Pure Substance Chemistry Example 4

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Example 4

hard
Both distilled water and steel are uniform in appearance and properties. Explain why distilled water is classified as a pure substance while steel is a mixture.

Solution

  1. 1
    Distilled water has a fixed chemical composition: every molecule is H2O\text{H}_2\text{O}. It has a sharp melting point (0°C0°\text{C}) and boiling point (100°C100°\text{C}).
  2. 2
    Steel is a homogeneous mixture (alloy) of iron and carbon (and often other metals). Its composition can vary — mild steel is about 0.2% carbon, high-carbon steel is up to 2%.
  3. 3
    The key test: a pure substance has a fixed, definite composition and sharp phase-change temperatures. A mixture can be combined in varying proportions.

Answer

Distilled water has fixed composition (H2O); steel’s composition varies — it is a mixture.\text{Distilled water has fixed composition (H}_2\text{O); steel's composition varies — it is a mixture.}
Uniformity alone does not make something a pure substance. Homogeneous mixtures also look uniform. The defining characteristic of a pure substance is a fixed, definite chemical composition with characteristic properties.

About Pure Substance

A sample of matter that has a fixed, definite chemical composition throughout, consisting of only one type of element or one type of compound.

Learn more about Pure Substance →

More Pure Substance Examples

Example 1 easy

Distinguish between a pure substance and a mixture. Give two examples of each.

Example 2 medium

How can you determine whether a clear liquid is a pure substance or a mixture?

Example 3 easy

Is orange juice a pure substance or a mixture? Explain.

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