Identification of an Unknown Liquid

 

Introduction

 

A substance can be identified by observing its chemical and physical properties. Physical properties are those that a substance can exhibit without undergoing a change in chemical composition. Chemical properties are those that a substance exhibits only by undergoing a change in chemical composition (i.e., a chemical reaction).

 

The identification of a substance by its physical properties is the more desirable method because the sample is not destroyed in the determination. Some of the more common physical properties are: color, odor, density, solubility, state (solid, liquid, or gas at 20°C), melting point, boiling point, and refractive index. Probably the major difficulty is that in order to determine accurate values, one must be using a pure substance. Most materials found in nature are not pure.

 

In this experiment you will identify an unknown liquid (a pure substance) after measuring values of the following physical properties:

 

1. density           

The density of a substance is a measure of its mass per unit volume. Density units for liquids may be expressed in various sources as g/mL, g/cm³, or g/cc, but all have the same numerical value. The density of a liquid will vary with the temperature but this change is usually negligible if the temperature change is small.

 

2. boiling point

 The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure, usually that of the atmosphere.

3. refractive index

The index of refraction, expressed as n_D²⁰, is the ratio of the constant velocity of light in a vacuum to the variable velocity of light in a medium. The number 20 represents the Celsius temperature of the sample while D represents the monochromatic D line of the sodium spectrum. The refractive index of a substance changes if the temperature changes, or if the color of the light used changes. Refraction is responsible for the bent spoon effect observed when a spoon is partially submerged in water. Refractive index measurements can be used to determine solution concentrations, ascertain purity and identify a compound.

 

   

Safety

 

Most organic liquids are toxic, so you should exercise care whenever handling them. Do not breathe the vapors. Do not allow unknown liquids to come in contact with your skin. If this happens flush the area with copious amounts of water.

 

The unknowns are flammable. Keep them away from all flames.

 

  

Procedure

 

1. Density determination of an unknown liquid.

 

1. Weigh a clean, dry, stoppered 50 mL Erlenmeyer flask as accurately as possible.
2. Pipet 10.00 mL of the unknown liquid into the Erlenmeyer flask. Replace the stopper avoiding any contact between the stopper and the liquid. Weigh the Erlenmeyer flask, stopper and added liquid on the same balance used for the initial measurement.
3. Calculate the mass of the 10.00 mL aliquot of unknown liquid added to the flask.
4.  Calculate the density of the unknown to four significant figures. This is necessary since many of the unknown liquids in Table #1 have very similar densities.
5. Once the density has been calculated, try to eliminate compounds that have densities far from this experimental value (of course, you need to be sure that your density determination is accurate).

 

2. Boiling point determination for an unknown liquid

 

1. Add approximately 5 mL of unknown liquid to a 25 X 200 mm pyrex test tube. A boiling chip may be added to the liquid in the test tube to avoid bumping or uneven boiling of the liquid.
2. Close the test tube with a rubber stopper fitted with a thermometer and a right -angle glass tube.
3. Run the rubber hose from the bent glass tube into the sink to keep flammable vapors away from the bunsen burner.  The liquid level should be so that the thermometer bulb is about 1 cm above the surface of the liquid. Do not attempt to slide the thermometer in the stopper; it will break!
4. Clamp the test tube to a ring stand. Support a 400 mL beaker half-full of water on an iron ring and a wire gauze. The test tube should be immersed in the beaker of water so that the unknown liquid is below the water level but the thermometer bulb is above the water level. Use care to assure that the thermometer does not touch the inside of the test tube and the test tube does not come in contact with the beaker. See Figure above right.
5. Heat the beaker of water gradually. Record the temperature at which the liquid in the test tube boils freely, then remove the burner.
6. If you wish to recheck this boiling point temperature, allow the water-bath temperature to fall until the unknown liquid stops boiling, then reheat the beaker of water. Again, using this experimental boiling point value, eliminate unknown possibilities that have boiling points far from this determined value.

 

3. Refractive index determination for an unknown liquid.

 

1. Before making a refractive index measurement of the unknown liquid, your instructor should brief you on the proper operating procedure for the refractometer. Next, for practice, determine the refractive index of distilled water. When you are confident determine the refractive index of your unknown liquid. The refractive index should be read to at least four decimal places.
2. Since the laboratory is not equipped with a constant temperature bath, the temp-erature of the refractometer will probably be higher than 20°C. The n_D values decrease as the temperature rises (n_D is sometimes called “optical density”). To compare your experimental value with the values given in Table #1, add 0.00045 for each degree above 20°C.

n_D²⁰ = n_D^T + (T – 20)(.00045).

 

4. Final determination of the unknown liquid

 

Using the data collected for density, boiling point, and refractive index of the unknown liquid identify the unknown liquid. See Table 1 below. If more than one unknown or if none of the unknowns agree with your data then your measurements were not accurate enough and should be repeated.

 

Table 1. Physical Properties of Some Common Liquids