Time, temperature, mass & volume

Time

• Time can be measured using a stopwatch or stopclock which are usually accurate to one or two decimal places

• The units of time normally used are seconds or minutes

  • Other units may be used for extremely slow reactions (e.g. rusting)

• Remember: 1 minute = 60 seconds

Examiner Tips and Tricks

Careful: Units of time often cause issues in results tables.

If the display on a stopwatch showed 1:30.

• The incorrect time to record would be 1.30 minutes.

• The correct time would be 1.5 minutes.

To avoid any confusion, if the time intervals are less than a minute, it is best / easire to change the recorded units to seconds.

• So, the same stopwatch display would be recorded as 90 seconds.

Temperature

• Temperature is measured with a thermometer or digital temperature probe

• Laboratory thermometers usually have a precision of a half or one degree

  • Digital temperature probes are available which are more precise than traditional thermometers and can often read to 0.1 oC

• Traditional thermometers rely upon the uniform expansion and contraction of a liquid substance with temperature

  • Digital temperature probes can be just as, if not, more accurate than traditional thermometers

• The units of temperature are degrees Celsius (ºC)

Mass

• Mass is measured using a digital balance which normally gives readings to two decimal places

  • Balances should be tared (set to zero) before use

  • Balances should also be allowed time to settle on a final measurement / reading before it is recorded

• The standard unit of mass in kilograms (kg)

  • However, in chemistry grams (g) are most often used

• Remember: 1 kilogram = 1000 grams

Volumes of liquid

• The volume of a liquid can be determined using different pieces of apparatus

  • The choice of apparatus depends on the level of accuracy needed

• Three common pieces of apparatus for measuring the volume of a liquid are:

  • Burettes

  • Volumetric pipettes

  • Measuring cylinders

• Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm3 and 50 cm3

  • They are most commonly used in titrations

  • Careful: Read the burette scale from top to bottom as 0.00 cm3 is at the top of the column

• Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid,

  • They have a scratch mark on the neck which is matched to the bottom of the meniscus to make the measurement

  • A pipette filler is used to draw the liquid into the volumetric pipette

  • The most common volumes for volumetric pipettes are 10 cm3 and 25 cm3

• Measuring cylinders are used when approximate volumes are required (accuracy is not an important factor)

  • These are graduated (have a scale so can be used to measure)

  • Measuring cylinders typically range from 10 cm3 to 1 litre (1 dm3)

• Whichever apparatus you use, you may see markings in millilitres, ml, which are the same as a cm3

Volumes of gas

• For some experiments, the volume of a gas produced needs to be measured

• This is typically done by using one of the following methods:

  • Using a gas syringe 

  • By downward displacement of water

• A gas syringe is more precise and accurate than downward displacement of water

Diagram of the set-up for an experiment involving a gas syringe

• Downward displacement of water is where a measuring cylinder is inverted in water to collect the gas produced

  • This method does not work if the gas is soluble in water

Diagram of the set-up for an experiment collecting gas by downward displacement of water

• If the gas happens to be heavier than air and is coloured, the cylinder does not need to be inverted

Advantages & disadvantages of methods & apparatus

• In the lab, we often have choices of different apparatus to do the same job

• Evaluating which piece of apparatus is the best one to use is part of good experimental planning and design

• This means appreciating some of the advantages and disadvantages of laboratory apparatus

Advantages and disadvantages of lab apparatus

Five pieces of apparatus that can be used to measure the volume of a liquid. They all have their pros and cons

Planning your method

• Good experimental design includes the answers to questions like

  • Have I chosen a suitable apparatus for what I need to measure?

  • Is it going to give me results in an appropriate time frame?

  • Is it going to give me enough results to process, analyse and make conclusions?

  • Does it allow for repetitions to check how reliable my results are?

  • Does my plan give a suitable range of results?

  • How can I be sure my results are accurate?

  • Have I chosen an appropriate scale of quantities without being wasteful or unsafe?

