8th Grade Lab Science (Cribb & Duane)
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Chapter 8

All Mixed Up: Mixtures and Solutions

 

PIZZA & SODA

Think of what might just be one of your favorite meals – pizza and a soda!  If that meal is one of your favorites, you are enjoying two foods that are types of mixtures.  Those foods are substances that are made up of more than two substances that can easily be separated by physical means.  The pepperoni, mushrooms, and peppers on a pizza can be easily removed from the dough and cheese.  The carbon dioxide (fizz) in the soda bubbles out over time.  You could also separate the water from the dyes and sugar and secret formulae that make up the rest of the soda.  Being able to isolate and separate the substances of a mixture by physical means is a characteristic of a mixture.

 

SEPARATING BY PHYSICAL MEANS

The substances that make up a mixture can be separated and identified by physical means.  That means, that the substances do not undergo a chemical change and therefore retain their characteristic properties. Those characteristic properties can be used to help separate mixtures into substances.  Think of an Oreo cookie.  Do you lick the icing before eating the cookie?  You are separating the substances of the cookie by physical means.  The icing retains its characteristic sweet taste! Physically separating these substances in this manner is effective when separating substances found in heterogeneous mixtures.  You simply rely upon the appearance of the mixture to separate out its substances.

Other physical properties can be used to separate the substances of a mixture.  One way involves using the physical property of magnetism to separate iron particles in a mixture.  Objects that contain iron, are attracted to magnets.  In a mixture that contains metal objects, a magnet can separate the magnetic particles from the non-metal particles.  If you have a mixture of iron filings and yellow sulfur powder, a magnet can separate the filings from each other.  Don’t heat the substances however, since heat would cause a chemical reaction that creates the substance iron sulfide!

Size is another physical property that can be used to separate substances.  Filters and sieves of varying sizes allow small sized particles to fall through, leaving behind the larger particles.  Separating by size can be an effective way to separate substances. 

A solution mixes gases, solids, and liquids.  Filtering is a means of separating solid particles from the liquid.  If the particles are clearly visible, simply decanting – pouring off the liquid portion, is a way to separate the solid from the liquid. 

Filtering or decanting will not separate substances that have dissolved in a solution however.  For that, you must rely upon using phase changes to separate.  Remember how substances retain their characteristic properties in a mixture?  This is useful for separating substances in a mixture.  This is because boiling point is one of those characteristic properties.  In other words, boiling off the liquid in a solution leaves behind the solid that dissolved in the mixture.  Re-condensing the vapor after boiling returns that substance to its liquid state.  This process is called distillation.

Characteristic properties of density and solubility are other means to separate substances.  In using this method, remember that water has a density of 1.0 g / mL.  If you have a mixture of solid particles, some whose density is less than the density of water, placing the mixture in water will separate the substances – the low density particles will float, the high density particles will sink.  This method also works if the particles have different solubilities.  In that case, insoluble particles will remain visible, where as the soluble particles will dissolve and “disappear”. 

 

 

MIXTURES

Mixtures consist of combinations of pure substances that remain separate from each other, but share the same physical space.  Mixtures do not have their own unique characteristic properties.  Instead, the substances that make up mixtures retain their own properties.  Those pure substances simply share the same physical space, and do not react or combine with each other. 

This differs from a compound.  As you know, the substances in a compound cannot be separated from each other by physical means.  They can by chemical means however.  Separating by chemical means changes the identity of the substance.  For example, separating water into hydrogen and oxygen requires electrolysis.  Other ways to separate a compound into its elements include chemical reactions.  Compounds also contain the same proportion of elements.  Water is always two hydrogen atoms bound to one oxygen atom.  Mixtures do not always contain the same proportion of the substances that make them up.  

Mixtures can be either homogenous or heterogeneous.  A mixture in which the materials can be easily distinguished is a heterogeneous mixture.  In heterogeneous mixtures, the particles are not evenly spaced.  They are random.  The pizza chef doesn’t measure accurately when he places the toppings on the dough.  As a result, pizza is a heterogeneous mixture.  Homogenous mixtures contain two or more gaseous, liquid, or solid substances blended evenly throughout.  Your favorite soda is an example of a homogenous mixture.  The carbon dioxide bubbles, the sugar, and the particles of the secret formula are evenly spaced throughout the soda.  A soda is an example of a solution.  Any solution is an example of a homogeneous mixture. 

