Identification of Substances by Physical Properties

Introduction Physical Properties – Taste (do NOT taste!!) – Color – Odor – Melting point – Boiling point (P atm = P vapor ) – Solubility – Density – Viscosity – Refractive index

Boiling point Boiling point: P atm = P vapor Lower P atm : liquid boils at lower temp. Higher P atm : liquid boils at higher temp. Ex: P atm = higher than 1 atm: water boils above 100°C P atm = 760 mmHg (1 atm): water boils at 100°C P atm = lower than 1 atm: water boils below 100°C

Nomograph Instead of using the nomograph, you can use the temperature correction equation from the Basic Lab Techniques Lab. TC= (760mmHg – P atm ) (0.037°C/mm) – Current barometric pressure: 29.92inHg – Don’t forget to convert to mmHg before plugging it into the equation!! – Show your work

Unknowns Obtain and keep the same solid and liquid unknowns for all parts of the lab Make sure you record your unknowns in the unknown blanks on your lab sheet

A. Solubility Qualitatively determine the solubility of naphthalene in: – Water – Cyclohexane – Ethyl alcohol Add 2-3 crystals to 2-3mL of each solvent in separate test tubes (clean and dry) – ~same amount of crystals in each one Place a cork on the test tube and shake briefly Cloudiness indicates insolubility

In three new, clean test tubes place 2-3mL of the same solvents used for mothballs – Water – Cyclohexane – Ethyl alcohol Add 4-5 drops of toluene Formation of two layers indicates immiscibility A. Solubility

Obtain your solid and liquid unknown Use same three solvents – Water – Cyclohexane – Ethyl alcohol Determine solubility of each A. Solubility

B. Density Determine the densities of your two unknowns Density of unknown solid Weigh 1.5g solid unknown, record Fill a 10mL graduated cylinder ½ full with solvent that your unknown is insoluble in, record volume Be careful not to get liquid on the inside walls because you don’t want your solid to adhere on the walls Add unknown to grad. Cylinder – Don’t lose any solid – Make sure that all solid is under then surface of the liquid Tap the side with your finger (flick) Add 1-2 drops of soap solution if not settling Use volume to calculate density

Density of unknown liquid Weight clean, dry 50mL erlenmeyer flask Obtain 15mL of unknown liquid in test tube Pipet 10mL of unknown into 50mL flask – Quickly weigh flask Calculate density SAVE LIQUID FOR BOILING POINT DETERMINATION!! B. Density

C. Melting Point of Solid Unknown Obtain a sealed capillary tube Pulverize a small portion of your solid unknown with end of a test tube on a clean watch glass Partially fill capillary with your unknown by gently tapping open capillary end in pulverized sample Turn the capillary over and drop into glass tube onto counter with sealed end down – Repeat until sample is at the bottom of the tube and 5mm in height

C. Melting Point of Solid Unknown Set up melting point apparatus Place a rubber band ~5cm above bulb of thermometer with attached capillary Place thermometer into beaker of water so that the sample is covered by water – Thermometer off bottom or walls of beaker – Capillary top above water level Heat water gently while stirring with stir rod Observe capillary tube, at the moment the solid melts record the temperature Important to record range: – When solid begins to melt and when it is completely melted

Use temperature correction equation given in beginning of slides to correct temperatures and obtain the true melting point C. Melting Point of Solid Unknown

Put 3mL of the unknown liquid (from when used in part B – density) into clean, dry test tube Fit the test tube into two-hole rubber stopper that has one slit Insert your thermometer into the hole with the slit and a right-angle-bend glass tubes into the other Add 1-2 small boiling chips Thermometer ~1cm above surface of unknown liquid Clamp test tube in ring stand and connect the right- angle-bend tubing to a length of rubber tubing that reaches to the sink D. Boiling Point of Liquid Unknown

YOUR UNKNOWN IS FLAMMABLE, KEEP AWAY FROM OPEN FLAMES! Heat water gradually and watch for changes in temperature Temperature will become constant at the boiling point Correct the boiling point, as explained previously D. Boiling Point of Liquid Unknown

E. Unknown Identification Your unknowns are going two of the unknowns listed in table 1 Compare the properties you have determined with those in table 1 Identify your unknowns Dispose of your unknowns in the waste containers that are marked

Precautions!! Cyclohexane is HIGHLY FLAMMABLE – Keep away from flames