Final answer:
The treatment in Sarah's experiment is the saltwater. Treatment refers to a condition expected to cause change in a study, and the saltwater, with its freezing point depression property, is being studied against plain water to determine its effect on freezing times.
Step-by-step explanation:
In the experiment you described, Sarah is investigating how the addition of salt to water affects its freezing point. The treatment in this experiment is the saltwater. A treatment in a scientific experiment refers to any condition applied to the subjects or units being studied that is expected to cause a change, which in this case is the saltwater mixture being compared to plain water to assess its impact on the freezing time.
When salt (NaCl) is dissolved in water, it breaks into its constituent ions, Na+ and Cl-, and this process is known as dissociation. The presence of these dissolved ions interferes with the formation of the solid ice structure, thereby causing a phenomenon known as freezing point depression. This means that saltwater has a lower freezing point compared to pure water, which typically freezes at 0°C.
In the context of making homemade ice cream, adding salt to ice lowers its temperature below the freezing point of water. This is crucial because for ice cream to freeze, the temperature must be lower than -3°C. According to the instruction for an ice cream maker, which suggest adding one part salt (NaCl) to 11 parts water by mass, the resulting solution will have a reduced freezing point significant enough to freeze the ice cream mixture.