Chemistry P4

What you should know:

Chemical Apparatus (measuring cylinder, pipette, burette, test tube, chromatography chamber)

Chemical techniques (how to measure, how to avoid common mistakes)

Separation techniques (for solids and fluids)

Preparation of salt (methods used for insoluble and soluble salts)

Titration (apparatus used, end-point, use of indicators)

Endo-Exothermic reactions (how to find out if a rxn is endo/exo)

Chromatography (Rf values, how deal with different dyes, identification of dyes)

Types of Chemical Reactions (displacement, double-displacement, acid-base, ammonia)

Tests for Anions, Cations and gases

Soluble Salts Insoluble Salts

All sodium, potassium and ammonium salts None
All nitrates None
Chlorides Except for silver and lead
Except for barium, lead and
Sulfates
calcium
Potassium, sodium and ammonium
All other carbonates
carbonates
Sodium, potassium and ammonium
Nearly all hydroxides
hydroxides
Common Apparatus

Common mistakes and corrections

Errors Corrections

No Repeats Repeat Three Times

Different Colour Changes Use a colorimeter
Use a burette because it is more
The measuring cylinder is inaccurate
accurate
Adding past endpoint Add volume in smaller quantities
Use a thermostatically controlled
Temperature/Heat Loss
water bath
The same measuring cylinder used in Use different measuring
multiple investigations cylinders
 SEPARATION TECHNIQUES
INSOLUBLE SOLID+LIQUID
 Filtration

Soluble Solid+ Liquid

 Crystallization
Common question: How to obtain pure crystals?

SOLUBLE LIQUID + SOLUBLE

IMPURITY
Simple Distillation

Used for liquids with LARGELY varying boiling points, like:

1. Water and Ink

2. Water and Ethanol
3. Seawater and Salt (desalination)
4. Water and Sugar (when dissolved)

Why is the inlet under the condenser?

So cool water travels in all of the condenser.

Sometimes, the flask might have a stopper on it, which is a common error in experiments. We
don’t put a stopper on the flask as it might break due to heat and pressure.
 SOLUBLE LIQUID + SOLUBLE
LIQUID
Fractional Distillation

 Removes a liquid from a mixture of liquids because liquids have different b.p.s

 The mixture is heated to evaporate the substance with the lowest b.p.

 Some of the other liquid(s) will evaporate too.

 Beads are heated to the boiling point of the lowest substance so that a substance

being removed cannot condense on beads.

 Other substances continue to condense and will drip back into the flask

 The beaker can be changed after every fraction.

 The thermometer is placed on the top to measure the temperature of the vapour

instead of the liquid

Used for liquids with similar boiling points like:

1. Alcohol and Water

2. Sulfur Dioxide and Chlorine

3. Ammonia and Water

4. Liquid air into respective gases

IMISCIBLE LIQUIDS
SEPARATING FUNNEL

Used for liquids with different densities that cannot mix, like:

1. Water and Oil

2. Water and Benzene
3. Honey and Oil

FOR A MIXTURE OF SOLIDS

THAT HAS A METAL, USE A
MAGNET
 Titration
1. Place known volume of alkali using volumetric pipette into a conical flask
2. Add indicator (e.g. thymolphthalein)
3. Titration: add acid using burette until end point has reached
4. Record volume of acid added
5. Repeat without indicator
6. Transfer to evaporating basin
7. Heat with bunsen burner
8. Leave to cool to crystallisation point
9. Wash crystals with distilled water
10. Dry crystals on filter paper

Chromatography
 Principle: Difference in solubility separates different pigments

o Drop the substance to centre of filter paper and allow it to dry

o Drop water on the substance, one drop at a time

o Paper + rings = chromatogram.

 Stationary phase: material on which the separation takes place

 Mobile phase: mixture you want to separate, dissolved in a solvent (water or ethanol)

 Interpreting simple chromatograms:

o Number of rings/dots = number of substances

o If two dots travel the same distance up the paper, they are the same substance.

o The pure substance only shows one dot on the chromatogram.

 You can calculate the Rf value to identify a substance given by the formula:

Rf Value= Distance moved by solvent/Distance moved by solute

 To make colourless substances visible

o Dry chromatogram in an oven

o Spray it with a locating agent

o Heat it for 10 minutes in the oven

 Downward Upward Collection Gas
Method
Delivery Delivery Overwater syringe
Gas is To
Gas is less
Use Gas more dense sparingly measure
dense than
when... than air soluble in the
air
water volume

Apparatus

Carbon dioxide,
Carbon
chlorine, sulfur
Ammonia, dioxide,
Examples dioxide, Any gas
hydrogen hydrogen,
hydrogen
oxygen
chloride

COMMON EXPERIMENTS
Showing that Oxygen and Water are needed for Rusting
Iron and that air is 21% Oxygen
 Shows that oxygen and water are needed for rusting iron

 Showing that air is 21% Oxygen

Planning an investigation
Example Question:

Complete six marks response:

Prepare all the apparatus needed for this investigation. Weigh the same mass of the
Cadmium metal and placed it in a beaker. Add 30cm^3 of dilute hydrochloric acid
into the beaker, whose concentration remains unchanged. Observe the reaction in a
set time of 2 minutes and record the gas produced using a gas syringe. Repeat the
investigation using Cobalt and Vanadium. A conclusion can be made by comparing
which metal produces the largest gas volume and placing it accordingly from least to
most reactive.

Example Question:

Baking powder is used to make cakes rise. When water is added to baking powder, carbon
dioxide gas is released. The longer the baking powder is stored, the less carbon dioxide it
releases when water is added. Plan an investigation to show which of two different samples
of baking powder has been stored for longer. Your plan should include the use of common
laboratory apparatus, the two samples of baking powder and water. No other chemicals
should be used.
Your plan should include:
• the apparatus needed
• the method to use
• the measurements to take
• the variables to control
• how to use the results to determine which sample has been stored for longer.
You may draw a diagram to help you answer the question.

Answer Key:
(1) Apparatus

 Balance

 Test-tube / conical flask / beaker to hold sample and water

 Equipment to measure volume of gas etc. e.g. measuring cylinder with water trough /
gas syringe

(2) Method and Measurements

 Measures a mass / amount of a sample

 Adds (excess) water

 Reweighs or measures volume of gas produced / counts bubbles / measures time to

turn limewater milky / (IGNORE one which goes milkier)

(2) Control

 Same mass of each sample / amount / volume OR calculate change in mass / volume
produced per gram OR same surface area of baking powder

 Same temperature

 Measure total volume of gas produced when no further change / Measure volume of
gas produced in a fixed time / Measure time taken to produce a fixed volume of gas

(3) Use of results

 Sample that produces more gas / loses more mass (weight) / has more bubbles / has
less time to turn milky / is milker is stored for less time or vice versa