1.9 Chemical Analysis

Pure substance

single element or compound not mixed with any other substance

Examples of pure substances

• Diamond, only element carbon

• Water, only the compound water

• Table salt, only the compound sodium chloride

Mixture (impure)

two or more substances mixed together, usually easy to separate

Examples of mixtures

• Air, mixture of oxygen, carbon dioxide, nitrogen and other trace gases

• Mineral water, mixture of water and dissolved salts

• Milk, mixture of water, lactose, fat and minerals

Melting point

temperature at which solid changes into liquid

Boiling point

temperature at which liquid changes into gas

Melting and boiling points of pure substances

specific points/ temperatures

Melting and boiling points of impure substances

• will melt/ boil over range of temperatures, gradually

• melts at lower temperature than expected

• boils at higher temperature than expected

Differences in melting and boiling points

allow us to distinguish between pure substances and mixtures e.g X is pure

Formulation

mixture designed as useful product

How are formulations made

mixing different substances in carefully measured quantities to ensure it has required properties

Alloys as formulations

mixture of two or more metals, usually through melting, mixing and cooling

Examples of alloy formulations

stainless steel, made from iron, chromium and other elements e.g carbon or nickel

Medicines as formulations

contain ingredients other than active drug, usually to prevent complication e.g timed release

Examples of medicine formulations

Calpol, made from paracetamol and liquid flavourings

Fertilisers as formulations

contain multiple compounds depending on plants used for

Examples of fertiliser formulations

NPK fertilisers, made from nitrogen, phosphorus and potassium

Separating mixtures

relatively easy (no chemical reactions needed) because they are not chemically joined to each other

Soluble

dissolves in water

Insoluble

does not dissolve in water

Solute

substance which dissolves

Solvent

liquid which dissolves the solute

Solution

mixture of solute dissolved in solvent

Miscible

liquids mix e.g water and ethanol

Immiscible

liquids do not mix e.g oil and water

Filtrate

filtered solution

Residue

solid which remains on filter paper

Distillate

liquid produced by distillation

Evaporation

when liquid is heated and changes state into gas

Condensation

when gas cools and changes state into liquid

Filtration

separating insoluble solid from liquid e.g sand from water

Crystallisation

separating soluble solid from solvent through evaporation, leaves behind saturated solution and cools to form crystals e.g obtaining pure soluble salts

Paper chromatography

separating of mixtures of soluble substances by running solvent through mixture on paper, causing different substances to move through at different rates

Why substances move at different speeds

strength of attraction to each phase

Phases of chromatography

• two phases with different properties

• paper is stationary phase

• solvent is mobile phase

Substances with strong attraction to paper

move slowly and only travel short distance

Substances with strong attraction to solvent

move quickly and travel further

Interpreting a chromatogram

pure substance has single spot whereas impure substances have two or more spots

Identifying same substances

• produce same number of spots

• spots travel at same distance and have same R_f value

• e.g brown contains all three colours

Rf value

measure of distance substance travels relative to the solvent

R_f value equation

distance moved by spot/ distance moved by solvent

Interpreting R_f value

value of 0 is not attracted to solvent, value of 1 if not attracted to paper

Simple distillation

separating solvent from solution or two miscible liquids with different boiling points using evaporation and condensation e.g salt and water

How simple distillation works

• solution is heated

• dissolved solute has much higher boiling point that solvent so only it evaporates

• gas moves away and cools in condenser

• it then drips into beaker

• remaining solution becomes more concentrated as amount of solvent decreases

Fractional distillation

separating mixture of liquids with boiling points that are close together

How fractional distillation works

• has fractionating column (hot at bottom and cooler at top)

• vapor condenses when reaching part of column below their boiling point going back into flask

• gas that makes it to top of column enters condenser to be changed into liquid

Fractionating column

gradually changes the temperature so liquid with lowest boiling point will evaporate first

Fractions

different components of the mixture

Boiling point of water and ethanol

100° C, 78° C

Where is fractional distillation mostly used

refining of crude oil as is complex mixture of many different compounds

Potable water

water that is safe to drink, must be treated first

How to make potable water from fresh water

• Filtration - to remove insoluble solids like stones and leaves, water is passed through layers of sand and gravel called filter beds

• Sedimentation - Aluminium sulfate is added to help tiny particles clump together so they settle to bottom and clean water can be drawn off the top

• Chlorination - Chlorine gas is bubbled through water to kill any harmful microorganisms, such as bacteria

Desalination

removal of salt from seawater

How to make potable water from seawater (desalination)

• salt water is heated so water is allowed to evaporate

• water vapour is collected rather than being lost

• it’s condensed to form pure water/ fresh water

• salt is left behind and can be used for other purposes

Potable water from fresh water vs sea water

desalination uses lots more energy and is more expensive

Uses of desalination of seawater

useful in countries that have coastlines but readily available fresh water sources however mostly those that are quite wealthy

Pure/ anhydrous copper (II) sulfate

when water is added changes colour from white to blue

Equation for testing water

• anhydrous copper(II) sulfate + water ⇌ hydrated copper(II) sulfate

• CuSO4 + 5H2O ⇌ CuSO4.5H2O

Testing for cations

• flame tests

• precipitate tests

Flame tests

metal ions produce strong colour in blue bunsen flame, can be used to identify it

Flame colour of lithium ion (Li⁺)

crimson

Flame colour of sodium ion (Na⁺)

yellow/ orange

Flame colour of potassium ion (K⁺)

lilac

Flame colour of calcium ion (Ca²⁺)

brick red

Flame colour of copper ion (Cu²⁺)

blue-green

Precipitate tests

when metal ions combine with hydroxide ions they form precipitates with characteristic colours

Percipiate colour of copper(II) ion (Cu²⁺)

blue perciptate, remains in excess sodium hydroxide but dissolves to form deep blue solution in ammonia solution

Percipitate colour of iron(II) ion (Fe²⁺)

Green percipitate, remains in excess

Percipitate colour of iron(III) ion (Fe³⁺)

brown percipitate, remains in excess

Percipitate colour of magnesium ion (Mg²⁺)

white percipitate, remains in excess

Percipitate colour of aluminium ion (Al³⁺)

white percipitate, dissolves to form colourless solution in excess sodium hydroxide solution but remains in ammonia solution

Percipitate colour of zinc ion (Zn²⁺)

white percipitate, remains in ammonia solution but dissolves to form colourless solution in excess sodium hydroxide solution