5.7 Unique Properties
Your fingerprint is a unique characteristic that distinguishes you
from others. Fingerprints are commonly used to identify people.
Your fingerprint will remain the same, even if other physical
properties--such as your height or weight--change.
Characteristic Physical Properties
Substances also have unique characteristics that can be used to
identify them. A characteristic property is a physical or
chemical property that is unique to a particular substance.
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- characteristic property: a property that is unique to a
substance and that can be used to identify the substance
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Freezing Point, Melting Point, and Boiling Point
The temperatures at which common physical changes like
freezing, melting, and boiling take place are characteristic
properties of a substance because they do not change.
- Freezing point is the temperature at which a substance turns
from liquid to solid. The freezing point of a substance is also the
same temperature as its melting point.
- Melting point is the temperature at which a substance turns
from a solid into a liquid. The freezing/melting point of pure water
is 0 °C.
- Boiling point is the temperature at which a substance forms
bubbles of vapour that escape into the air. Pure water boils at
100 °C.
Pure Substances
Carbon has the highest melting point of all pure substances. It
melts at 3550 °C. Helium has the lowest melting point of all pure
substances. It melts at -272.2 °C. Tungsten has the highest
boiling point. It boils at 5927 °C.
Gallium is a metal that is so soft, it can be cut with a knife. Its
melting point of 29.8 °C is unusually low for a metal. The
temperature of the human body is 37 °C. That means that if you
hold gallium in the palm of your hand, as shown in Figure 1, the
gallium will melt. Despite its low melting point, gallium has an
extremely high boiling point of 2403 °C. The range of
temperatures at which gallium is a liquid is the greatest of any
pure metallic substance.
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Figure 1 The unusual melting and boiling points of gallium make
it useful for special high-temperature thermometers.
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Figure not reproduced.
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Mixtures
The freezing/melting point of a mixture of substances is different
than the freezing/melting point of a pure substance. For example,
dissolving another substance into water forms a mixture with a
lower freezing point than pure water. For example, a salt and
water mixture can have a freezing point as low as -10 °C. This is
why salt is used to make icy roads safer to drive on (Figure 2).
Spreading salt on the roads allows the saltwater solution to
remain liquid below 0 °C. Liquid water is safer to drive on than
ice.
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Figure 2 Icy roads are dangerous. A salt spreader adds salt to
icy roads. This creates a mixture with a freezing point lower than
0 °C, making roads safer at low temperatures.
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Figure not reproduced.
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Liquid water is also important in a car engine. You could use
water to cool your car's engine, as long as its temperatures
stayed between the freezing and boiling points of water.
However, once the temperature drops below 0 °C, the cooling
system freezes solid. Adding antifreeze to the water forms a
mixture with a lower freezing point, and a higher boiling point,
than pure water. This prevents the cooling system from freezing
in the winter and "boiling over" in the summer.
Density
Density is also a characteristic physical property. Density is the
mass in a given volume of a substance. Density is expressed as
mass per volume, or mass/volume. Each substance has its own
characteristic density. For example, the density of pure water is 1
g/mL. This means that 1 mL of pure water has a mass of 1 g.
Densities are quite variable. For example, the density of lead is
11.36 g/cm3 while the density of aluminum is only 2.7 g/cm3.
Gold has a density of 19.32 g/cm3.
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- density: a measure of how much mass is contained in a given
volume of a substance; you calculate density by dividing the
mass of a sample by its volume
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Lifejackets
The density of the average human is just about the same as that
of water. We can sink or float, depending on how much air we
hold in our lungs. A lifejacket is very helpful when we are unable
to hold enough air in our lungs.
An inflatable lifejacket like the one in Figure 3 has a tiny cylinder
of compressed carbon dioxide gas attached inside it. When you
pull the ripcord, shown on the lower left in the photo, the
compressed gas rushes out of the cylinder and inflates the
lifejacket. The mass of the jacket does not change, but its volume
increases significantly. That means the density of the lifejacket is
much lower than the density of the water you are in. This fact
helps to keep you safely floating on the water instead of sinking
to the bottom.
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Figure 3 In Canada, the law requires every boat to carry a
personal flotation device (PFD) for each passenger.
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Figure not reproduced.
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5.7 Wrap Up
- A characteristic property is a property that is unique to a
substance.
- Characteristic properties can be used to identify a substance.
- Freezing point, melting point, boiling point, and density are
characteristic physical properties.
- Adding another substance to a pure substance produces a
mixture with a lower freezing point.
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CHECK YOUR LEARNING
1. Explain why the boiling point of water is a characteristic
physical property, but the temperature of water is not. K/U, T/I
2. Describe what happens to the freezing point of pure water
when salt is added to it. K/U
6. A drinking glass at a crime scene contains a clear, colourless
liquid that could be pure water. You are the investigator. Before
you send the liquid to a lab for testing, you want to rule out the
possibility that the glass contains water. Use what you know
about the characteristic properties of water to design a simple
test. T/I, C