Chemical Tests etc. Flashcards
What is a pure substance?
A substance which is a single element or compound
What is a mixture?
A substance that contains two or more substances not chemically joined and easily separated using a physical property
What is important about the boiling and melting points of pure elements and compounds?
Pure compounds and elements melt and boil at fixed, specific temperatures
What is the test for pure water?
If the water boils at 100 degrees centigrade
What is important about the boiling and melting points of impure elements and compounds?
They have a range of melting and boiling points
What is the melting point?
The temperature at which heat gives energy to the particles so they move further apart and the forces of attraction weaken
What is the boiling point?
The temperature at which heat gives more energy so the particles have lots of energy and the forces of attraction between them completely break to let the particles move freely
What is condensation?
When the particles have less energy so they move slower so forces of attraction become greater and particles stick together in an irregular way. They become a liquid
What is freezing?
When particles have a lot less energy so the forces of attraction become strong and the particles stick together in a regular way to make a solid
What do impurities do to the melting point?
lower the melting point and increase the melting point range
What do impurities do to the boiling point?
increase the boiling point and give a boiling point range
What is a formulation?
A mixture that has been designed as a useful product. Typically complex mixtures in which each chemical has a particular purpose
How are formulations made?
By mixing the components in carefully measured quantities to ensure that the product has the required properties
Descibe how you carry out the chromatography experiment?
1) Use a ruler to draw a horizontal pencil line 2cm from the bottom of the chromatography paper
2) mark five pencil spots at equal intervals across the origin line
3) using a different capillary tube for each coloring, put a small spot of each colouring on four of the pencil spots
4) Use another capillary tube to put a small spot of the unknown mixture on the last pencil spot
5) pour water into the beaker to a depth no more than 1cm
6) clip the top of the paper to a wooden spill and rest on top of beaker. The bottom of the paper should dip in the solvent
7) wait for the water solvent to travel at least 3/4 of the way up the paper
8) remove the paper from the beaker and draw another pencil line as close to the wet edge as possible, this is the solvent front line
9) hang the paper up to dry thoroughly
How do you calculate an Rf value?
distance moved by solute / distance moved by solvent
Why must a lid be put on top of the beaker in chromatography?
to prevent the evaporation of the solvent
Why should the origin line be drawn in pencil?
as ink (pen) would dissolve in the solvent
Why should the pencil line and spots be above the level of the solvent?
so that the spots do not dissolve into the solvent in the beaker
What is chromatography used for?
to separate mixtures which can give information to help identify substances
How many spots will a pure substance generate?
one
How many spots will a mixture generate?
more than one spot (one for each substance in the mixture)
What is the stationary phase?
The chromatography paper
What is the mobile phase?
The solvent
How does chromatography work?
1) mixture dissolves in the mobile phase as it moves through the stationary phase
2) the mixture is also interacting with the stationary phase (constantly switiching between phases)
3) the more soluble a component of the mixture, the further it will travel
4) different components have different Rf values
Why do different substances travel different distances up the chromatography paper?
- susbtances that have a stronger attraction to the solvent (more soluble), move quickly and travel a long way up the paper
- substances that have a stronger attraction to the paper, move slower and travel a short distance up the paper
What is an Rf value?
the ratio of the distance moved by a compound to the distance moved by the solvent
Why would a permanent marker have an Rf value of 0?
Because they are insoluble so don’t dissolve in the solvent and travel up the paper
What does a large Rf value indicate?
- the substance is very soluble
- the substance spent a longer time in the mobile phase
Why would substances be heated in an oil bath, not directly in a Bunsen burner flame to calculate melting and boiling points?
A Bunsen burner would heat the substance too quickly, making it difficut to judge the temperatures at which substances start and finish melting
How can a more reliable result be given for chromatography experiments?
- repeating the experiment using a different solvent
- calculate the different Rf values and compare with the previous
- the Rf values should match relative to the other substances
How does chromatography separate substances?
