Chemical Analysis Flashcards
Element
An element is a substance that consists of only one type of atom. They cannot be broken down into simpler substances by chemical means
What is filtration used to separate?
Solids from liquids
What is the substance left on the filter paper after filtration?
Residue
The liquid that filters through filtration is
Filtrate
What is evaporation used to separate
Evaporation is used to separate a soluble solid from a liquid.
What is simple crystallisation used to separate?
To separate a solution from the solvent and make crystals.
What is fractional distillation used to separate?
Separating miscible liquids with different boiling points
Potable water is
Water that is safe to drink
Desalination is
The process of removing dissolved substances from sea water
Sea water can be made potable by distillation where the sea water is boiled and water is condensed. What are the disadvantages of producing drinking water this way? (3)
- it is expensive because large amounts of energy are needed to heat the sea water
- it increases the use of fossil fuels which are non-renewable resources
- carbon dioxide emissions from burning fossil fuels contribute to global warming
What must be treated to kill microorganisms which may cause disease. How is this done?
Chlorination
Test for water: add water to anhydrous copper(ii) Sulfate crystals and these will change from…
White to blue
Flame test colours:
Cation Flame colour
Lithium. Crimson Sodium. Yellow/orange Potassium. Lilac Calcium. Brick red Copper. Blue/green
Precipitate definition
A solid which may be formed on mixing two solutions
Sodium hydroxide solution will form ….. coloured metal hydroxide …… when added to different salt solutions
Gelatinous
Precipitates
Give the colour of solution, colour of precipitate and colour in excess NaOH when cations are present Cu2+ Fe2+ Fe3+ Mg2+ Al3+ Zn2+
Cation. Solution. Precipitate. In excess
Cu2+. Blue. Blue. Nochange
Fe2+. Green. Green. No change
Fe3+. Orange. Red/brown. No change
Mg2+ colourless White. No change
Al3+. Colourless. White. White ppt dissolves to form colourless solution
Zn2+. Colourless. White. White ppt dissolves to form colourless solution
Silver Sulfate
2Ag+ (aq) + SO4 2- (aq) -> Ag2SO4 (s)
Silver chloride precipitate colour
White
Silver bromide precipitate colour
Cream
Silver iodide precipitate colour
Yellow
Barium Sulfate precipitate colour
White
Iron hydroxide precipitate colour
Green
Colour of halide precipitate in silver nitrate
Cl-
Br-
I-
Cl- White
Br- cream
I- yellow
Test for Sulfate ions will give a
White precipitate
Test for copper ions
- Take a nichrome wire with a loop on the end and clean in concentrated HCl and place in a hot blue Bunsen flame.
- Dip the clean wire into concentrated acid and then solid metal salt and place in the hottest part of the hot Bunsen flame.
- If copper is present it will burn with a blue-green colour
Test for Sulfate ions (3)
- Form a solution of salt and water
- Pour a few cm3 of the solution into a test tube and add barium chloride solution drop wise
- Observe a white ppt forming if sulfate ions are present in the sample
How would you carry out a flame test (4)
- Light a hot Bunsen flame.
- Dip the nichrome wire with a loop into the conc HCl and clean in the blue hot Bunsen flame.
- Dip in the HCl again and then in the metal solid.
- Hold under the blue flame and observe the colour of the flame
Why does an impure substance have a lower melting point ?
The melting point is lower because impurities will disrupt the regular lattice arrangement that is present in most solids and so the bonds between the particles will be weaker
The …. amount of impurity the … the melting point
Greater, lower
Why is salt (with grit for grip) added to roads in winter ?
To lower the melting point of water so that ice cannot form at 0 degrees
The … the amount of impurity the .. the boiling point
Greater, higher
How come the boiling point is higher if substances such as ionic compounds dissolve in water ?
The boiling point is higher because the water molecules are attracted to the ions and so more energy is required to make the water molecules escape from the water
Boiling point definition
The temperature at which a liquid changes into a gas
Melting point definition
The temperature at which a solid changes into a liquid
Pure substance definition
A single element or compound that is not mixed with any other substance
Mixture
Defined as two or more substances mixed together, which are usually easy to separate
Formulation definition
A mixture that has been designed as a useful product and is formed by mixing together several different substances in carefully measured quantities to ensure the product had the required properties
Give three examples of formulations
- Alloys
- Medicines
- Fertilisers
Describe chromatography
The separation of mixtures of soluble substances by running a solvent (mobile phase) through the mixture on the paper (stationary phase), which causes the substances to move at different rates over the paper;
What substance is added to the contents of the flask during simple distillation to promote smooth boiling ?
Anti-bumping granules
Why is pencil used in chromatography ?
It will not dissolve in the solvent
The result of a chromatography experiment is called a ..
