TOPIC 2: EXPERIMENTAL CHEMISTRY

Experimental Design

 

Volumes of Liquids

• SI unit: cubic metre (m³)
• Large volume measurement: decimetres (dm³) 
• 1 dm³ = 1 000 cm³ 
• Daily life measurement: millilitres (ml) or litres(l) 
• 1 litre = 1 000 ml 
• Apparatus for measuring liquids depends on, the volume being measured and how accurate the measurement needs to be. 
• Beaker hold approximate volume of 100 cm³ and 250 cm³. 
• Conical flask hold approximate volume of 100 cm³ and 250 cm³. 
• Measuring cylinder has accuracy to 1 cm³. 
• Reading to be taken nearer to the meniscus (bottom line). 
• If reading is 23 cm³, should not write 23.0 cm³ as the ‘0’ means accurate to 0.1 cm³. 
• Burette has long scale of 0 – 50 cm³, accurate to 0.1 cm³. 
• Liquid level to be measured before and after tap opening. The difference of volume gives the liquid volume poured off. 
• Bulb pipette measures exact volumes such as 20.0, 25.0 or 50.0 cm³, not odd volumes such as 31.0 cm³. 

Volumes of Gases

Measured with gas syringe, up to 100 cm³

 

Temperature

• Measured with thermometer. 2 types are:  a) Mercury-in-glass  b) Alcohol-in-glass.

• SI Unit: Kelvin (K)

• Daily life measurement: degree Celcius (^oC) , K = ^oC + 273

 Time

• SI Unit: seconds (s) Other Units: minutes (min)/hour (h)

• Measured with: (a) Clock (b) Digital stopwatch 

Mass

• Mass – the measure of amount of matter in a substance

• SI Unit: kilogram (kg),Other Units: grams (g)/milligrams (mg)

• Large volume measurements: tonnes (t)
• 1 tonne = 1 000 kg 

Methods of Purification and Analysis 

 

 

 

• Pure substance – single substance not mixed with anything else

E.g: white sugar, copper sulfate crystals, distilled water

• Mixture – contains two or more substances. Its quantity is more on Earth.

E.g: seawater (salt, water & dissolved solids), milk (fats & dissolved solids)

 Checking the Purity of Substances 

• Pure substances have FIXED MELTING AND BOILING POINTS.Example, Pure water boils at 100^oC and melts at 0^oC.

• Impure substances have NO FIXED MELTING AND BOILING POINTS. They melt and boil at a RANGE OF TEMPERATURES. e.g. starts boil at 70^oC, completes boil at 78^oC. Also, it can VARY melting and boiling points of pure substances. e.g. pure water boil at 100^oC, but with salt is at 102^oC

1. Filtration

• Filtration – separates insoluble solid from a liquid.
• Mixture is poured through a filter with tiny holes made of paper.
• Large solid particles cannot pass through the pores and trapped in it as residue while tiny liquid particles pass through as filtrate.

2. Crystallisation & Evaporation 

Crystallisation – separation of dissolved solid from a solution as well-formed crystals
Evaporation to Dryness – seperation of dissolved solid from a solution as crystals of salt by evaporating all the liquid off.

Why crystallisation occur?
- Solubility of most solutes decrease as temperature decrease, when solution cools, solution can’t hold more solute (saturated) so the extra solute separates as pure crystals.

3. Simple Distillation

Simple Distillation – separation of pure liquid from a solution. Condensed pure liquid – distillate.

Process of Distillation: Solution is heated, and steam (pure vapour) is produced. The steam is cooled in condenser to form pure liquid. Solute remains in the flask.

4. Fractional Distillation

• Fractional Distillation – separates mixture of miscible (soluble) liquids with widely differing boiling points. Use of fractionationg column separates them.
• Process of Fractional Distillation: E.g. ethanol and water. 
• Mixture of ethanol and water is placed in flask and heated. Ethanol with lower boiling point boils and vaporises first and reach fractionating column then cools and condenses into ethanol as it passes through condenser. Temperature will stay constant until all ethanol is distilled. Water will distil the same way after all ethanol is distilled.

 Uses of fractional distillation:

• Separates pure oxygen and pure nitrogen from liquefied air
• Separates substances in petroleum (crude oil) into fractions
• Separates alcohol to produce alcoholic drinks

 

 5. Chromatography

  Chromatography – a method of separating and identifying mixtures.

• The need for Chromatography;

       - Separates and identify mixtures of coloured substances in dyes
       - Separates substances in urine, drugs & blood for medicinal uses.
       - To find out whether athletes have been using banned drugs.

Separating Mixtures of Coloured Substances

Obtain a dye sample then put a drop of the sample on a pencil line drawn on the filter paper then dip the paper into a solvent with the level below the spot. The dye will dissolve in solvent and travel up the paper at different speed. Hence they are separated.

Identifying Mixturees of Coloured Substances

In the diagram on top, drop of sample dye is placed on pencil line. The result shows that:
       - The sample dye is made of 3 colours.
       - 2 comparison dyes are of one of the compositions of the original dye as the spots are of same colour and distance.
       - a comparison dye isn’t part of sample.

 Separating and Identifying Mixtures of Colourless Substances


To do this a locating agent is to be sprayed on filter paper. Locating Agent – a substance that reacts with substances (e.g. sugars) on paper to procuce a coloured product.

 Rf Values

To identify unknown dye in the diagram at the very top:

                                     Rf value =x/y.

Where x = distance moved by the substance and;
y = distance moved by the solvent