• You may be asked about experimental methods in exam questions and your experience and knowledge of practical techniques in chemistry should help you to spot mistakes and suggest improvements

Solutions

• You need to know all the following terms used when describing solutions:

Terminology about solutions table

Acid-base titrations

• Titrations are a method of analysing the concentration of solutions

• They can determine exactly how much alkali is needed to neutralise a quantity of acid – and vice versa

• You may be asked to perform titration calculations to determine the moles present in a given amount or the concentration / volume required to neutralise an acid or a base

• Titrations can also be used to prepare salts

Apparatus

• 25 cm3 volumetric pipette

• Pipette filler

• 50 cm3 burette

• 250 cm3 conical flask

• Small funnel

• 0.1 mol / dm3 sodium hydroxide solution

• Sulfuric acid of unknown concentration

• A suitable indicator

• Clamp stand, clamp & white tile

The steps in performing a titration

Method

1. Use the pipette and pipette filler and place exactly 25 cm3 sodium hydroxide solution into the conical flask

2. Using the funnel, fill the burette with hydrochloric acid placing an empty beaker underneath the tap. Run a small portion of acid through the burette to remove any air bubbles

3. Record the starting point on the burette to the nearest 0.05 cm3

4. Place the conical flask on a white tile so the tip of the burette is inside the flask

5. Add a few drops of a suitable indicator to the solution in the conical flask

6. Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm3 portions, while swirling the flask vigorously

7. Quickly close the tap when the end-point is reached 

   • The endpoint is when one drop causes a sharp colour change

8. Record the volume of hydrochloric acid added, in a suitable results table as shown below

   • Make sure your eye is level with the meniscus

9. Repeat the titration with a fresh batch of sodium hydroxide

10. As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour

11. Record the volume to the nearest 0.05 cm3 

12. Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy

Examiner Tips and Tricks

Common errors during a titration include:

• Not removing the funnel from the burette

  • This can lead to some liquid dripping into the burette and cause false / high readings

• Not filling the jet space of the burette

  • The jet space is the part of the burette after the tap

  • Not filling this space can lead to false readings

• Reading the volume from the burette incorrectly

  • Readings should be taken from the bottom of the meniscus

  • Careful: The scale on the burette has 0.0 cm3 at the top and 50 cm3 (typically) at the bottom

Indicators

• Indicators are used to show the endpoint in a titration

• Wide range indicators such as litmus are not suitable for titration as they do not give a sharp colour change at the endpoint

  • However, methyl orange and phenolphthalein are very suitable

• Some of the most common indicators with their corresponding colours are shown below:

Common acid-base indicators

Paper chromatography

• Chromatography is used to separate substances and provide information to help identify them

• The components have different solubilities in a given solvent

  • E.g. Different coloured inks that have been mixed to make black ink

• A pencil line is drawn on chromatography paper and spots of the sample are placed on it

  • A pencil is used for this as ink would run into the chromatogram along with the samples

• The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don’t wash into the solvent container

  • The solvent used is usually water but it can be other substances such as ethanol

• The solvent travels up the paper by capillary action, taking some of the coloured substances with it

• Different substances have different solubilities so they will travel at different rates, causing the substances to spread apart

  • Those substances with higher solubility will travel further than the others

How to carry out chromatography

The pigments in ink can be analysed using paper chromatography

Interpret simple chromatograms

• We can use a chromatogram to compare the substances present in a mixture to known substances and make assumptions

  • Pure substances will produce only one spot on the chromatogram

  • Impure substances will produce more than one spot on the chromatogram

  • If two or more substances are the same, they will produce identical chromatograms

  • If the substance is a mixture, it will separate on the paper to show all the different components as separate spots

• It is common practice to include a known compound as a reference spot

  • This can help match up to an unknown spot or set of spots in order to identify it

Example chromatogram results

The brown ink has separated showing a spot of red ink, blue ink and yellow ink

• We can draw several conclusions from this chromatogram:

  • The brown ink is a mixture as there are three dots

  • Red, yellow and blue are pure as there is only one dot for each 

  • The brown ink contains red, blue and yellow as the dots are in line with one another horizontally

Examiner Tips and Tricks

Chromatograms in exams will be in black and white so to identify whether a mixture contains a known sample, the dots need to be in line with one another. Locating agents

Extended tier only

• For chromatography to be useful, the chemist needs to be able to see the components move up the paper

  • This is not the case for colourless substances such as amino acids or sugars

• Locating agents can be used to see the spots

  • These are substances which react with the sample and produce a visible / coloured spot for the product(s) 

• The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye

Retention factor (Rf) values

Extended tier only

• Rf values are used to identify the components of mixtures

• The Rf value of a particular compound is always the same

  • However, it does depend on the solvent used

  • If the solvent is changed then the Rf value changes

• Calculating the Rf value allows chemists to identify unknown substances because it can be compared with the Rf values of known substances under the same conditions

• The retention factor, Rf, is calculated by the equation:

Rf =  

• The Rf value:

  • Is a ratio

  • Has no units

  • Will always be less than 1

Worked Example

A student obtained the following chromatogram when carrying out chromatography. 

Calculate the Rf value of the substance. 

