Identifying Pure Substances via Melting Point

Hi Gülen, let's work through these summary questions on pure substances together. We will explore how to identify pure substances using scientific definitions and temperature graphs.

Let's start with Question One. We need to choose the best meaning of the word pure in science from the three options: A, B, or C.

In science, a pure substance is made of only one type of atom or one type of molecule, meaning all the particles in it are identical. Option B describes natural substances, which are often complex mixtures, and Option C is too narrow because compounds and elements can be mixed. Therefore, Option A is the correct definition.

Now, let's look at Question Two. Tim heats a sample and plots its temperature over time, as shown in Figure Seven. Let's recreate Tim's graph so we can analyze it.

Notice the flat, horizontal section in the middle of Tim's graph. Let's highlight this region, where the temperature stays constant for a period of time.

This horizontal line, or plateau, shows that the substance has a fixed, sharp melting or boiling point. Since its temperature remains constant while changing state, we can conclude that the sample is a pure substance.

Let's summarize this decision in writing. The sample is a pure substance because the graph shows a clear horizontal plateau, meaning it has a fixed melting point.