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C1 - Elements Compounds Mixtures

I can distinguish between elements, compounds, and mixtures

Substances and Mixtures

Matter can be classified into two broad categories: pure substances and mixtures. A pure substance is a form of matter that has an uniform composition and properties that are constant throughout the sample. Elements and compounds are both example of pure substances.

Mixtures are physical combinations of two or more elements and/or compounds. The term “physical combination” refers to mixing two different substances together where the substances do not chemically react. The physical appearance of the substances may change but the atoms and/or molecules in the substances do not change.

Compounds and Chemical Formulas

Chemical symbols are used not only to represent the elements, but also to write chemical formulas for the millions of compounds formed when elements combine. The law of constant composition states that the ratio by mass of the elements in a chemical compound is always the same, regardless of the source of the compound. The law of constant composition can be used to distinguish between compounds and mixtures. Compounds have a constant composition, and mixtures do not. Pure water is always 88.8% oxygen and 11.2% hydrogen by weight, regardless of the source of the water. Brass is an example of a mixture. Brass consists of two elements, copper and zinc, but it can contain as little as 10% or as much as 45% zinc.

The formula for a compound uses atomic symbols to indicate the type of atoms involved and uses subscripts to indicate the number of each atom in the formula. For example, aluminum combines with oxygen to form the compound aluminum oxide. To form aluminum oxide requires two atoms of aluminum and three atoms of oxygen. Therefore, we write the formula for aluminum oxide as Al2O3. The symbol Al tells us that the compound contains aluminum, and the subscript 2 tells us that there are two atoms of aluminum in each molecule. The O tells us that the compound contains oxygen, and the subscript 3 tells us that there are three atoms of oxygen in each molecule. It was decided by chemists that when the subscript for an element is 1, no subscript would be used at all. Thus the chemical formula MgCl2 tells us that one molecule of this substance contains one atom of magnesium and two atoms of chlorine. In formulas that contain parentheses, such as Ca(OH)2, the subscript 2 applies to everything inside the parentheses. Therefore, this formula (calcium hydroxide) contains one atom of calcium, two atoms of oxygen, and two atoms of hydrogen.

Homogeneous Mixtures

A solution is a homogeneous mixture of substances. When you consider that the prefix “homo” means “same,” this definition makes perfectly good sense. Solutions carry the same properties throughout. Take, for example, vinegar that is used in cooking is approximately 5% acetic acid in water. This means that every teaspoon of vinegar that is removed from the container contains 5% acetic acid and 95% water.

A point should be made here that when a solution is said to have uniform properties throughout, the definition is referring to properties at the particle level. Well, what does this mean? Let's consider brass as an example. Brass is an alloy made from copper and zinc. To the naked eye a brass coin seems like it is just one substance but at a particle level two substances are present (copper and zinc). So the brass represents a homogeneous mixture. Now, consider a handful of zinc filings and copper pieces. Is this a homogeneous solution? The properties of any scoop of the “mixture” you are holding would not be consistent with any other scoop you removed from the mixture. The ratio of copper and zinc may be different. Additionally, you would see differences in the color at different places in the mixture. Thus the combination of zinc filings and copper pieces in a pile does not represent a homogeneous mixture.

The solvent and solute are the two basic parts of a solution. The solvent is the substance present in the greatest amount. The solute, then, is the substance present in the least amount. Let’s think for a minute that you are making a cup of hot chocolate. You take a teaspoon of cocoa powder and dissolve it in one cup of hot water. The amount of cocoa powder is less than the amount of water, so the cocoa powder is in the solute and the water is the solvent.

The point should be made that because solutions have the same composition throughout does not mean you cannot vary the composition. If you were to take one cup of water and dissolve \frac{1}{4} teaspoon of table salt in it, a solution would form. The solution would have the same properties throughout, the particles of salt would be so small that they would not be seen and the composition of every milliliter of the solution would be the same. But you can vary the composition of this solution to a point. If you were to add another \frac{1}{2} teaspoon of salt to the cup of water, you would make another solution, but this time there would be a different composition than the last. You still have a solution where the salt particles are so small that they would not be seen and the solution has the same properties throughout, thus it is homogeneous.

Sample question: Name the solute and solvent in each of the following mixtures.

(a) salt water

(b) air

Solution:

(a) solute = salt; solvent = water

(b) solute = oxygen; solvent = nitrogen

Colloids and Suspensions

Recall that a solution is a mixture of substances in such a way that the final product has the same composition throughout. Remember the example of vinegar that is 5%, by mass, acetic acid in water. This clear liquid is a solution since light easily passes through it and it never separates.

