
- Atoms are in a fixed ratio by number.
- Can only be separated into elements by chemical reactions.
- The properties of a compound are different from those of the elements it contains.
Example Water (H2O)
Mixtures
A mixture is a physical combination of two or more substances (elements and/or compounds) that are not chemically bonded.
- Substances retain their original properties.
- The composition can vary – there is no fixed ratio.
- Can be separated by physical methods.
Mixtures can be homogeneous or heterogeneous:
Homogeneous mixtures:
- Uniform composition throughout.
- Also called solutions.
- Individual components are not visible.
Examples: Saltwater, air.
Heterogeneous mixtures:
- Non-uniform composition.
- Different components are visible or can settle out.
Examples: Sand in water, oil and vinegar.
Separation Techniques for Mixtures
As mixtures are made up of substances only physically held together (rather than by chemical bonds), they can be separated using physical separation methods.
You need to know the following methods of separation:
Filtration
Used to separate an insoluble solid from a liquid. A filter paper allows the liquid to pass through while the solid remains.
Example: Sand separated from water.
Evaporation
Used to remove a solvent from a solution, leaving the solute behind. Works when the solute does not decompose upon heating.
Example: Salt obtained from evaporated seawater.
Distillation
Separates components based on different boiling points.
- Simple distillation: Used to separate a solvent from a solution (e.g., water from ink).
- Fractional distillation: Separates mixtures of liquids with close boiling points (e.g., components of crude oil).
Recrystallization
Used to purify solid compounds. Involves dissolving the impure solid in hot solvent and then cooling it slowly. The desired compound crystallizes out, leaving impurities in solution.
Example: Purifying aspirin in a lab.
Paper Chromatography
Separates substances based on their solubility in a solvent and affinity for the stationary phase (the paper). Components move at different speeds and separate into distinct spots.
Example: Identifying dyes in ink samples.
Solvation
The process of surrounding solute particles with solvent molecules. When the solvent is water, it's called hydration.
Solvation allows soluble substances to dissolve and form a solution, leaving insoluble substances as solid (which can be separated by filtration).
Example: Dissolving NaCl in water — Na⁺ and Cl⁻ ions are surrounded by water molecules.
Summary
- Elements are made of one type of atom and cannot be broken down further.
- Compounds are formed by atoms chemically bonded in fixed ratios.
- Mixtures are physical combinations of substances with variable ratios.
- Mixtures can be separated by methods such as filtration, evaporation, distillation, recrystallisation and chromatography.
- Solvation explains how solutes dissolve in solvents.
What factors are considered in choosing a method to separate the components of a mixture?
The method depends on the physical properties of the components, such as particle size, solubility, boiling point, and magnetic or density differences. For example, filtration separates solids from liquids, distillation separates based on boiling point, and chromatography separates by solubility and polarity differences. The goal is to select a technique that exploits a measurable difference between the components.
How can the products of a reaction be purified?
Purification depends on the product’s physical and chemical properties. Common methods include recrystallisation for solids (using a solvent where the product is only soluble when hot), distillation for liquids (based on boiling point differences), solvent extraction for separating immiscible liquids, and drying or washing to remove impurities. The purity can then be checked using melting point, boiling point, or spectroscopy.
How do intermolecular forces influence the type of mixture that forms between two substances?
Substances with similar types and strengths of intermolecular forces tend to mix uniformly (miscible), as their interactions are compatible – for example, ethanol and water both form hydrogen bonds. When intermolecular forces differ greatly (e.g. polar vs non-polar), the substances are immiscible and form separate layers because the energy cost of disrupting stronger forces is too high.
Why are alloys generally considered to be mixtures, even though they often contain metallic bonding?
Alloys are physical mixtures of metals (and sometimes non-metals) rather than compounds with fixed ratios. Their atoms are mixed in a metallic lattice but not chemically bonded in defined proportions. The metallic bonding extends throughout the structure, but the composition can vary – for example, brass (Cu and Zn) or steel (Fe and C) — so alloys are classified as mixtures rather than pure substances.