Answer:

• The Rf value of the substances in the chromatogram above can be calculated by:

  • Rf =  =  = 0.5

Examiner Tips and Tricks

When you calculate Rf values in exams, make sure to use your ruler carefully to measure the distance moved by the solvent and the substance as mark schemes can be strict about the values accepted for these. 

Filtration & crystallisation

• The choice of separation technique depends on the substances being separated

• All techniques rely on a difference in properties of the chemicals in the mixture

  • This is usually a physical property such as boiling point 

Separating a mixture of solids

• Differences in solubility can be used to separate solids

• For a difference in solubility, a suitable solvent must be carefully chosen

  • Only the desired substance should dissolve in the solvent

  • Other substances or impurities in the mixture should not dissolve in the solvent

• For example, to separate a mixture of sand and salt:

  • Water is a suitable solvent because salt is soluble in water, but sand is insoluble in water

Filtration

• This technique is used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water)

  • Centrifugation can also be used for this mixture

• A filter paper is placed in a filter funnel above another beaker

• The mixture of insoluble solid and liquid is poured into the filter funnel

• The filter paper will only allow small liquid particles to pass through in the filtrate

• Solid particles are too large to pass through the filter paper so will stay behind as a residue

Filtration of a mixture of sand and water

Crystallisation

• This method is used to separate a dissolved solid from a solution

  • A simple application of this is to heat a solution to boiling, remove the heat and leave the solvent to evaporate

• A more common application of this is sometimes called crystallisation 

  • This is when the solid is more soluble in hot solvent than in cold, e.g. copper sulphate from a solution of copper(II) sulphate

• The solution is heated, allowing the solvent to evaporate and leaving a saturated solution behind

• You can test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution

  • If the solution is saturated, crystals will form on the glass rod when it is removed and allowed to cool

• The saturated solution is allowed to cool slowly

  • Solids will come out of the solution as the solubility decreases

  • This will be seen as crystals growing

• The crystals are collected by filtration

• They are then washed with distilled water to remove any impurities

• Finally, they are allowed to dry

  • Common places to dry crystals are between sheets of filter paper or in a drying oven

The process of crystallisation

The solution is slowly heated to remove around half of the liquid. The remaining liquid will evaporate slowly 

Examiner Tips and Tricks

In exams, you need to be specific that no more than half of the solution is removed by direct heating or you may lose a mark.

Distillation: simple & fractional

Simple distillation

• Distillation is used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) or a pure liquid from a mixture of liquids

• The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask

• The vapour passes through the condenser, where it cools and condenses, turning into pure water which is collected in a beaker

• After all the water is evaporated from the solution, only the solid solute will be left behind

Simple distillation apparatus

Diagram showing the distillation of a mixture of salt and water

• Simple distillation can be used to separate the products of fermentation, such as alcohol and water

• However, fractional distillation is a more effective separation technique, commonly used when the boiling points of the liquids are close and/or a higher degree of purity is required, such as crude oil

Fractional distillation

• Used to separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two)

• The solution is heated to the temperature of the substance with the lowest boiling point

  • This substance will rise and evaporate first

  • The vapours will pass through a condenser, where they cool and condense

  • The condensed liquid is then collected in a beaker

  • All of the substance is evaporated and collected, leaving behind the other component(s) of the mixture

• For water and ethanol:

  • Ethanol has a boiling point of 78 ºC

  • Water has a boiling point of of 100 ºC

  • The mixture is heated until it reaches 78 ºC, at which point the ethanol distills out of the mixture and into the beaker

  • When the temperature starts to increase to 100 ºC heating should be stopped as the water and ethanol are now separated

Fractional distillation of a mixture of ethanol and water

• An electric heater is safer to use when there are flammable liquids present

• The separation of the components in petroleum is achieved by fractional distillation on an industrial scale

• Fractional distillation of crude oil is not carried out in school laboratories due to the toxic nature of some of the components of the crude oil, but it can sometimes be simulated using a synthetic crude oil made specially for the demonstration

Worked Example

A student is given a mixture of calcium sulfate,  magnesium chloride and water. The table below shows some information about calcium sulfate and  magnesium chloride.          

How does the student obtain magnesium chloride crystals from the mixture?

1. Crystallisation followed by distillation

2. Crystallisation followed by filtration

3. Distillation followed by crystallisation

4. Filtration followed by crystallisation

Answer

The correct answer is D because:

• The difference in solubility in water means the first step is to make a solution

• The magnesium chloride will dissolve, but the solid calcium sulfate will be left behind

• The mixture is filtered to remove the calcium sulfate and then evaporated and crystallised to obtain magnesium chloride crystals

Examiner Tips and Tricks

You may be asked how to separate a mixture of gases:

• One method involves cooling the gaseous mixture sufficiently to liquefy all of the gases, which are then separated by fractional distillation. 