- different substances will be distributed differently between the mobile phase and the stationary phase
- the substances will separate because they spend different amounts of time in the mobile phase depenedent on their solubility
- and therefore they move different distances up the paper
How can paper chromatography be used to identify what impurity is present in the substance/mixture
- run the mixture alongside pure samples of the possible impurity
- if the spot from one of the possible impurities moves the same distance as the mixture, then it indicates that that is the impurity
- this is the same if the calculated Rf values match
What is the colour of the flame for lithium testing?
crimson
What is the colour of the flame for sodium testing?
yellow
What is the colour of the flame for potassium testing?
lilac
What is the colour of the flame for calcium testing?
orange-red
What is the colour of the flame for copper testing?
green
What are the limitations to the flame tests?
- it is diffucult to judge the colour
- individuals see colour differently
- results quoted (expected flame colours) are only for pure substances
- if a mixture of ions is in the flame test, they can mask the colour of one ion
What is the test and result for carbon dioxide?
bubble gas into limewater and if carbon dioxide, the limewater will turn cloudy
What is the test and result for hydrogen?
adding a lit splint to the gas, and if hydrogen, the gas will burn rapidly with a squeaky pop sound
What is the test and result for chlorine?
add damp litmus paper to the gas and if chlorine, it bleaches the paper white
What is the test and result for oxygen?
put a glowing splint into the gas and if oxygen, the splint will relight
What are cations?
Positively charged metal ions
What must react to form a solid precipitate?
Two liquids
What is added to the solution of metal ions to form the coloured precipitates?
Sodium Hydroxide
What is formed when NaOH is added to Calcium?
White precipitate
What is formed when NaOH is added to magnesium?
White precipitate
What is formed when NaOH is added to aluminum (3+)?
White precipitate
What is formed when NaOH is added to iron (2+)?
Green precipitate
What is formed when NaOH is added to iron (3+)?
Brown precipitate
What is formed when NaOH is added to copper (2+)?
Blue precipitate
Which metal ion’s precipitate dissolves when in excess NaOH?
Aluminium (III)
What is the general ionic formula for testing cations?
Metal Ion + Hydroxide = Metal(Hydroxide)
What is an anion?
Negatively charged ion
What is the test for a sulfate anion?
- add barium chloride and hydorchloric acid to a metal sulfate
What is the test result for a sulfate anion?
White precipitate formed
What is the test for a carbonate anion?
Add an acid to a metal carbonate. Bubble the gas produced into limewater
What is the test result for a carbonate anion?
Limewater turns from clear to cloudy showing carbon dioxide gas has been made
What is the test for halide anions?
Add halide solution to a test tube
Add Nitric acid
Add silver nitrate solution
What is the test result for a chloride ion?
white precipitate formed
What is the test result for a bromide ion?
cream precipitate formed
What is the test result for an iodide ion?
yellow precipitate formed
What can occur when a sample containing a mixture of ions is used in a flame test?
The colour of the flame can be masked
Why is HCl added in the sulfate test and HNO3 added in the halide tests?
To remove impurities
What are the uses of instrumental analysis?
- analyse blood, tissue and urine samples
- analyse the content of food
- monitor the quality of water and air
- monitor the purity of water
What are the advantages of instrumental analysis compared to chemical tests?
- rapid, information ca be gained very quickly
- sensitive, only a small amount of a substance needs to be used
- informative - provides a lot of valuable information about a substance
- very accurate
What are the disadvantages of instrumental analysis compared to chemical tests?
- equipment required is expensive
- requires an operator with special training
- results have to be compared with results from known samples which have to have been obtained
What is flame emission spectroscopy?
An instrumental analysis method used to identify metal ions in a solution as well as their concentration
What is the flame emission spectroscopy method?
- place the sample into a flame
- as the ions in the sample heat up, their electrons become exited and rise to a higher energy level
- when the electrons drop back down to their original energy level, they transfer energy as light which passes through a spectroscope
- the spectroscope detects different wavelengths of light to produce a line spectrum specific to that ion
How can the ions be identified from a line spectrum?
Each ion gives a unique pattern of lines so the spectra of the mixtures of ions can be identified by comparing it to known spectra of metal ions
What does the intensity of the spectrum indicate?
The concentration of an ion in a sample (more intense = more concentrated)
What indicates a solution is acidic?
If blue litmus paper turns red in it
What happens to the pH of water when carbon dioixide is in it?
It goes acidic
What type of wire is used in flame tests?
A nichrome wire
