MU #1 Differences in properties such as density, particle size, molecular polarity, boiling point and freezing point, and solubility permit physical separation of the components of the mixture.

Density

Example

In a mixture of oil, water, and sand the density of oil < the density water < the density sand

Particle Size

Example

In a mixture of clay, sand, and pebble the particle size of clay < the particle size of sand < the particle size of pebble.

Example

In a mixture of sand and water the sand can be filtered out.

Molecular polarity

Molecules that are polar attracts one another and other polar molecules. Those attractions are called intermolecular forces. Molecular polarity ranges from none to high. The higher the molecular polarity the stronger the intermolecular force is.

Example

Consider a mixture of two substances where one has molecular polarity and the other has none. If a substance with molecular polarity is introduced in the mixture it will connect to the substance that has molecular polarity and could be removed from the mixture.

Boiling and freezing points

Distillation is the process of separating substances from a mixture according to their respective boiling points.

Example

In a mixture of alcohol and water the alcohol will boil first since it has a lower boiling point than water. The alcohol vapor is then condensed and collected.

Example

Petroleum contains gasoline, kerosene, fuel oil, and many useful other substances. When petroleum is heated different substances evaporate at different temperature, therefore each substance can be collected according to its boiling point. This process is called fractional distillation.

Solubility

Example

Sodium chloride NaCl(s) is soluble in water while calcium carbonate, CaCO3(s) is not. If a mixture of both is made water can be added to separate the two substances then the insoluble substance is filtered out of the mixture.

chromatography is the process of separating substances from a mixture that rely on the differential affinities of substances for a gas or liquid mobile medium and for a stationary adsorbing medium through which they pass, such as paper, gelatin, or magnesia.

Example

Paper chromatography

When the end of a piece of paper is dipped into water the water molecules keep finding new places (polar regions) to stick to and so the water molecules climb up the paper being replaced by new water molecules below. Other molecules which might be dissolved in the water will also be carried along up the paper.

A spot of dye is placed on the paper above the level of the water. As the water moves up, the dye molecules will move with it if they are more strongly attracted to the water molecules than to the paper molecules. If the dye molecules are more strongly attracted to the paper than to the water, they will move more slowly than the water or even not at all. What if the dye is a mixture? If two or more dyes have been mixed to form an ink, then they may move at different rates as the water moves up the paper. If this happens, they will separate and we can identify them.

TEST YOUR UNDERSTANDING

8/02

Which mixture can be separated by using the equipment shown below?

            (1) NaCl(aq) and SiO₂(s)  (2) NaCl(aq) and C₆H₁₂O₆(aq)   (3) CO₂(aq) and NaCl(aq)  (4) CO₂(aq) and C₆H₁₂O₆(aq)

1/03

1 When a mixture of water, sand, and salt is filtered, what passes through the filter paper?

(1) water, only             (2) water and sand, only            (3) water and salt, only     (4) water, sand, and salt

2  Which physical property makes it possible to separate the components of crude oil by means of distillation?

                (1) melting point    (2) conductivity             (3) solubility               (4) boiling point

3  At equilibrium, nitrogen, hydrogen, and ammonia gases form a mixture in a sealed container. The data table below gives some characteristics of these substances.

Describe how to separate ammonia from hydrogen and nitrogen.

1/04

12 A bottle of rubbing alcohol contains both 2-propanol and water. These liquids can be separated
by the process of distillation because the 2-propanol and water

    (1) have combined chemically and retain their different boiling points (3) have combined physically and retain their different boiling points
    (2) have combined chemically and have the same boiling point (4) have combined physically and have the same boiling point

6/05

Base your answers to questions 80 on the information and diagram below and on your knowledge of chemistry.

Crude oil is a mixture of many hydrocarbons that have different numbers of carbon atoms. The use of a fractionating tower allows the separation of this mixture based on the boiling points of the hydrocarbons. To begin the separation process, the crude oil is heated to about 400°C in a furnace, causing many of the hydrocarbons of the crude oil to vaporize. The vaporized mixture is pumped into a fractionating tower that is usually more than 30 meters tall. The temperature of the tower is highest at the bottom. As vaporized samples of hydrocarbons travel up the tower, they cool and condense. The liquid hydrocarbons are collected on trays and removed from the tower. The diagram below illustrates the fractional distillation of the crude oil and the temperature ranges in which the different hydrocarbons condense.