• They can also be separated by diffusion, where the boiling points are very close or it is impractical or expensive to use fractional distillation.

Assessing purity

• Pure substances melt and boil at specific and sharp temperatures

  • For example, water has a boiling point of 100°C and a melting point of 0°C

• Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures

  • Therefore, melting and boiling point data can be used to distinguish pure substances from mixtures

• An unknown pure substance can be identified by experimentally determining its melting point and boiling point and comparing them to literature values / data tables

  • Boiling points are commonly determined by distillation

• Melting point analysis is routinely used to assess the purity of drugs for example

  • This is done using a melting point apparatus which allows you to slowly heat up a small amount of the sample, making it easier to observe the exact melting point

Melting point test using an oil bath 

• This is then compared to data tables

• The closer the measured value is to the actual melting or boiling point, the purer the sample is

• If the sample contains impurities:

  • The boiling point may appear higher than the sample's actual boiling point

  • The melting point may appear lower than the sample's actual melting point

Identification of anions

• Negatively charged non-metal ions are known as anions

• You must be able to describe the tests for the following ions:

  • Carbonate ions, CO32– 

  • Halide ions, Cl– , Br– , I– 

  • Nitrate ions, NO3–

  • Sulfate ions, SO42– 

  • Sulfite ions, SO32– 

Test for carbonate ions

• Carbonate compounds contain the carbonate ion, CO32-

• The test for the carbonate ion is:

  • Add dilute acid 

  • Bubble the gas released through limewater

  • Limewater turns cloudy if the carbonate ion is present

• If a carbonate compound is present then fizzing / effervescence should be seen as CO2 gas is produced, which forms a white precipitate of calcium carbonate when bubbled through limewater:

CO32- (aq) + 2H+ (aq) → CO2 (g) + H2O (l)

CO2 (g) + Ca(OH)2 (aq) → CaCO3(s) + H2O(l)

• The white precipitate turns limewater cloudy 

Testing for carbonate ions

Limewater turns milky in the presence of carbon dixoide caused by the formation of insoluble calcium carbonate

Examiner Tips and Tricks

• If you are asked to describe the test for carbonate ions, make sure that you say:

  • Bubble the gas produced through limewater, which turns cloudy if the carbonate ion is present

•   Just saying that limewater turns cloudy is not enough

  • This isn't describing the test, it is stating the result

Test for halide ions

• Halide ions are the negative ions / anions formed by the elements in Group 7 

• The test for the halide ions is:

  • Acidify the sample with nitric acid

  • Add silver nitrate solution, AgNO3,  

  • A silver halide precipitate forms if a halide ion is present

    • The precipitate is indicated by the state symbol (s)

• The colour of the silver halide precipitate depends on the halide ion:

  • The chloride ion forms a white precipitate of silver chloride 

potassium chloride +  silver nitrate   →  potassium nitrate + silver chloride 

KCl (aq)   +     AgNO3 (aq)   →  KNO3 (aq)  +  AgCl (s) 

• The bromide ion forms a cream precipitate of silver bromide 

potassium bromide +  silver nitrate   →  potassium nitrate + silver bromide

KBr (aq)   +     AgNO3 (aq)   →  KNO3 (aq)  +  AgBr (s) 

• The iodide ions forms a yellow precipitate of silver iodide 

potassium iodide +  silver nitrate   →  potassium nitrate + silver iodide

KI (aq)   +     AgNO3 (aq)   →  KNO3 (aq)  +  AgI (s) 

Testing for halide ions

Each silver halide produces a precipitate of a different colour

Examiner Tips and Tricks

The acidification step in the halide ion test must be done with nitric acid rather than hydrochloric acid.

HCl contains the chloride ion which would interfere with the results.

Test for nitrate ions

• Nitrate compounds contain the nitrate ion, NO3–

• The test for the nitrate ion is

  • Add aqueous NaOH and aluminium foil

  • Warm gently and test the gas released

  • The gas given off is ammonia, NH3

• Ammonia is a gas with a characteristic sharp choking smell that turns damp red litmus paper blue

Test for sulfate ions

• Sulfate compounds contain the sulfate ion, SO42-

• The test for the sulfate ion is:

  • Acidify the sample with dilute nitric acid 

  • Add a few drops of barium nitrate solution

  • A white precipitate of barium sulfate is formed, if the sulfate ion is present

Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

• The test can also be carried out with barium nitrate solution

Testing for sulfate ions 

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Examiner Tips and Tricks

Nitric is added first to remove any carbonates which may be present which would also produce a precipitate and interfere with the results.

