12. Experimental techniques and chemical analysis Flashcards
Flame test - lithium, Li+
Red
Flame test - sodium, Na+
Yellow
Flame test - potassium, K+
Lilac
Flame test - copper(II), Cu2+
Blue-green
Flame test - barium, Ba2+
Green
Flame test - calcium, Ca2+
Orange - red
How to conduct a flame test.
In a flame test, a nichrome wire is dipped in concentrated hydrochloric acid and then into the salt you want to test. The wire is then held at the outer edge of the blue portion of the Bunsen burner flame and the colour observed.
Pure substance
Contains only one type of particle. As a result, the particles within this substance are found in a very particular arrangement.
Impure substance
Contain more than one type of particle. Impurities result in the original arrangement of the particles being disrupted –> alters the boiling and melting point of a substance.
Purification
The act of separating substances in a mixture to obtain a pure substance.
Testing for Oxygen, O2
Glowing splint relights
Test for Hydrogen, H2
Lighted (burning) splint ignites gas with a “pop” sign
Test for Carbon Dioxide, CO2
Limewater turns milky
Test for Ammonia, NH3
Warm ammonia and then hold damp red litmus paper over it. If positive it turns blue
Test for Chlorine, Cl2
Damp red litmus paper bleaches white
Test for Sulfur Dioxide, SO2
Turns acidified aqueous potassium manganate (VII) from purple to colourless.
Dip filter paper with potassium manganate (VII). If positive it will turn from purple to colourless.
Solvent
A liquid substance within which a solid can dissolve.
Solute
A soluble solid that has dissolved in a solvent.
Solution
The name given to a liquid substance made up of a solvent and solute dissolved in it.
Suspension
A mixture of substances; usually a liquid that may or may not be a solution, and an insoluble solid.
Filtrate
The solution that passes through the filter in filtration.
Residue
The insoluble solid caught in the filter paper after a mixture has been filtered.
Saturated solution
A solution which contains as much of the solute as possible at a particular temperature in that solvent.
Precipitate
An insoluble substance that forms in a solution.
Testing for Aluminium (Al³⁺)
(Effects on Sodium Hydroxide)
White precipitate, soluble in excess – gives a colourless solution.
Testing for Ammonium(NH⁴⁺)
(Effects on Sodium Hydroxide)
Ammonia gas produced on heating. Turn red litmus blue.
Testing for Calcium(Ca²⁺)
(Effects on Sodium Hydroxide)
White precipitate, insoluble in excess
Testing for Chromium(III) (Cr³⁺)
(Effects on Sodium Hydroxide)
Green precipitate, soluble in excess – gives a green solution
Testing for Copper(II) (Cu²⁺)
(Effects on Sodium Hydroxide)
Light blue precipitate, insoluble in excess
Testing for Iron(II) (Fe²⁺)
(Effects on Sodium Hydroxide)
Green precipitate, insoluble in excess
Testing for Iron(III) (Fe³⁺)
(Effects on Sodium Hydroxide)
Red/brown precipitate, insoluble in excess
Testing for Zinc(II) (Zn²⁺)
(Effects on Sodium Hydroxide)
White precipitate, soluble in excess – gives a colourless solution
Testing for Aluminium (Al³⁺)
(Effect of aqueous ammonia)
White precipitate, insoluble in excess
Testing for Calcium (Ca²⁺)
(Effect of aqueous ammonia)
No precipitate, or very slightly white precipitate
Testing for Chromium(III) (Cr³⁺)
(Effect of aqueous ammonia)
Grey-green precipitate, soluble to some extent in excess – gives blue solution
Testing for Copper(II) (Cu²⁺)
(Effect of aqueous ammonia)
Light blue precipitate, soluble in excess – gives a dark blue solution
Testing for Iron(II) (Fe²⁺)
(Effect of aqueous ammonia)
Green precipitate, insoluble in excess
Testing for Iron(III) (Fe³⁺)
(Effect of aqueous ammonia)
Red-brown precipitate, insoluble in excess
Testing for Zinc(II) (Zn²⁺)
(Effect of aqueous ammonia)
White precipitate, soluble in excess – gives a colourless solution
Testing for Carbonate (CO3 ²⁻)
Add dilute acid
Result = Effervescence (gas bubbles), carbon dioxide produced.
Testing for Chloride (Cl⁻)
Acidify with dilute nitric acid, then add aqueous silver nitrate.
Result = White precipitate
Testing for Bromide (Br⁻)
Acidify with dilute nitric acid, then add aqueous silver nitrate.
Result = Cream precipitate
Testing for Iodide (I⁻)
Acidify with dilute nitric acid, then add aqueous silver nitrate.
Result = Yellow precipitate
Testing for Nitrate (NO3⁻)
Add aqueous sodium hydroxide then aluminium foil; warm carefully.
Result = Ammonia produced (turns damp red litmus paper blue)
Testing for Sulfate (SO4 ²⁻)
Acidify, then add aqueous barium nitrate or barium chloride.
Result = White precipitate
Testing for Sulfite (SO3²⁻)
Add dilute hydrochloric acid, warm gently and test for the presence of sulfur dioxide.
Result = Sulfur dioxide produced will turn acidified aqueous potassium manganate (VII) from purple to colourless.
