DExperiment #1
DENSITY OF SUBSTANCES
Prepared by Paul Okweye and Malinda Gilmore
Purpose of the Experiment
To learn about the properties of matter such as density that are used as a method of physical identification. In this experiment the objectives are:
Background Information
Density, like boiling point, color, odor, solubility, and melting point, is a physical property of matter. Therefore, density may be used in identifying matter. Density is defined as mass per unit volume and is expressed mathematically as d = m / v (Equation 1: d is density, m is mass, and v is volume). The density of a sample of matter represents the mass contained within a unit volume of space in the sample. The units of density, therefore, are quoted in terms of grams per milliliter (g/ml) or grams per cubic centimeter (g/cm3) for most solid and liquid samples of matter. The density of a sample represents the mass of the specific sample divided by its volume.
density (g/ml) = mass (g) ÷ volume (ml or cm3) Eqn. 1
Often, a density varies with temperature because of the volume of the sample such as gases. Therefore, densities are usually determined and reported at room temperature (about 25oC; see Table 1). References such as chemical handbooks always specify the temperature at which a density was measured.
As previously stated, density can be used as a method of identification. Various things that density can be useful for are listed below:
Table 1. Densities of various substances at room temperature, 25oC.
Several techniques are used for the determination of density of substances. In general, a density determination involves the determination of the mass of the sample divided by the determination of the volume of the sample. However, the method used for determining mass or volume depends on whether or not the sample is a solid or a liquid.
For solid samples, the volume of the solid can be determined using Archimedes’s principle, which states that an insoluble, nonreactive solid will displace a volume of liquid equal to its own volume. Typically, a solid is added to a liquid in a volumetric container (such as a graduated cylinder) and the change in the liquid level is determined.
For liquids, very precise values of density may be determined by measuring an accurate volume of liquid in a container that can then be weighed and then determining the mass of the liquid that was measured. A convenient container for determining the volume of a liquid is to weigh a particular volume of liquid in a graduated cylinder.
The density of substances is very important especially when talking about buoyancy – the tendency or capacity to remain afloat in a liquid or rise in air or gas. Often one asks the question, “Why does ice float in water?” The answer to that question depends totally on density of the substances involved. When dealing with water, water can be in the form of ice, liquid or solid (Table 2). The density of ice is 0.917 g/cm3 and then density of water in its liquid state at 25oC (room temperature) is 0.999 g/cm3. Therefore, the density of ice is less than the density of water so that is why ice floats in water.
Temperature (oC) |
Density of Water (g/cm3) |
0 (ice) |
0.91700 |
0 (liquid water) |
0.99984 |
2 |
0.99994 |
4 |
0.99997 |
10 |
0.99970 |
25 |
0.99707 |
100 |
0.95836 |
Table 2. Temperature Dependence of Water Density
Safety Precautions
Materials and Chemicals
Procedures
A. Determination of the Density of Solids
Note:
Percent Error = Experimental Value – Actual Value x 100%
Accepted Value
B. Density of Pure Liquids
Pure Water (Distilled Water)
Rubbing Alcohol
C. Density of Solutions
Chemical solutions are often described in concentrations and most times in terms of the solutions’ percent composition on a weight basis. For example, a 1% sodium chloride (NaCl) solution contains 1 g of NaCl in every 100 mL of solution (which corresponds to 1 g of NaCl for every 99 mL of water (H2O) present).
Name_______________________________________________________________________________
Lab Partner____________________________Section/Day/Time_______________________________
Experiment #1
DENSITY OF SUBSTANCES
DATA SHEET
A. Determination of the Density of Solids
Sample Name ______________________________
Appearance of Solid ______________________________
Weight (g) of the Solid ______________________________
Initial Volume (mL) of Water for the Solid ______________________________
Final Volume (mL) of Water for the Solid ______________________________
Volume (mL) of the Solid ______________________________
Experimental Value of Density (g/mL) of Solid______________________________
Actual Density (g/mL) of the Solid ______________________________
Percent Error of Solid ______________________________
B. Determination of the Density of Pure Liquids
Pure Water (Distilled Water)
Initial Weight (g) of the Graduated Cylinder (Water) ______________________________ Final Weight (g) of the Graduated Cylinder (Water) ______________________________
Weight (g) of Water Sample______________________________
Volume (mL) of Water ______________________________ Experimental Value of Density (g/mL) of Water ______________________________ Actual Density (g/mL) of the Water ______________________________ Percent Error of Water ______________________________
Name_______________________________________________________________________________
Lab Partner____________________________Section/Day/Time_______________________________
Experiment #1
DENSITY OF SUBSTANCES
DATA SHEET
Rubbing Alcohol
Initial Weight (g) of the Graduated Cylinder (Rubbing Alcohol)______________________________ Final Weight (g) of the Graduated Cylinder (Rubbing Alcohol) ______________________________
Weight (g) of Rubbing Alcohol Sample______________________________
Volume (mL) of Rubbing Alcohol ______________________________ Experimental Value of Density (g/mL) of Rubbing Alcohol ______________________________
Actual Density (g/mL) of the Rubbing Alcohol ______________________________ Percent Error of Rubbing Alcohol ______________________________
C. Determination of the Density of Solutions
% NaCl |
Mass (g) |
Volume (mL) |
Density (g/mL: Calculated) |
Density (g/mL: Actual) |
% error |
5 |
|||||
10 |
|||||
15 |
|||||
20 |
|||||
25 |
Note: Show calculations in your lab report.
Name____________________________________________________________________________
Lab.Partner____________________________Section/Day/Time_____________________________
Experiment #1
DENSITY OF SUBSTANCES
HOMEWORK SHEET
1. Explain density in words.
2. What error would be introduced into the determination of the density of the solid if the solid were hollow? Would the density be too high or too low?
3. An insoluble, nonreactive metal sphere weighing 18.45 g is added to 21.7 ml of water in a graduated cylinder. The water level rises to 26.8 ml. Calculate the density of the metal.
4. An empty graduated cylinder weighs 34.4257 g. A 10-ml pipet sample of an unknown liquid is transferred to the graduated cylinder. The graduated cylinder weighs 40.1825 g when weighed with the liquid in it. Calculate the density of the unknown liquid.
5. Your data for the density of the NaCl (sodium chloride) solutions should have produced a straight line when plotted. How could this plot be used to determine the density of any concentration of sodium chloride solution?
6. Examine your graph and determine the density for each of the following percents of NaCl: 3%, 9%, 15%, 21%, and 45%.
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