Properties of Pure Substances and Mixtures Lesson 11

Properties of Pure Substances and Mixtures The Melting point of frozen water is 0º C. This is also referred to as the freezing point of water. Like water, all substances have their own melting point. Scientists use this property to help them identify the substance. Example: If a substance looks like pure water but does not melt at 0º C, then it is not pure water.

Antifreeze and water are mixed and used in car radiators Antifreeze and water are mixed and used in car radiators. Sometimes more or less water is used, depending on the weather. During the winter, some people use antifreeze with a freezing point of – 40 º C. During the summer, more water can be added to the antifreeze to produce a mixture with a higher freezing point.

Mechanics can tell how much antifreeze is mixed by testing the density of the solution. A gauge is used to suck up a bit of antifreeze. A float inside the gauge records the density of the mixture.

Questions: 1. Name two physical properties that are used to test mixtures and pure substances. Temperature Density 2. Which mixture would have more water: antifreeze guaranteed to – 40 º C or antifreeze guaranteed to -20 º C. Explain how you know. The -20 º C antifreeze would have more water because it is closer the freezing point of water.

A more common mixture containing water is salt water which normally has a 3.5 % weight to volume. Salt water has some interesting properties compared to regular water. Salt water is more viscous. This means that it does not flow as easy as water, it sticks to itself more. The density of salt water is higher. This means it has more mass than the same volume quantity of pure water. 3.5% Salt water’s freezing point is at -2 º C which is slightly lower than pure water. Salt water also has a slightly higher boiling point than pure water.

If salt increases the temperature of water before it boils, why do chefs put salt in their water? It allows the water to get hotter which cooks the food faster.

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