Chromatogram
How to determine the most soluble sample in the solvent on the chromatography paper ?
It moves furthest up the chromatography paper
How is an Rf value calculated for chromatography
Distance moved by spot
——————————-
Distance moved by solvent
What is the solvent front ?
How far the solvent has travelled up the paper
What does an Rf value of 0 mean ?
The spot has not moved
What is the mobile phase ?
The phase that is moving, e.g solvent or dye
What is the stationary phase
The one through which the mobile phase moves, e.g the paper
How water is made potable (3)
- water is filtered through gravel and sand to remove different-sized solid particles. The gravel and sand are equivalent to different-grade filters. The gravel removes larger particles and the sand smaller particles
- Water is treated with coagulant to Sediment any remaining, very small, solid particles and it is again filtered using a carbon filter
- Water is finally chlorinated by adding chlorine to kill microorganisms
Describe the process of desalination for seawater using distillation (2)
- The water is evaporated and then condensed
- Any dissolved solids do not boil and the water is separated from the dissolved sodium chloride and any other dissolved substances
Describe the test for Cu2+, Fe2+, Fe3+, Al3+, Zn2+ and Mg2+ ions in solution using sodium hydroxide solution and ammonia solution;
(6)
- Dissolve The cation in water to form a solution
- Add a few drops of sodium hydroxide solution or ammonia solution to the test solution containing the cation
- Observe The results
- Continue to add the sodium hydroxide solution or ammonia solution until it is in excess
- The colour of ppt and whether or not it redissolves in excess of either solution identifies the cation present
- The ppt formed on addition of sodium hydroxide solution and ammonia solution is the same - it is the metal hydroxide
Describe the tests for the following
• chloride, bromide and iodide (5)
- Dissolve The sample in water to form a solution
- Pour a few cm3 of the solution into a test tube and add a few cm3 of silver nitrate solution
- Observe The formation of The precipitate
- Chloride anion forms white ppt, bromide cream ppt, iodide yellow ppt
- White ppt is silver chloride, cream ppt is silver bromide, yellow ppt is silver iodide
Test for carbonate ions (3)
- Dilute acid such as HCl is added to the solid and bubbles of gas are observed if carbonate ions are present
- Gas is carbon dioxide
- Bubbling this Gas through limewater results in the limewater changing from colourless to milky
Ionic equation for the halide tests (3)
Ag+ + Cl- —> AgCl
Ag+ + Br- —> AgBr
Ag+ + I- —> AgI
Why are formulated fertilisers referred to as NPK fertilisers ?
- They proved plants with the correct minerals in suitable proportions for growth such as nitrogen, phosphorous and potassium compounds
Define solute
A solid that dissolves is called a solute
What is a solvent
The liquid in which the solute dissolves
What is a solution
The resulting mixture of a solute dissolved in a solvent
What are liquids that do not mix called ?
Immiscible
What is recrystallisation ? (2)
- If the solid to be obtained from evaporation contains water of crystallisation, the solution is heated to evaporate to half volume and left aside to cool and crystallise
- The crystals form in the solution and they can be filtered off
What is a separating funnel used to separate ?
Immiscible liquids based on a difference in their densities
What is simple distillation used to separate ?
A solvent from a solution or one of two miscible liquids
What happens if a liquid in the mixture for simple distillation is flammable?
- a water bath or electric heating mantle is used to heat the flask rather than a Bunsen burner
Why is simple distillation not as accurate as fractional distillation for separating water and ethanol
- Some water will evaporate and the ethanol collected will not be pure
How does the condenser in simple distillation work? (2)
- Gases pass through the middle tube and cold water runs between the cylinders, providing a cold surface for the gases to condense on
- The cooling water must go in at the bottom of the condenser and out at the top to ensure that the condenser is filled with water at all times
Why is fractional distillation better? (2)
- This method provides better separation of miscible liquids than simple distillation because evaporated liquids below their boiling point do not reach the condenser
- they condense on the glass beads in the fractionating column and return to the flask
What are distillates collected at different temperatures called ?
Fractions
How to separate two solids - one soluble in water, the other insoluble (3)
- Dissolve in water
- filtration to obtain the insoluble solid
- Evaporation or recrystallisation to obtain the soluble solid from the filtrate
Examples of components separated by filtration
Sand and water
Sulfur and water
Examples of components separated by simple distillation
Salt solution
Sugar solution
Examples of components separated by evaporation or recrystallisation
Salt solution
Sugar solution
Examples of components separated by fractional distillation
Ethanol and water
Crude oil
Liquid air
When to use simple distillation ?
When two liquids have substantially different boiling points
Examples of components separated by separating funnel
Oil and water
Examples of components separated by chromatography
Dyes, food colourings, chemical indicators
