C12 Chemical analysis Flashcards
What is a pure substance?
A pure substance is one that is made up of just one substance. The substance can be either an element or a compound
What is a formulation?
A mixture that has been designed to produce a useful product
How to calculate retention factor (Rf)
distance moved by substance / distance moved by solvent
What is chromatography?
- Used to separate pictures of substances that are soluble in the same solvent
- The substances in the mixture get separated because more soluble substances spread further along the paper
How does chromatography separate a mixture?
- Particles in the mixture start off in one place
- Some are less soluble and are adsorbed onto the paper more than others, so travel less far
- Ones which are more soluble stay in the solvent for longer and are carried further up the paper
What is the retention factor?
It is a measure of how easily the substance can be carried through the stationary phase
what is the solvent front?
The distance that the solvent moves
What is the difference between the melting and boiling points of pure and impure substances?
- Pure substances have a very distinct melting and boiling point
- If impurities are added, the melting point is lowered and the boiling point is raised
- Impure substances tend to melt and boil over a range of temperatures
What is the melting and boiling point of pure water?
Melts at exactly 0 degrees and boils at exactly 100 degrees
How is salt used as an impurity?
- If salt is added to icy roads, the ice melts at a lower temperature
- If salt is added to water for cooking, the water boils at a higher temperature
- A range of temperatures may occur, depending on how much salt is added
describe the melting point and boiling point of a mixture
- Will vary, depending on the composition of the mixture
- A mixture does not have a sharp melting point or boiling point. It changes state over a range of temperatures
What is the relationship between the purity and melting/boiling point of a substance?
The purer the compound, the smaller the deviation from the actual value of the pure substance
How could we check if the water in the tap was pure?
- Heat a sample of the water to boiling point
- Check if the boiling point is exactly 100 degrees
- If it is then the water is pure
what are some examples of formulations?
- Paint
- Fuels
- Alloys
- Ready meals
- Medicines
- Fertilisers
- Pesticides
describe the meaning of formulations
- A mixture made to a set recipe for a particular use
- All components are carefully measured and then mixed together to make a formulation
- This ensures consistency in the product
Why is it important that these are formulated consistently?
So that the product remains the same, for example paint must maintain the correct colour in each formulation
Examples of purposeful substances added to formulations
Every substance included in a formulation has a purpose, for example:
- Medicines are often coated in sugar to make them taste better while swallowing
- Cleaning agents or fuels may have chemicals added to make them taste or smell bad so that they aren’t consumed
- Fuels may be formulated to be easy to store or be easier to burn
What is an active ingredient in formula?
The ingredient that delivers the effect e.g. the medicine product
Why might tablets contain an inactive ingredients?
- Preservatives, to change the taste, to allow the body to absorb the medicine more effectively
- Cannot sell the small amount of active ingredient on its own, carrier/increase size, filler materials
Why do tins of the same colour paint need to have the same ingredients?
Because otherwise the colour would be different
How can melting and boiling points be used to check if a substance is pure?
Melting point and boiling point can be checked against published values - they should match, if not, the substance is impure
what is the test for hydrogen?
How to carry out the test: React a metal compound (metal must be more reactive than hydrogen) with hydrochloric acid inside a test tube. Insert a lit splint into the test tube.
Positive result: The lighted splint produces a ‘pop’ sound
what is the test for oxygen?
How to carry out the test: Hydrogen peroxide naturally decomposes to form oxygen and water. Reaction is sped up using the catalyst, manganese(IV) oxide inside a test tube. Insert glowing splint into the test tube.
Positive result: The glowing splint relights
what is the test for carbon dioxide?
How to carry out the test: React calcium carbonate with hydrochloric acid inside a test tube, to form calcium chloride, water and carbon dioxide. Bubble carbon dioxide gas through limewater
Positive result: Limewater turns cloudy
what is the test for chlorine?
How to carry out the test: Hold damp blue litmus paper near the gas
Positive result: The litmus paper will be bleached white
Flame tests - ions and the colour flame they produce
Lithium (Li+) - Crimson
Sodium (Na+) - Yellow
Potassium (K+) - Lilac
Calcium (Ca2+) - Orange-red
Copper (Cu2+) - Green
What type of charge do metal ions have?
Positive
What is a precipitate?
An insoluble solid formed from a solution
How can precipitates be removed from a liquid?
Filtration
describe the flame test method
- A nichrome wire loop should be dipped in dilute hydrochloric acid and then heated to clean it first
- Then it should be dipped in the acid again before dipping it into the metal compound that is being tested
- Then hold the loop in the roaring blue flame of a Bunsen burner
- Use the colour of the bunsen flame to identify the metal ion in the compound
- However, if the sample being tested contains a mixture of metal ions, then some flame colours can be masked. For example, the intense yellow colour of sodium ions can dominate other colours
Coloured precipitates with sodium hydroxide
- Copper (II) ions, a blue precipitate appears
- Iron (II) ions, a green precipitate is produced
- Iron (III) ions, a brown precipitate is formed
How to identify halide ions
- Add dilute nitric acid and then silver nitrate solution
- If a precipitate forms, there are halide ions present
Why is nitric acid added when identifying halide ions?
To dissolve the compound and to remove any carbonate ions, as they would also form a precipitate with the silver ions and therefore interfere with the test
Coloured precipitates of halide ions
- Iodide ions (I-) give a yellow precipitate
- Bromide ions (Br-) give a cream precipitate
- Chloride ions (Cl-) give a white precipitate
How to identify sulfate ions
- Add dilute hydrochloric acid, followed by barium chloride solution
- If it is a positive result a white precipitate is formed with barium sulfate
Why is dilute hydrochloric acid added when trying to identify sulfate ions?
To remove carbonate ions that would form a precipitate with the barium ions
What does the formulation for paint usually contain?
- A pigment, to provide colour
- A binder, to help the paint attach itself to an object and to form a protective film when dry
- A solvent, to help the pigment and binder spread well during painting by thinning them out
what is flame emission spectroscopy?
an instrumental method used to analyse metal ions in solutions to identify the metal ions and measure their concentrations
what are cations and anions?
cations are positive ions, anions are negative ions
