Introduction:
A mixture is a combination of two or more substances that are not chemically combined. It can be separated into its constituents through physical processes. Energy may or may not be given out during the preparation of a mixture. The composition of a mixture is variable, meaning that it can vary in proportion and amounts of its components. However, a mixture does not have a fixed melting point.

Explanation:
1. Separation into constituents:
A mixture can be separated into its constituents through various physical processes. This is one of the fundamental characteristics of a mixture. For example, if we have a mixture of sand and water, we can separate them by filtration.

2. Energy in the preparation of a mixture:
Energy may or may not be given out during the preparation of a mixture. It depends on the specific process involved in mixing the substances. For instance, when we dissolve sugar in water, energy is required to break the intermolecular forces between sugar molecules and water molecules. On the other hand, when we mix two gases together, no energy is given out or absorbed.

3. Variable composition:
The composition of a mixture is variable, meaning that it can vary in proportion and amounts of its components. This is another characteristic of a mixture. For example, if we mix salt and water, we can vary the amount of salt dissolved in the water to create a more concentrated or diluted solution.

4. No fixed melting point:
Unlike pure substances, a mixture does not have a fixed melting point. The melting point of a substance is the temperature at which it changes from a solid to a liquid state. In a mixture, each component may have its own melting point, and the overall mixture will not have a specific melting point.

Conclusion:
In conclusion, the statement "A mixture has a fixed melting point" is not true. A mixture does not have a fixed melting point as it is a combination of different substances with their own individual melting points.