Physical Properties of Two Types of Solids

11 Chemistry Lab Name: ______

Physical Properties of Two Types of Solids

Purpose: To study some physical properties of two types of solids - molecular and ionic.

Introduction

Some solids consist of molecules in which the atoms are held together by covalent bonds. The attractive forces between molecules (intermolecular force) hold the molecules together as a solid. These intermolecular forces are called van der Waals forces of attraction and these solids are molecular solids.

Other solids consist of an array of positive and negative ions, arranged in a crystal lattice in such a way that every positive ion has only negative neighbours and vice versa. The solid is held together because of the electrostatic attractions between ions of opposite charge. Such solids are called ionic solids.

In this experiment, you will examine five physical properties (odour, hardness, melting point, solubility, and electrical conductivity when mixed with water) of a molecular and ionic solid. You will attempt to relate these properties to the strength of the forces of attraction between the particles in each solid. The solids are sodium chloride, camphor and unknowns.

Apparatus

Materials

sodium chloride(NaCl) camphor(C10H16O) Unknown A Unknown B

Procedure

Neatly record all observations on the data table provided in the chart at the bottom of this page.

A.  On their own piece of paper towel place separate ~ 0.2 g samples of sodium chloride, camphor and the unknowns. Observe the odour of each solid.

B.  Check the hardness by attempting to crush a few small crystals of each solid between a scoopula and a watch glass. Note whether each feels soft or hard.

C.  You are going to decide whether each of the substances have a high or low melting point. Place a few crystals of sodium chloride on an inverted crucible cover. Support the crucible cover on a clay triangle on an iron ring and heat it gently for approximately one minute, or when the substance melts. After the substance has melted, remove the heat. Be Careful: The melted solid can probably catch fire and burn with a smoky flame. (For this reason, only a few crystals of each solid should be used.)

D.  Put the remaining crystals of each of the solids in their own test tubes. Make sure you know which substance is in which test tube. You should have nearly 0.1 g of solid in each test tube. Add approximately 3 mL of distilled water and shake vigorously, use a stir stick if necessary.

E.  Teacher Demonstration: The teacher will use a conductivity apparatus to see if the substance will conduct electricity when mixed with water.

Analysis Questions

1.  Which of the substances did you indicate as hard? What does hardness indicate about attractive forces between particles?

2.  All solids can be thought of as being composed of particles. Which of the substances did you indicate as having a strong odour? What does a strong odour indicate about the ease with which the particles in a solid leave its surface and go up your nose?

3.  In your own words explain why ionic substances are brittle. What is happening at the atomic level?

4.  From your observations of hardness and melting points, how does the strength of forces of attraction in a molecular compound compare with the strength of the attractions between ions of opposite charge?

5.  Which solutions conducted electricity? What must be present in order to conduct electricity when dissolved in water?