Course Outline


Properties of Matter



 

After completing this unit you should be able to:


Density

By definition, matter has mass.  The mass of a sample tell us how much we have but on its own it tells us nothing about what the sample is.  Matter also has volume, and as the volume of a sample increases so does the mass.  For a sample that is homogeneous, i.e. uniform throughout, the mass of any part of the sample will be directly proportional to its volume, and the ratio of mass divided by volume will always give the same value, i.e. the ratio is a constant. This constant, called the density, is a characteristic property of the sample, and is useful because it has a single value independent of the quantity of the sample.
Measurement of a single property usually cannot tell you what a sample of matter is, but it can tell you what it might be, and what it is not.

Common units of density are g/mL (or equivalently g/cm3) for solids and liquids, and g/L for gases.  Air for example has a density of 1.2 g/L at 20 oC and 1 atmosphere pressure, and the densities of aluminum and gold are 2.7 g/mL and 19.3 g/mL respectively.  Values for many other substances can be found in reference tables.

You should learn the density of liquid water = 1.00 g/mL (to 3 sf., at 20 oC and 1 atm pressure).

The density of a sample of matter is determined by measuring its mass and volume and performing a calculation.  For example, a 'gold' ring was found to have a mass of 5.94 g and a volume of 0.39 mL.  The mass was measured using an electronic balance, and the volume was measured by noting the increase in volume when the ring was placed into a measuring cylinder containing water.
Notice that the answer is correctly rounded to 2 sf., and the answer includes the units!  If you have any doubt that including units is important when reporting a value, read this story:
The Crash of Flight 143; more details here.

Since the density of pure gold is 19.3 g/mL, we can conclude that the ring described above is not pure gold.  Read this story for the first application of this principle: Archimedes and King Hiero's Crown.

When the density of a substance is known, it can be used to calculate the expected volume of a given mass, or the mass of a given volume.  Simple algebra can be used to rearrange the equation that defines density so that the unknown is isolated on one side of the equation.  Alternatively, we can think of using density as a conversion factor (derived from, for example, 19.3 g gold = 1 mL gold) to convert a given mass to a volume, or vice versa.  Remember that if the units of the given quantity do not match the units in the density you are using, additional conversion factors will be required so that units cancel out correctly.

Problem: What is the volume of a gold ingot that has a mass of 13.4 lb?
(The density of gold is 19.3 g/cm3 and 1 kg = 2.205 lb).

Solution: The density of gold is given as 19.3 g/cm3 so we will need to convert pounds to grams using the given conversion factor to convert pounds to kilograms, and the conversion factor we have learned to convert kilograms to grams.  Using the density value provided will give an answer in cubic centimeters.

The value of the answer is rounded to 3 sf since the original measurement has 3 sf and all conversion factors used have at least 3 sf.  Notice how the units in the conversions cancel out properly.

Temperature

Values for density should always be reported at the particular temperature and pressure at which they were measured.  Density varies with temperature because volume varies with temperature.  Daniel Fahrenheit made use of this property in inventing the mercury thermometer.  When the mercury in the bulb is warmed it expands up a narrow glass tube, and calibrations on the tube allow us to read the temperature.  On the Fahrenheit scale water freezes at 32 oF and boils at 212 oF, a difference of 180 F degrees.  Although this is a familiar temperature scale in the US, for scientific measurements we use the scale invented by Anders Celsius.  On the Celsius scale water freezes at 0 oC and boils at 100 oC, a difference of 100 C degrees.

To convert a temperature measured in degrees Fahrenheit to degrees Celsius we subtract 32 (to account for the different zero point) and multiply by a conversion factor because 100 C degrees = 180 F degrees.  For example, to express the body temperature of a healthy person, 98.6 oF, in degrees Celcius:

Using simple algebra, this relationship can be put into the familiar form of a linear equation:

where F is the temperature in oF, C is the temperature in oC, 9/5 is the slope, and 32 is the intercept (the point where C = 0 and the straight line crosses the F axis).

For calculations in chemistry we frequently need to use temperature on a scale measured from absolute zero, the lowest theoretical temperature for matter.  This is the Kelvin scale, and absolute zero is 0 K.  On the Celsius scale this temperature is defined as -273.15 oC.  Degrees on the Kelvin scale are the same size as degrees on the Celsius scale (made so by defining the temperature of the triple point of water as 273.16 K and 0.01 oC), so to convert Celsius to kelvins we add 273.15, or more commonly 273, a precise enough value for most situations.  You should learn the conversion equation: K = C + 273, eg

Changes of State

The familiar states of matter are solid, liquid, and gas.  Solids hold their shape, liquids flow, and gases expand to fill their containers.  Individual substances change from one state to another at characteristic temperatures under defined conditions.  For example, when heated in an open container at atmospheric pressure, the temperature of pure isopropyl alcohol (the alcohol that is mixed with water in rubbing alcohol) at its boiling point is 82 oC.  The liquid will stay at this temperature until it has all been converted to vapor.  If we bring the vapor into contact with a surface colder than 82 oC, it will condense back to the liquid state and be indistinguishable from the original liquid.  Similarly, to melt sucrose (table sugar) we must heat it at 185 oC, and if we cool melted sucrose below 185 oC it will solidify.

Melting point and boiling point are physical properties that can be used to characterize a pure substance.  The fundamental chemical nature of the substance is not changed by a change of state.  Melting, boiling, and other processes that do not alter the chemical identity of a substance are called physical changes.  On the other hand, if we heat melted sucrose strongly it turns black, and after cooling we have a black solid that is obviously not sucrose.  If we heat this black solid, it does not melt at 185 oC.  A process that alters the chemical identity of a substance is called a chemical change, and the characteristic chemical changes that a substance undergoes are its chemical properties.

 

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