SCN1288-ABED_1: Lesson 3.3 - Pure Substances and Mixtures

Lesson 3.3 - Pure Substances and Mixtures

You will now learn to classify substances as either pure substances or mixtures.

Read in your textbook:

the section titled Classifying Matter on the bottom of page 15
the section titled Pure Substances and Mixtures on page 16

Then, answer the following questions.

Question 1. What is the definition of a pure substance?

Question 2. What is the definition of a mixture?

Then, return here to continue this lesson.

Look at the pictures of substances in Fig 1.8 on page 16. Identifying a substance as a pure substance or a mixture may be difficult because the scientific meaning is different than the common meaning. Using the scientific definition, pure baking powder is not a pure substance because baking powder is a mixture of a base and an acid. For this science course, be sure to use the scientific definition and not the common definition.

You know it is not easy to tell the difference between a pure substance and a mixture just by looking at them.

Mixtures have different chemical and physical properties than the pure substances that make up the mixtures.

This may sound obvious to you, but consider it carefully. Think about a mixture that is very important to Canadians-"antifreeze". You know that pure water freezes at 0°C and expands by about 10%. If you used pure water in your car's radiator, the water will freeze if the temperature drops below 0°C; the expansion would likely ruin the radiator and might even damage the engine. Therefore, we have antifreeze.

Pure antifreeze will freeze at about -13°C. From your general knowledge, perhaps you think that a mixture of water and antifreeze would freeze at a temperature somewhere between 0°C and -13°C.

Hmmm, winter temperatures can reach -40°C, but our cars are not damaged. Why do the automobile radiators not all freeze when it gets that cold? Something strange happens when water and antifreeze are mixed!

The freezing point gets lower and lower as the percentage of antifreeze in the mixture increases. Then, at about 70% antifreeze and 30% water, the minimum freezing point is reached. If more antifreeze is added, the freezing point increases instead of decreasing. Therefore, the 70/30 ratio is the optimum ratio; more antifreeze than this is not good. In fact, it is bad. Look at the following table to see the freezing points of some antifreeze mixtures.

	Pure Water	Pure Antifreeze ( C₂H₆O₂ )	50/50 C₂H₆O₂/Water	70/30 C₂H₆O₂/Water
Freezing Point	0°C	-13°C	-37°C	-55°C

The radiator in an automobile is made to get rid of excess heat from the motor. What happens to the boiling point of the antifreeze mixture? Pure water boils at 100°C and pure antifreeze boils at 197°C. What do you think will happen to the boiling point of the water/antifreeze mixture? Will it increase in the way the freezing point decreased?

No! You have been foiled again! This mixture behaves strangely. The boiling point is hardly increased at all from the boiling point of water.

	Pure Water	Pure Antifreeze ( C₂H₆O₂ )	50/50 C₂H₆O₂/Water	70/30 C₂H₆O₂/Water
Boiling Point	100°C	197°C	106°C	113°C

Point of interest:

To do its job, the coolant in the radiator must be in liquid form at all times. If it reaches the boiling point and turns into vapour, the cooling ability is lost and the engine will overheat. Have you ever seen a car with its hood up and steam billowing from the radiator?

The operating temperature of a gasoline engine is about 125°C. Hmmm, this is much higher than the 113°C boiling point of a 70/30 antifreeze mixture. How do they increase the boiling point of the antifreeze?

The radiator cap is also a pressure cap. As the antifreeze heats, it expands. The pressure increases until the pressure cap rating is reached (about 15 pounds per square inch). The boiling point of the pressurized antifreeze is now about 138°C, higher than the operating temperature of the motor. If there is too much pressure, the radiator cap has a valve that releases some of the antifreeze into an overflow container. Interesting? You might be aware of the plastic "overflow coolant tank" next to the engine in your car-and now you will know why it is important to the motor!

Question 3. Alanna has a 4 L container of what looks and smells like antifreeze, but she is not sure if it is pure antifreeze or a mixture of antifreeze and water. What would you tell her to do to find out if it is pure antifreeze or a mixture of antifreeze and water?

Question 4. The instructions on the radiator cap of a car tell you not to remove the cap when the radiator is hot. Why must you never remove the radiator cap when the radiator is hot?

Then, return here to continue this lesson.

Answers to Questions:

Question 1. What is the definition of a pure substance?

A pure substance is made up of only one kind of particle. For example, table sugar that you put in your coffee or tea is a pure substance because it contains only particles of sugar.

Question 2. What is the definition of a mixture?

A mixture contains more than one kind of particle. For example, if you accidentally mixed some sugar and salt together, you have a mixture because it now contains both sugar and salt particles.

Click to return to where you left off in this lesson.

Answers to Questions:

Question 3. Alanna has a 4 L container of what looks like antifreeze but she is not sure if it is pure antifreeze or a mixture of antifreeze and water. What would tell her to do to find out if it is pure antifreeze or a mixture of antifreeze and water?

You might suggest one of several ways to find out it the substance is pure antifreeze or a mixture of antifreeze and water.

The simplest is to use an antifreeze tester, which tells you the freezing point of the mixture. If the freezing point was -13°C, then the liquid is pure antifreeze. If the freezing point is more or less -13°C, then it is a mixture.

You could try freezing the liquid in your freezer and noting the temperature.

You could try boiling the liquid and noting the temperature.

You could determine the density and compare it to that of pure antifreeze.

Question 4. The instructions on the radiator cap of a car tell you not to remove the cap when the radiator is hot. Why must you never remove the radiator cap when the radiator is hot?

The antifreeze in a hot radiator is pressurized. If you even loosen the cap, the hot antifreeze will spray out under pressure and you might be burned severely.

Go to the next page to learn more about mixtures.