Elements, Compounds, and Mixtures
Understanding the fundamental differences between pure substances and mixtures
Building Blocks of Matter
Elements, compounds, and their mixtures
All matter can be classified into three main categories: elements, compounds, and mixtures. Understanding these classifications is fundamental to chemistry.
An element is a pure substance made of only one type of atom. There are 118 known elements, each represented by a unique symbol (e.g., H for hydrogen, O for oxygen, Fe for iron). Elements cannot be broken down into simpler substances by chemical means.
A compound is formed when two or more elements chemically bond together in a fixed ratio. For example, water (H₂O) always contains hydrogen and oxygen in a 2:1 ratio. Compounds have different properties from their constituent elements—sodium (a reactive metal) and chlorine (a toxic gas) combine to form sodium chloride (table salt), which is safe to eat.
A mixture contains two or more substances that are physically mixed but not chemically bonded. Unlike compounds, mixtures can have variable proportions and each component retains its original properties. Air is a mixture of gases, primarily nitrogen and oxygen, while salt water is a mixture of salt dissolved in water.
Compounds have fixed melting and boiling points because their composition is constant. They can only be separated into their elements by chemical reactions, which require breaking the chemical bonds between atoms. The properties of a compound are completely different from the properties of its constituent elements.
Mixtures, in contrast, have variable melting and boiling points that depend on their composition. They can be separated using physical methods such as filtration, evaporation, distillation, or chromatography—no chemical reactions are needed. Each substance in a mixture retains its original properties, making separation possible.
Common separation techniques include: filtration (removing insoluble solids from liquids), evaporation (obtaining dissolved solids by heating), distillation (separating liquids by boiling point differences), and chromatography (separating substances based on how they move through a medium like paper). Understanding which method to use depends on the physical properties of the mixture components.
Classify each substance as an element, compound, or mixture
Oxygen (O₂)
Explore different techniques to separate mixtures
Filtration
Separates insoluble solid from liquid
Suitable for separating:
• sand and water
• chalk and water
Question:
You have a mixture of sand and salt dissolved in water. Describe how you would separate all three components and obtain pure samples of each.
Solution:
Step 1: Filtration - First, use filtration to separate the insoluble sand from the salt solution. Pour the mixture through filter paper in a funnel. The sand particles are too large to pass through the paper and will be trapped, while the salt solution (filtrate) passes through.
Step 2: Evaporation or Crystallisation - To obtain the salt from the solution, heat the filtrate in an evaporating basin. As the water evaporates, salt crystals will form. Continue heating gently until all water has evaporated, leaving pure salt crystals.
Step 3: Distillation (optional) - If you also need the pure water, use distillation instead of evaporation in Step 2. Heat the salt solution to boiling, collect and condense the water vapour, leaving salt behind in the flask.
Result: Pure sand (from filter paper), pure salt crystals (from evaporation basin), and pure water (from distillation condenser).
Question 1 of 6
Which of the following is an element?