Test for sulfite ions

• Sulfite compounds contain the sulfite ion, SO32-

• The test for the sulfite ion is:

  • Add dilute acid

  • Warm the mixture gently

  • Bubble the gas released through potassium manganate(VII) solution

  • The potassium manganate(VII) solution changes from purple to colourless if the sulfite ion is present

Examiner Tips and Tricks

For qualitative inorganic analysis, there will be one test for the metal cation and another test for the non-metal anion. 

If you are an extended level student you may be asked to write balanced ionic equations for cation and anions tests, so make sure you know the formulae of all the ions and precipitates formed.

Identification of cations

Test for ammonium ions

• Ammonium ions, NH4+, can be identified by gently warming a solution containing the ions with sodium hydroxide solution

  • The sodium hydroxide solution is a source of hydroxide ions, OH–, for the test

• This releases ammonia gas which turns damp red litmus paper blue

Testing for ammonium ions

 Heating ammonium ions with sodium hydroxide solution releases ammonia gas which turns damp red litmus blue 

• Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide and ammonia

  • Most transition metals produce hydroxides with distinctive colours

Test for metal ions with sodium hydroxide solution

• If a small amount of sodium hydroxide solution is used, the resulting metal hydroxide normally precipitates out of solution

• If excess sodium hydroxide solutionis used, some of the precipitates may re-dissolve

  • For this reason, just a few drops of sodium hydroxide solutionare added at first and very slowly

• The sodium hydroxide test for the metal ion is:

  • Add a few drops of sodium hydroxide solution

  • Record any colour changes or precipitates formed 

  • Add excess sodium hydroxide solution

  • Record any colour changes or changes to precipitates

Test for metal ions with ammonia solution 

• If a small amount of ammonia solution is used, the resulting metal hydroxide normally precipitates out of solution

• If excess ammonia solution is used, some of the precipitates may re-dissolve

  • For this reason, just a few drops of ammonia solution are added at first and very slowly

• The ammonia test for the metal ion is:

  • Add a few drops of ammonia solution

  • Record any colour changes or precipitates formed 

  • Add excess ammonia solution

  • Record any colour changes or changes to precipitates

Metal ion tests summary

• Initially, sodium hydroxide solution and ammonia solution give the same results for 2 - 3 drops

  • This is because they both contain the hydroxide ion, OH–

Analysing results

• The tables above contain the results for all metal cations included in the syllabus

• If a precipitate is formed from either sodium hydroxide or ammonia solution, then the hydroxide is insoluble in water

• For example, zinc chloride:

ZnCl2 (aq) + 2NaOH (aq) →  Zn(OH)2 (s) + 2NaCl (aq)

• There are 3 metal ions that all form white precipitates:

  • Aluminium ions, Al3+ 

  • Calcium ions, Ca2+ 

  • Zinc ions, Zn2+ 

• Calcium ions, Ca2+, can be easily distinguished from Zn2+ and Al3+

  • The white precipitate of calcium hydroxide does not dissolve in excess sodium hydroxide solution

  • The white precipitates of zinc hydroxide and aluminium hydroxide dissolve in excess sodium hydroxide solution

• Zinc ions, Zn2+, can then be distinguished from Al3+ ions as

  • The white precipitate of zinc hydroxide dissolves in excess ammonia solution

  • The white precipitate of aluminium hydroxide does not dissolve in excess ammonia solution

Examiner Tips and Tricks

The ammonia or sodium hydroxide solution must be added very slowly. If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate, which dissolves as quickly as it forms if excess solution is added.

Be sure to distinguish between the term “colourless” and “clear”. A solution that loses its colour has become colourless. A clear solution is one that you can see through such as water. Solutions can be clear and have colour eg. dilute copper sulphate.

Flame tests for metal ions

• The flame test is used to identify the metal cations by the colour of the flame they produce

  • Ions from different metals produce different colours

• Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid and then hold it in the blue flame of a Bunsen burner until there is no colour change

  • This is an important step as the test will only work if there is just one type of ion present

    • Two or more ions means the colours will mix, making identification erroneous

    • This cleans the wire loop and avoids contamination

• A small sample of the compound is placed on an unreactive metal wire loop such as nichrome or platinum

• Dip the loop into the solid sample / solution and place it in the edge of the blue Bunsen flame

  • Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour

Diagram showing the technique for carrying out a flame test

• The colour of the flame is observed and used to identify the metal ion present:

Metal ions form distinctive coloured flames