Concept Explanation: Pure Substances

A pure substance is a material that has a constant composition and distinct chemical properties. It consists of only one type of particle, either an element or a compound. Pure substances cannot be separated into other substances by physical means.

Step 1: Analyze the Options

Let's evaluate each option:

• Milk: A mixture of water, fats, proteins, lactose, and minerals. It is not pure.
• Soil: A complex mixture of minerals, organic matter, water, and air. It is not pure.
• Sea water: A homogeneous mixture of water, salts (like NaCl), and other dissolved substances. It is not pure.
• Gold: An element (Au) with a fixed composition and consistent properties. It is a pure substance.

Step 2: Identify the Pure Substance

Among the options, gold is an element and thus a pure substance. The others are mixtures.

Final Answer

Gold is the substance present in pure form.

Related Topics and Formulae

Pure Substance: A form of matter that has a fixed chemical composition and characteristic properties. Examples include elements (e.g., gold, iron) and compounds (e.g., pure water, sodium chloride).

Mixture: A combination of two or more substances that are not chemically bonded. Mixtures can be homogeneous (uniform composition, like sea water) or heterogeneous (non-uniform composition, like soil).

Separation Techniques: Mixtures can be separated using physical methods such as filtration, distillation, or chromatography, but pure substances cannot be separated further by physical means.

Chemical Formula (for Gold): Gold is represented by the symbol $\mathrm{Au}$ (from Latin "aurum").