Course Outline


Mixtures



 

After completing this unit you should be able to:


Heterogeneous vs Homogeneous

A sample of matter is called a heterogeneous mixture when we can see two or more phases separated by boundaries.  The phases might be a solid and a liquid, such as dietary fiber suspended in water, or two liquids, like an oil and vinegar salad dressing, or other combinations of solids, liquids, and gases.  The components of heterogeneous mixtures can usually, but not always, be separated by simple mechanical processes such as filtering or decanting.  For example, we could separate the dietary fiber from a suspension in water by pouring the mixture into a coffee filter.  The water passes through and the fiber is collected on the paper.  The oil and vinegar of our salad dressing can be separated by allowing the mixture to stand, and then carefully decanting the less dense oil layer from the more dense water layer.

Some mixtures are only recognized as heterogeneous when observed through a microscope.  Mayonnaise, for example, is an emulsion consisting of very fine droplets of oil and water.

When a sample of matter appears to be the same throughout, i.e. a single uniform phase even at very high magnification, it is referred to as homogeneous.  A homogeneous sample may consist of a single chemical substance, or it may be a mixture of two or more different substances.  Homogeneous mixtures are also called solutions, a term that is most familiar when one of the components is a liquid.  For example, a solution of sodium chloride (table salt) in water is homogeneous.  Any sample taken from the original solution would taste the same, and would have salt and water in the same proportions.  Air is another well known homogeneous mixture, consisting primarily of the gases nitrogen and oxygen.  Solid homogeneous mixtures of metals are called alloys.  'Gold' used in jewelry is usually a mixture of gold with silver and other metals, and stainless steel is a mixture of iron, chromium, and nickel.  Homogeneous mixtures often have their own unique properties.  Bronze, for example, is harder than either of the metals, copper and tin, from which it is composed, a property which led to the development of useful tools during the age that is named for it.

The components of a homogeneous mixture can usually be separated by taking advantage of the different properties of the individual components.  Distillation is a common technique in which a mixture is heated until the component that boils at the lowest temperature becomes a vapor and can be separated.  The vapor can be cooled and collected in a separate vessel, and the other components of the mixture are left in the original flask.  Another important separation process is chromatography.  You may have used paper from a coffee filter to separate the components of an ink or dye for example (see this experiment).

Homogeneous mixtures can have variable composition.  Let's consider salt solutions.  We can make salt solutions by dissolving salt in water in just about any proportions we choose.  The exact properties of the solution, how salty it tastes for example, will depend on the proportions.  A solution of salt in water conducts electricity, but the amount of current that can be carried depends on the amount of salt dissolved in a given volume of water.  To describe a mixture completely, we need to give not only the identity of the components of which it is composed but also the relative amounts of the components.

Percentage Composition

One way to describe a mixture quantitatively is to report the percentage by mass of the components.  Percentages are related to fractions, but where fractions are parts of a whole (one), percentages are parts per hundred.  Fractions are converted to percentages simply by multiplying by 100.

For example, when 25.0 g of  a particular salt solution is evaporated to dryness (heated so that all of the water is boiled away), the residual salt is found to have a mass of 1.32 g.   Percentage by mass of salt in the original solution is calculated by dividing the mass of the salt by the total mass of the original solution, which gives the fraction of the total that is salt, and then multiplying by 100 to express this fraction as %:

Notice that the units of the original measurements cancel out.  The percentage by mass of water in the original solution must be 100 - 5.28 = 94.72 %, because the total of all components is always 100%.

When the percentage by mass is known, it can be used to calculate the amount of each component in a given amount of the mixture.  For example, how much salt would be contained in 15.3 g of a salt solution that is 5.28 % sodium chloride by mass?  To solve, we use the percentage to build a conversion factor, 5.28 g of salt per 100 g of solution.

As a final example, let's calculate the volume of water that would be needed to make a 5.28 % solution from 12.4 g of sodium chloride.  First, we calculate what the mass of the solution will be:

Of this mass of solution, 12.4 g is the salt, so the mass of water required is 234.85 - 12.4 = 222.45 g of water.  Finally , we can use the density of water (1.00 g/mL) to convert this mass to a volume, 222 mL.

Proportions can also be reported as percentage by volume.  The common unit 'proof' used for alcoholic beverages is twice the percentage by volume of ethyl alcohol in the drink.  For example, 80 proof vodka is 40 % ethyl alcohol by volume.  To calculate the volume of ethyl alcohol in a 1.5 fluid ounce shot of vodka:

When we build a conversion factor from a percentage we can use any unit that is appropriate to the calculation at hand, so long as it is of the correct type (in this case a volume unit).  40 gallons per 100 gallons and 40 microliters per 100 microliters are additional representations of 40 % by volume.

Pure Substances

When all attempts to separate a homogeneous sample of matter into components with different properties using physical processes fail, we must conclude that the sample is not a mixture but a pure substance.  A pure substance has a characteristic density, undergoes phase changes at particular temperatures (melting point and boiling point), and has other fixed and unchanging properties such as solubility, taste, odor, and appearance that are independent of the source of the substance.  The physical properties of a substance are characteristics that are observed without causing any change in its chemical identity.

Pure substances can be changed into different substances by chemical processes.  For example, if an electric current is passed through water, two colorless, odorless gases are produced (hydrogen and oxygen).  The total mass of the gases is found to equal the mass of water that disappears.  The gases are clearly not water but two new substances with their own unique properties.  We can mix them together, but the mixture is still a gas at room temperature, not liquid water.  The passage of electricity has caused a chemical process to occur.  By passing a spark through the mixture of hydrogen and oxygen we can cause another chemical reaction to occur; there is an explosion and the mixture is converted back to water.  We can say hydrogen reacts with oxygen to produce water.  The characteristic reactions that a substance undergoes are its chemical properties.

Taken all together, the physical and chemical properties of a substance define what it is.  While different substances may have some properties in common, no two substances will have the same complete set of properties.  Chemists make use of the differences in properties between substances to separate and characterize them.  Each unique substance is given a unique name.  Early in the development of chemistry these names were descriptive of a source or property, Chile saltpeter and oil of vitriol for example, but such names have been replaced by much more useful systematic names as we shall see shortly.

 

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