Experimental Chemistry

Experimental Chemistry Purification of Substances

The Need for Pure Substances: Definitions A pure substance… is a singlesubstance not mixed with anything else A mixture.. contains two or moresubstances

The Importance of Pure Substances • Example #1: Impurities in medicine may cause undesirable side effects.

The Importance of Pure Substances • Example #2: Impurities in metals weaken its structure more than it should be.

The Importance of Pure Substances • Example #3: • Chemicals are added to food and beverages to make them last longer, taste better or look more attractive/appealing. • Safe-for-consumption chemicals

Recognizing Pure Substances • Checking the melting and boiling points of the substance • Performing chromatography

Effect of impurities on M.P. and B.P. Presence of impurities: • Lowers the melting point (greater % of impurity = lower m.p.) • Raises the boiling point (greater % of impurity = higher b.p.)

Concept check! An impure substance of X melts at around 90 °C. What is most likely to be the melting point of X? • between 90 °C to 110 °C • between 80 °C to 90 °C • below 80 °C • cannot be determined

Separation Techniquesmethods of purification • Filtration • to separate insoluble solid from liquid

Separation Techniquesmethods of purification • Filtration Filters are used to purify water at water purification plants.

Separation Techniquesmethods of purification • Daily applications of filtration

Separation Techniquesmethods of purification • Evaporation (to dryness) • to separate soluble solid from liquid evaporating dish

Separation Techniquesmethods of purification • Evaporation (to dryness) Limitations: • Some substances decompose strongly when heated strongly. • Soluble impurities may be left behind after all the water has been removed.

Separation Techniquesmethods of purification • Crystallization • to separate soluble solid from liquid as crystals BETTER METHOD!

Separation Techniquesmethods of purification • Crystallization • Heating is stopped when a hot saturated solution is formed.

How do we test for a saturated solution? Procedure: • A clean glass rod can be used to test whether a solution is saturated. • It is dipped into the solution and removed. There will be a small amount of solution on the rod. • If small crystals form on the rod as the solution cools, the solution is saturated.

Separation Techniquesmethods of purification • Decanting • just pouring the liquid away! • to separate a dense insoluble solid from liquid

THINK! How can you separate a mixture containing salt and sand? Add a suitable solvent, and filter

Separating Salt and Sand • Add some distilled water into the mixture of salt and sand. Stir and make sure all the salt has dissolved. • Filter the mixture. Sand Salt solution

Separating Salt and Sand • Wash the residue with a little distilled water to remove all the salt solution from it. • Evaporate the filtrate to dryness. Salt solution Sand

Separation Techniquesmethods of purification • Sublimation • to separate a mixture of solids, one of which sublimes

Separation Techniquesmethods of purification Some substances that sublime: • solid carbon dioxide • anhydrous iron (III) chloride • anhydrous aluminium chloride • iodine • ammonium chloride • naphthalene

THINK! How would you purify solid sodium nitrate contaminated with solid lead(II) sulfate? sodium nitrate is soluble in water, but lead sulfate` is not

Separation Techniquesmethods of purification • Simple distillation • to separate a pure liquid from a solution • the process of boiling the liquid and condensing the vapour

Separation Techniquesmethods of purification • Simple distillation • solution is heated until it boils • pure liquid turns into vapour and leaves the flask • vapour is then cooled by a condenser, which changes it back into a liquid • the liquid is collected in the conical flask, and is called the distillate

Separation Techniquesmethods of purification • Simple distillation Where is the bulb of the thermometer placed? This ensures that the thermometer measures the boiling point of the substance being distilled. It is placed beside the side arm of the distillation flask, and should not be dipped in solution.

Separation Techniquesmethods of purification • Simple distillation Where does the water enter and leave the condenser? Cold running water enters from the bottom of the condenser and leaves from the top.

Around 97% of the water on Earth is seawater.

Obtaining Pure Water from Seawater: Desalination • by distillation of seawater [one way] • disadvantage: expensive method because oil is the main fuel used to heat the water in the distillation process, and a lot of oil is required, which makes the process very costly Chemistry in Daily Life: Pure Water for Singapore page 25

Separation Techniquesmethods of purification • Fractional distillation • to separate mixtures of miscible liquids with widely differing boiling points

Temperature / oC Time / s Separation Techniquesmethods of purification How does the temperature change as a solution of ethanol and water undergoes fractional distillation? 1. The temperature of the mixture increases as it is heated.

Temperature / oC Time / s Separation Techniquesmethods of purification How does the temperature change as a solution of ethanol and water undergoes fractional distillation? 2. At 78 °C, ethanol distils over. The temperature remains constant until all the ethanol has distilled out of the round-bottomed flask.

Temperature / oC Time / s Separation Techniquesmethods of purification How does the temperature change as a solution of ethanol and water undergoes fractional distillation? 3. The temperature then increases until 100 °C. At 100 °C, water distils over.The temperature remains unchanged as water is being distilled.

Separation Techniquesmethods of purification • Fractional distillation the liquid with the lower boiling point will be distilled first Why is the fractionating column filled with glass beads (or plates/spirals)?

Separation Techniquesmethods of purification • Use of separating funnel • to separate immiscible liquids

Separation Techniquesmethods of purification • Use of separating funnel • pour the mixture into the separating funnel, ensuring the tap is closed

Separation Techniquesmethods of purification • Use of separating funnel • support the separating funnel with a retort stand, placing a clean beaker below it

Separation Techniquesmethods of purification • Use of separating funnel • allow some time for the liquids to separate completely • open the tap to allow the denser liquid at the bottom to drain into the beaker * close the tap before the liquid in the top layer runs out! the denser liquid

Separation Techniquesmethods of purification • Use of separating funnel • place another beaker below the separating funnel and allow a little of the liquid in the top layer into it * dispose this liquid collected The mixture is separated!

QUIZ! How would you separate… • red dye from a mixture of red and blue dyes in solution? • oxygen from air? • barium sulfate from a mixture of (insoluble) barium sulfate and water?

QUIZ! How would you separate… • water from oil and water? • petrol from crude oil? • iodine crystals from iodine and copper(II) oxide? • salt from sea water?

Experimental Chemistry