Filtration
A purification method used to remove an insoluble solid from liquids by pouring through a filter, such as filter paper.
Distillation
A purification method used to separate a mixture of liquids by boiling and condensing.
Chromatography
A method used to separate and identify substances by dissolving them in a solvent and allowing them to be carried by the solvent through a sheet of filter paper. The dissolved solutes are deposited along the sheet of paper according to their solubility and mass.
Chromatogram
A sheet of chromatography paper, obtained after chromatography has finished, showing separated solutes (coloured substances).
Compound
A substance made of two or more elements chemically bonded together.
Mixture
Two or more chemical substances found together but not chemically bonded.
Rf value
Distance travelled by substance / Distance travelled by solvent
Chemical tests for water
Cobalt (II) Chloride paper = blue when dry (anhydrous).
Exposed to water –> paper changes from blue to pink. (Heating paper returns it to blue)
Copper (II) Sulfate = white when anhydrous
Exposed to water = copper (II) sulfate changes from white to blue. (When dried becomes white again)
Physical tests for water
Water freezes at exactly 0C
Water boils at exactly 100C (at normal air pressure)
(If impure there is a heating range)
How chromatography is set up
- Reference line at the bottom of the chromatography paper.
- Solutes placed along the reference line
- Chromatography paper placed into beaker with solvent (solvent placed up to reference line)
- Solutes travel up at different speed depending on mass + solubility
- Measure distance from reference line to solvent front (where solvent has travelled to on paper)
- Measure distance from reference line to solutes
What is the Rf value
Unique value for each solute which measures how far the substance has travelled from the reference line.
Rf value will be the same in the same solvent, but for the same solute in a different solvent, it can be different.
Rf values will have been worked out for pure substances in certain solvents. By comparing Rf value of solutes to these standard Rf value, you can identify the substance.
(Value will always be less than one bc the solvent will have travelled further than the solute)
Rf value - Locating agents
- Solutes on a chromatogram can be colourless –> Therefore can’t measure the Rf value
- Locating agents are used
(a chemical substance that will react with the solute(s) separated by the chromatogram to produce a coloured substance)
–> This coloured substance will be clearly visible on the chromatogram.
–> Can then accurately calculate the Rf value.
How to separate a soluble solid and an insoluble solid
- Add a solvent to the mixture of solids
- (Suspension formed)
- Pour it through filter paper
- Evaporate filtrate (get rid of the solvent)
- Evaporate residue as well bc some solvent may be left in it
Residue = Insoluble solid
Filtrate = Soluble solid
Crystallisation - How it works
- A method allowing pure crystals of a solid to be obtained from a solution as it cools.
- Can occur with any liquid with a soluble solid dissolved in it.
(Works best when there is a large difference between the solubility of a solid at a high temperature and a low temperature)
- As the liquid part of the solution cools and becomes denser, there is less space for the solid particles to occupy.
- These particles are excluded from the liquid and form crystal arrangements on the side of the vessel containing them.
(A slowly cooling liquid forms larger crystals than a rapidly cooling liquid)
Simple distillation - Def
Simple distillation is used to obtain a solvent from a solution by heating and then condensing.
Simple distillation - How it works
- (Uses the fact that different substances have varying fixed points (eg. boiling points))
- The process of simple distillation uses a round bottomed flask connected to a long tube, called a condenser, through which cold water flows aroud it.
- Liquid with a lower boiling point will be the first to change the state to a gas. –> enters the condenser –> is condensed into liquid and the distillate is collected in a conical flask.
- Simple distillation is used to separate a mixture of liquids whose boiling point temperature difference is at least 50 °C.
Fractional distillation - Explained
- Used to separate a mixture of liquids whose boiling points are approximately within 25 °C of each other.
- Bunsen burner heats solution in flask.
- Liquids begin evaporating.
- Liquid with lowest boiling point goes thorguh fractionating column and condenses in the condenser so it can be collected in the flask.
- Liquids with higher boiling points condense on glass beads and fall down into flask again.
(Temperature decreases going up the fractionating column bc further away from flask + have had to travel through more glass beads)
Simple distillation - Set up
1) Distillation flask containing solution, with a bung at the top.
2) The ‘arm’ of the distillation flask is attatched to a condenser (glass tube which can be surrounded with water to decrease the temperature.
3) The condenser has a tube at the bottom wehre water enters at a pressure, pushing through the surroudning of the condesner and exiting at the top of it.
4) The condenser ends above a conical flask where the distillate is collected
Fractional distillation - Set up
1) Flask containing solution, with a bung at the top.
2) A fractionating column it attatched to the bung on the flask.
3) The fractionating column contains many small glass beads (larger surface area) on which solvents that have evaporated (and shouldn’t have bc the temp is too low) condense.
3) The ‘arm’ of the fractionating column is attatched to a condenser (glass tube which can be surrounded with water to decrease the temperature.
3) The condenser has a tube at the bottom wehre water enters at a pressure, pushing through the surroudning of the condesner and exiting at the top of it.
4) The condenser ends above a conical flask where the distillate is collected
How can the purity of a substance be measured?
By measuring the melting and boiling point.
Pure substances have fixed melting and boiling points.
On a heating / cooling graph the temperauter stays constant during a state change bc fixed points.
In an impure substance there is a melting range –> not a fixed value –> substance melts over a range of values.
