MU #1 Matter is classified as a pure substance or as a mixture of substances. 

Example

Hydrogen gas, the air, the chairs, the planets, etc.

MU #2 A pure substance (element or compound) has a constant composition and constant properties throughout a given sample, and from sample to sample.

Example

Hydrogen gas and  water

MU #3 Elements are substances that are composed of atoms that have the same atomic number (see periodic table of elements). Elements cannot be broken down by chemical change. 

Elements can be monatomic or polyatomic.

Example

A compound is a pure substance made of different elements. Substances can only be decomposed chemically. Binary compounds contain two elements.

Example

Example

Electricity is used to chemically decompose water into oxygen and hydrogen gas.

2 H₂O(l) ---electricity---> 2 H₂(g) + O₂(g)

MU #4 Mixtures are composed of two or more different substances that can be separated by physical means. When different substances are mixed together, a homogeneous or heterogeneous mixture is formed. The proportions of components in a mixture can be varied. Each component in a mixture retains its original properties.

Example 

A homogeneous mixture or solution has the same properties throughout.

Example 

Air is a gaseous solution composed of N₂(g) + O₂ (g) + Ar(g) + CO₂ (g) + other rare gases.

Example

Salted water, NaCl(aq), is a liquid solution.

Note: (aq) = aqueous or H₂O(l)

Therefore, NaCl(aq) means that NaCl(s) is dissolved in water.

Example

Brass is a solid solution composed of Cu(s) and Zn(s)

A heterogeneous mixture does not have the same properties throughout.

Example

Concrete, sand in water, pizza, etc.

TEST YOUR UNDERSTANDING

6/02 Given:

Which diagram represents a mixture?

8/02 Which species represents a chemical compound? (1) N₂               (2) NH₄⁺         (3) Na             (4) NaHCO₃

8/02 On a field trip, Student X and Student Y collected two rock samples. Analysis revealed that both rocks contained lead and sulfur. One rock contained a certain percentage of lead and sulfur by mass, and the other rock contained a different percentage of lead and sulfur by mass. Student X stated that the rocks contained two different mixtures of lead and sulfur. Student Y stated that the rocks contained two different compounds of lead and sulfur. Their teacher stated that both students could be correct.

Draw particle diagrams in each of the rock diagrams provided in your answer booklet to show how Student X’s and Student Y’s explanations could both be correct. Use the symbols in the key provided in your answer booklet to sketch lead and sulfur atoms.

1/03

1 One similarity between all mixtures and compounds is that both

(1) are heterogeneous  (2) are homogeneous (3) combine in a definite ratio  (4) consist of two or more substances

2 Draw two different compounds using the representations for atoms of element X and element Z given below.

3  Draw a mixture of these two compounds.

6/03

1 Which substance can be decomposed by a chemical change? (1) Co  (2) CO  (3) Cr (4) Cu

2  Which of these terms refers to matter that could he heterogeneous? (1) element    (2) mixture  (3) compound  (4) solution

3 Given the reaction between two different elements in the gaseous state:

Box A below represents a mixture of the two reactants before the reaction occurs. The product of this reaction is a gas. In another box draw the system after the reaction has gone to completion, based on the Law of Conservation of Matter.

8/03

14 Which substance can not be decomposed by a chemical change? (1) Ne (2) N₂O (3) HF (4) H₂O

18 Which of these contains only one substance? (1) distilled water (2) sugar water (3) saltwater (4) rainwater

43 Given the diagrams X, Y, and Z below:

Which diagram or diagrams represent a mixture of elements A and B? (1) X, only (3) X and Y (2) Z, only (4) X and Z

1/04

Base your answers to questions 65 through 67 on the particle diagrams below, which show atoms and/ or molecules in three different samples of matter at STP.

65 Which sample represents a pure substance?
66 When two atoms of y react with one atom of z, a compound forms. Using the number of atoms shown in sample 2, what is the maximum number of molecules of this compound that can be formed?

67 Explain why xx does not represent a compound.

6/04

17 At STP, the element oxygen can exist as either O₂ or O₃ gas molecules. These two forms of the element have

    (1) the same chemical and physical properties    (2) the same chemical properties and different physical properties

    (3) different chemical properties and the same physical properties  (4) different chemical and physical properties

8/04

7 Which substance represents a compound?   (1) C(s) (2) Co(s) (3) CO(g) (4) O₂(g)

Base your answers to questions 54 through 57 on the particle diagrams below. Samples A, B, and C contain molecules at STP.

54 Explain why the average kinetic energy of sample B is equal to the average kinetic energy of sample C.

55 Explain, in terms of the composition, why sample A represents a pure substance.

56 Explain why sample C could represent a mixture of fluorine and hydrogen chloride.

57 Contrast sample A and sample B, in terms of compounds and mixtures. Include both sample A and sample B in your answer.

1/05

39 Which particle diagram represents one pure substance, only?

6/05

10 Which substance can be decomposed by a chemical change?  (1) calcium (2) potassium (3) copper (4) ammonia 

13 Which terms are used to identify pure substances? 

                    (1) an element and a mixture (2) an element and a compound (3) a solution and a mixture (4) a solution and a compound 

43 Given the simple representations for atoms of two elements: 

Which particle diagram represents molecules of only one compound in the gaseous phase? 

1/06

6/06

8/06

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