Final answer:
A mercury thermometer contains mercury which is a pure substance in the form of an element; however, the thermometer in its entirety is not a pure substance because it combines multiple materials. Pure substances, like mercury, have fixed, consistent properties and composition.
Step-by-step explanation:
A Mercury Thermometer is considered a pure substance when referring to the mercury contained inside it. Mercury in its elemental form is a pure substance and falls under the category of elements, which are substances made up of only one type of atom and have a constant composition throughout. Mercury is often used in devices like thermometers due to its properties like uniform expansion over a wide temperature range and its ability to stay in liquid form at room temperature.
However, it is important to note that a mercury thermometer as a whole is not a pure substance because it contains other components, such as the glass tube which is a separate material. Each component within the thermometer has its distinct identity and composition.
Pure substances have fixed properties, like sharp melting and boiling points, and can be identified through methods like chromatography. In contrast, mixtures can be homogeneous or heterogeneous and retain the properties of their individual components, without a uniform composition throughout.