 

SOLUTIONS

A solution is a type of mixture that forms when one substance dissolves in another.  It is a homogeneous mixture because all the dissolved particles are evenly spread out throughout the liquids.  In a soda, the dissolved particles include both solids (sugar etc.) and gases (carbon dioxide).  When a particle dissolves, it seems to “disappear”.  In reality, the particles don’t “disappear”.  Instead, they become so small that they are no longer visible.  When particles dissolve, they break apart into individual molecules.  Dissolving of one substance into another substance is dependent upon a number of factors.  These variables include temperature, pressure, and surface area.

All solutions consists of two parts.  One part is the substance that is being dissolved – the solute.  The other part is the substance doing the dissolving – the solvent.  In the example of a salt water solution, salt – the thing that is dissolved, is the solute, and the water – the thing that does the dissolving, is the solvent.  Liquid water is a very useful solvent since many substances are able to dissolve in water.  Because of this, water is often referred to as the “universal solvent”.   Water is a good solvent because it is a polar molecule whose shape makes spaces between molecules when it exists in the liquid state.

Water cannot dissolve all particles however.  Particles that are non-polar will not dissolve in water.  Oils and fats are non-polar substances that do not mix with water and cannot be dissolved by the “universal solvent”.  Soaps are substances that can dissolve oils.  Some vitamins that are an essential part of your diet also are unable to dissolve in water. They are soluble in fat however.  Because of this, some fat in your diet is necessary to maintain a proper balance of vitamins.

There are many kinds of solutions depending upon what dissolves into what.  A common solution involves solid substances dissolving in liquids.  Many kinds of solid particles dissolve in liquids.  Table salt (sodium chloride) and table sugar (sucrose) are two common solids that easily dissolve in water.  When adding salt or sugar to a cup of water, the solid “disappears”, creating a salt water solution or sugar water solution is created.

Gases can also dissolve in a liquid.  As noted above, soda is an example.  Carbonated sodas have dissolved carbon dioxide gas.  The carbonation gives the soda its “fizz”.  Local rivers and streams also have dissolved oxygen and nitrogen in the water. 

In addition, liquids can dissolve in other liquids, gases can dissolve other gases, and solids can dissolve in other solids.  Metal alloys are types of mixtures where solids are “dissolved” in other solids. 

 

Table 8.1:  Examples of Common Solutions

Solute

Solvent

Example

Gas

Gas

Air (oxygen & nitrogen)

Gas

Liquid

Soda (CO2 in water)

Liquid

Gas

Humid air (water in air)

Liquid

Liquid

Anti-freeze (ethylene glycol in water)

Liquid

Solid

Dental filling (mercury in silver)

Solid

Gas

Soot (carbon) in air

Solid

Liquid

Sea water (Salt in water)

Solid

Solid

Gold jewelry (copper in gold)

 

SOLUBLE SUBSTANCES

The ability for a substance to dissolve in a substance is an additional characteristic property.  If a substance is “able to be dissolved”, that substance is called a soluble substance.  For example, salt and sugar are both soluble in water, since both of those substances dissolve when mixed in water.  If a substance is unable to dissolve, that substance is called insoluble.  If you recall, oils and some vitamins do not dissolve in water.  They are insoluble in water.  Oils are soluble in soap and some vitamins are soluble in fats however.  It all depends of the relationship between the solute and the solvent. 

If a substance is soluble –“able to be dissolved”, how much of the substance can dissolve? To tell how much, we need to understand the characteristic property of solubility.  Solubility is a measure of how much of a solute can dissolve in a given amount of a solvent under certain conditions.  The main condition that we will investigate is temperature.  To understand solubility, we must first review what is meant by concentration and saturated solutions.

 

CONCENTRATION

Let’s make sugar water!  Table sugar (sucrose) is a substance that is soluble in water.  To make sugar water, add a teaspoon of sugar to a cup of water.  If you taste it, it might not be sweet enough.  Add another, and another, until the taste has the right sweetness.  By adding sugar to the solution, you have changed the concentration of the sugar in the water.  A concentrated solution is one in which there is a large amount of solute (sugar) dissolved in the solvent (water).  A dilute solution is one in which there is only a small amount of solute in the solvent. 