On the other hand, colloids are mixtures in which the size of the particles is between 1 \times 10^3 pm and 1 \times 10^6 pm. In meters, these sizes translate to 1 \times 10^{-9} \ m to 1 \times 10^{-6} \ m – between 10 and 1,000 times smaller than a small grain. A common example of a colloid is milk. One way to tell that milk is a colloid is by the Tyndall effect. The Tyndall effect is the scattering of light by particles. This involves shining a light through the mixture: when the light is not allowed to pass through the mixture, that is, the light is scattered, the mixture is considered a colloid. This is why milk appears “cloudy” – or what we think of as “milky.” When light is passed through a true solution, the particles are so small that they do not obstruct the light. However, when light is passed through a colloid, since the particles are larger, they will act as an obstruction to the light and the light is scattered. The particles in a colloid, while able to scatter light, are still small enough so that they do not settle out of solution. Let’s look at a table of some common colloid products that are formed when different phase solutes and solvents are mixed.

Some Common Colloidal Materials

Solid SolventLiquid SolventGaseous Solvent
Solid SoluteRuby, brass, steel Butter, cheese, mustard, Jell-o Marshmallow
Liquid SolutePaint, milk of magnesia Milk, mayonnaise, face creams Whipping cream, shaving cream
Gaseous SoluteAirborn viruses, car exhaust, smoke Fog, clouds Aerosol Sprays

Suspensions are mixtures where the particles settle to the bottom of the container. This means that the particles in a suspension are large enough so that gravity pulls them out of solution. With suspensions, filtration can usually be used to separate the excess particles from the solution. A common example of a suspension is muddy water. If you had a beaker of water and added a handful of fine dirt, even if you stirred it, when you let it stand, dirt would settle to the bottom.

Neither colloids nor suspensions are classified as solutions. In order to be as solution, you must have very small particles evenly distributed, so that the mixture has the same properties throughout. Colloids and suspensions have particles that are too big to be considered a solution.

Lesson Summary

  • Matter can be classified into two broad categories: pure substances and mixtures.
  • A pure substance is a form of matter that has a uniform composition and properties that are constant throughout the sample.
  • Mixtures are physical combinations of two or more elements and/or compounds.
  • Elements and compounds are both example of pure substances.
  • Generally speaking, in a solution, a solute is present in the least amount (less than 50% of the solution) whereas the solvent is present in the greater amount (more than 50% of the solution).
  • A solution is a mixture that has the same properties throughout.
  • Common examples of colloids include milk, butter, Jell-O, and clouds.
  • Suspensions are mixtures in which the particles are large enough so that they settle to the bottom of the container and can be filtered using filter paper.

Vocabulary

Law of constant composition
The ratio by mass of the elements in a chemical compound is always the same.
Mixture
A combination of two or more elements or compounds which have not reacted to bond together; each part in the mixture retains its own properties.
Compound
A substance that is made up of more than one type of atom bonded together.
Solution
a homogeneous mixture of substances
Solvent
the substance in a solution present in the greatest amount
Solute
the substance in a solution present in the least amount
Colloid
type of mixture in which the size of the particles is between 1 \times 10^3 \ pm and 1 \times 10^8 \ pm
The Tyndall effect
the scattering of light by particles
Suspension
type of mixture in which the particles settle to the bottom of the container and can be separated by filtration

Review Questions

Classify each of the following as an element, compound, or mixture.

  1. Salt
  2. Vegetable oil
  3. Gold
  4. Sugar
  5. Salad dressing
  6. Salt water
  7. Water
  8. Copper
  9. Air
  10. Milk
  11. What makes a solution homogeneous?
  12. Distinguish between a solution, a colloid, and a suspension.
  13. What is one true way to tell you have a colloid solution?
  14. The biggest difference between a colloid and a suspension is that:
    1. In colloids, the solute is permanently dissolved in the solvent.
    2. In colloids the particles eventually settle to the bottom.
    3. In suspensions the particles eventually settle to the bottom.
    4. None of these are correct
  15. Karen was working in the lab with an unknown solution. She noticed that there was no precipitate in the bottom of the beaker even after it had been on the lab bench for several days. She tested it with a light and saw that light scattered as it passed through the solution. Karen concluded that the liquid was what type of a mixture?
    1. colloid
    2. suspension
    3. solution
  16. List two examples of colloids.