AQA GCSE FOUNDATION Chemistry: Pure Substances vs. Mixtures

AQA GCSE Foundation Chemistry: Pure Substances vs. Mixtures

Introduction

This tutorial will explore the fundamental difference between pure substances and mixtures in Chemistry. You'll learn how to classify substances, understand their properties, and discover techniques for testing purity.

Pure Substances

• Definition: A pure substance is a single element or compound that has a fixed chemical composition. It's made up of only one type of particle.
• Examples:
  • Elements: Gold (Au), Oxygen (O2)
  • Compounds: Water (H2O), Sodium Chloride (NaCl)
• Properties:
  • Fixed melting and boiling points: A pure substance melts and boils at specific temperatures.
  • Unique chemical properties: Pure substances exhibit consistent chemical reactions.

Mixtures

• Definition: A mixture is a combination of two or more substances that are not chemically bonded. The substances retain their individual properties.
• Types of Mixtures:
  • Homogeneous mixtures: Components are evenly distributed throughout the mixture. (e.g., saltwater, air)
  • Heterogeneous mixtures: Components are unevenly distributed. (e.g., sand and water, oil and water)
• Properties:
  • Variable melting and boiling points: The melting and boiling points depend on the composition of the mixture.
  • Components can be separated: The components of a mixture can be separated using physical methods like filtration, evaporation, or distillation.

Techniques for Purity Testing

• Melting Point Determination: A pure substance has a specific melting point. Impurities lower the melting point and broaden the melting range.
• Boiling Point Determination: Similar to melting point, a pure substance has a specific boiling point. Impurities affect the boiling point and can cause the mixture to boil over a range of temperatures.
• Chromatography: This technique separates mixtures based on their different affinities for a stationary phase and a mobile phase. Pure substances travel as a single spot, while mixtures appear as multiple spots.
• Distillation: This technique separates mixtures based on their different boiling points. Pure substances will distill at specific temperatures.

Summary

Understanding the distinction between pure substances and mixtures is crucial in Chemistry. Pure substances have fixed compositions and specific properties, while mixtures are combinations of substances that retain their individual properties. Purity testing techniques help us determine the purity of a substance by analyzing its physical and chemical characteristics.