However, the terms “concentrated” and “dilute” are too broad a measure of the specific concentration.  Concentrations of solutions can be described more precisely. To be more precise, you must tell how much of the solute dissolved in what volume of the solvent.  In the above example, dilute sugar water has a concentration of 1 teaspoon of sugar in 1 cup of water.  A concentrated solution has 3 teaspoons of sugar in 1 cup of water.  Teaspoons and cups are not metric measurements however.  You will measure concentration in grams and milliliters instead.  Typically, you will measure of concentrations of substances that are soluble in water.  Therefore, the standard unit for concentration is grams (g) of solute in 100 mL of water.  This is expressed as g / 100 mL.

 

SATURATED SOLUTIONS

Dissolving a teaspoon of sugar in a cup of water makes a solution.  It is a solution because the sugar particles “disappeared” as they dissolved in the water.  If you add more sugar, those additional sugar particles may also “disappear”.  There comes a point however when no more sugar can be dissolved in the solution.  If you add more sugar at this point, the additional particles will not dissolve.  Instead, they settle and remain as visible solid on the bottom of the container.  At this point, the solution is saturated.  A saturated solution is a solution that contains all the solute as can possibly hold.  In a saturated solution, no more solute can be dissolved.

 

SOLUBILITY

Solubility is the measure of the maximum amount of a solute that can be dissolved in a given amount of solvent at a given temperature.  In other words, solubility is the amount of solute needed to make a saturated solution – the concentration of solute in a saturated solution.  Since temperature is a variable that can effect how much solute can dissolve, comparing the solubility of substances keep the temperature constant at room temperature, 20C.  An unsaturated solution is any solution that can dissolve more of a solute at a given temperature.

 

Table 8.2: Solubility of Common Substances & Compounds (in water at 20C)

Substance

Chemical Notation

Solubility (g / 100 mL) in Water

Sodium chloride (table salt)

NaCl

35.9 g / 100 mL

Sucrose (table sugar)

C12H24O12

203.9 g / 100 mL

Sodium bicarbonate (baking soda)

  NaHCO3

9.6 g / mL

Sodium hydroxide (lye)

NaOH

109.0 g / 100 mL

Copper sulfate

CuSO4

32.0 g / 100 mL

Potassium nitrate

KNO4

31.6 g / 100 mL

Sodium nitrate

Na SO4

87.6 g / 100 mL

 

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GLOSSARY:

Concentration: The amount of a solute that dissolves in a solvent.  Concentration can be exressed as a percentage (%) or as g / mL

Dissolve: The break down of particles to atoms and molecules that cause the particles to "disappear".

Insoluble:  Describes a substance that is unable to dissolve in a solvent.

Saturated Solution: A solution in which no more solute is able to dissolve in a solvent.  Any solution that contains all the solute it can possibly hold at a given temperature.

Solubility: The measure of how much solute can dissolve in a given amount of solvent.  The maximum amount of a solute that can be dissolved in a given amount of solvent at a given temperature.  Factors that have an effect on solubility are the nature of the solute and solvent, temperature and pressure. 

Soluble: Describes a substance that is able to dissolve in a solvent.

Solute: In a solution, the substance that is dissolved.

Solution: A mixture that appears to have the same composition, color, density, and taste, and is mixed at the atomic or molecular level.

Solvent:  In a solution, the substance in which the solvent is dissolved.  

 

FORMULAE:

Calculating Solublity (concentration of a saturated solution):

formula symbols standard units
concentration = mass solute / volume solvent con = m / v g / mL or g / cm3
Solubility = (Mass solute / Volume solvent) x 100

solubility = conc. x 100

sol = (m / v) x 100 g / 100 mL or g / 100 cm3

 

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ON-LINE RESOURCES:

http://www.sciencebyjones.com/solubility.htm - general information about dissolving & solubility

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/solut.html - describes solutions and the dissolving process.

http://www.geocities.com/proofpurchaseweb/solubility.html - General definition of Solubility

http://www.chem.lsu.edu/lucid/tutorials/solubility/Solubility.html - This site discusses factors that affect solubility

 

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BIBLIOGRAPHY:

Haber-Schaim, Abegg, Dodge, and Walter, Introductory to Physical Science.  Prentice Hall Inc, Englewood, NJ, 1982.

Hurd, Silver, Bacher, and McLaughlin, Prentice Hall Physical Science.  Prentice Hall Inc, Englewood Cliffs, NJ, 1988.

McLaughlin, Thompson, and Zike, Glencoe Science Physical Science.  Glencoe / McGraw Hill, Columbus, OH, 2002.