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Academic Word List: Exercise 4

Read the following text, paying particular attention to the highlighted words.

AN EXPERIMENT IN CHEMISTRY

PRELIMINARY

The chemistry laboratory is a place where you will learn by observation what the behaviour of matter is. Forget preconceived notions about what is supposed to happen in a particular experiment. Follow directions carefully, and see what actually does happen. Be meticulous (very exact and careful) in recording the true observation even though you 'know' something else should happen. Ask yourself why the particular behaviour was observed. Consult your instructor (teacher) if necessary. In this way, you will develop your ability for critical scientific observation.

EXPERIMENT I: DENSITY OF SOLIDS

The density of a substance is defined as its mass per unit volume. The most obvious way to determine the density of a solid is to weigh a sample of the solid and then find out the volume that the sample occupies. In this experiment, you will be supplied with variously shaped pieces of metal. You are asked to determine the density of each specimen and then, by comparison with a table of known densities, to identify the metal in each specimen. As shown in Table E1, density is a characteristic property.

Table E1: Densities of Some Common Metals, g/cc.

Aluminium

2.7

Lead

11.4

Magnesium

1.8

Monel metal alloy

8.9

Steel (Fe, 1% C)

7.8

Tin

7.3

Wood's metal alloy

9.7

Zinc

7.1

PROCEDURE

Procure (obtain) an unknown specimen from your instructor. Weigh the sample accurately on an analytical balance.

Determine the volume of your specimen by measuring the appropriate dimensions. For example, for a cylindrical sample, measure the diameter and length of the cylinder. Calculate the volume of the sample.

Determine the volume of your specimen directly by carefully sliding the specimen into a graduated cylinder containing a known volume of water. Make sure that no air bubbles are trapped. Note the total volume of the water and specimen.

Repeat with another unknown as directed by your instructor.

QUESTIONS

1. Which of the two methods of finding the volume of the solid is more precise? Explain.

2. Indicate how each of the following affects your calculated density: (a) part of the specimen sticks out of the water; (b) an air bubble is trapped under the specimen in the graduated cylinder; (c) alcohol (density, 0.79 g/cc.) is inadvertently substituted for water (density, 1.00 g/cc) in the cylinder.

3. On the basis of the above experiment, devise a method for determining the density of a powdered solid.

4. Given a metal specimen from Table E1 in the shape of a right cone of altitude 3.5cm with a base of diameter 25 cm. If its total weight is 41.82 g, what is the metal?

(Experimental Chemistry, by Sienko and Plane).

Now try the exercises. Exercise a

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