Properties, Classifications and the Periodic Law

Properties, Classifications and the Periodic Law

Introduction

Substances, both elements and compounds, are described in terms of their chemical and physical properties. Physical properties are those characteristics that can be measured or observed without the substance changing its identity. Characteristics such as melting point, color, hardness, etc. are physical properties. Note that after observing or determining such properties the substance is still the same substance. Chemical properties on the other hand are those that describe the action of a substance as it loses its identity and forms a new substance. The statement "sodium reacts with chlorine to form a new substance, sodium chloride" describes a chemical change. Both the sodium and the chlorine change their identity and form a new substance that has a new set of characteristics.

People have always used classifications as means of communicating to each other information about members of a class. For example, two classifications of plants are trees and shrubs. Once we know the characteristics of the class then we can associate those characteristics to members of that class. Thus, if you were told that a tree is growing in my yard, you immediately conjure up in your mind the idea of a "tree"; you don't think of an object that we call a "shrub".

The elements and their compounds can be classified according to their chemical and physical characteristics. A couple of major classifications that you are no doubt already familiar with are: acids vs. bases, metals vs. nonmetals, organic compounds vs. inorganic compounds, etc. Furthermore, it is also known that the properties of elements vary in a regular way with the atomic number of the elements. This observation is known as the Periodic Law, which states that "Similar physical and chemical properties recur periodically when the elements are listed in order of increasing atomic number"¹. Note that the Periodic Law states that the property will change in a regular way; either increase OR decrease. It doesn't predict the direction of the change. The periodic law can also be extended to certain compounds of the elements.

The objective of the experiment is to:

a) determine the physical properties of some elements (namely sodium (Na), copper (Cu), carbon (C), and sulfur (S)) and classify the elements as metals or nonmetals

b) determine whether or not the periodic law can be applied to the solubility of a series of compounds for which the only difference between the compounds is the atomic number of one element in the compound. For this portion of the experiment we will be using: lithium carbonate (Li₂CO₃), sodium carbonate (Na₂CO₃), and potassium carbonate (K₂CO₃).

Materials Available

4 samples of elements, hammer, volt meter, spatula, steel wool

Experimental (work in pairs)

a) Physical Properties of Metals and Nonmetals

i. What are the physical properties that are used to determine if a substance is a metal or a nonmetal? What properties describe a metal? A nonmetal?

ii. Once you have determined the answers above you are ready to test the samples provided. Your instructor will point out their location and safety precautions.

b) Solubility of Metal Carbonates

i. Weigh 0.50 g ± 0.05 g of sodium carbonate, lithium carbonate, and potassium carbonate and place each sample in a separate test tube. Make sure that you label each test tube.

ii. Add to each test tube 5 drops of water and stir with a stirring rod for a couple of minutes.^*

iii. If the solid has not dissolved remove the stirring rod and add an additional 5 drops of water and reinsert the stirring rod and stir for a couple of minutes.

iv. Repeat step iii until the solid has just barely dissolved. Record how many drops of water it took to dissolve each solid.

^* It is important that you stir the solid - water mixture for 2-3 minutes because the salts dissolve at different rates. Your goal is to compare the absolute solubilities of the salts (lithium carbonate, sodium carbonate, and potassium carbonate) and not the rates at which they dissolve. The solid is considered totally dissolved only when the solution is relatively clear and there are no longer particles of the solid present. If you are not sure if the substance is dissolved, please ask your instructor BEFORE disposing of the solution.

Properties, Classifications and the Periodic Law

Name ______________________________ Section _________ Date _____________

a) Physical Properties of Metals and Nonmetals

Physical Properties Carbon Copper Sodium Sulfur

__________(Y/N) __________ __________ __________ __________

Based on your observations:

Which elements would you classify as metals? ___________________

Which elements would you classify as nonmetals? _________________

Where on the periodic table do the elements that you classified as metals appear?

(left side, right side, top, bottom, etc.) ______________________

Where on the periodic table do the elements that you classified as nonmetals appear?

(left side, right side, top, bottom, etc.) ______________________

b) Solubility of Metal Carbonates

Record the number of drops of water that it took to dissolve the 0.50 g samples of each salt.

Salt Number of Drops of Water Required to Dissolve 0.50 g

Lithium carbonate ______

Potassium carbonate ______

Sodium carbonate ______

List the elements lithium, potassium , and sodium in order of increasing atomic number (lowest atomic number on the left, highest atomic number on the right):

____________, ____________, ______________

List the compounds in order of increasing solubility (least soluble on the left and most soluble on the right):

________________________, _____________________, ____________________

Does the solubility of these compounds obey the Periodic Law? _____ Explain your response