chemical analysis Flashcards
What is a pure substance ?
-a single element or compound, not mixed with any other substance.
How can you distinguish pure substances from mixtures?
- Pure substances have specific melting and boiling points while mixtures melt and boil over a range of temperatures
what is a pure substance in everyday?
-a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
What is a formulation and how is it made?
-a mixture designed as a useful product, made by mixing chemicals in carefully measured quantities to ensure the product has specific properties.
Can you name some examples of formulations?
-fuels, cleaning agents, paints, medicines, alloys, fertilizers, and foods.
What is chromatography used for?
- to separate mixtures and it can help identify substances based on how they move between a stationary and mobile phase.
What is the formula for Rf value?
- Rf = (Distance moved by substance) / (Distance moved by solvent)
How can Rf values help identify compounds?
- Different compounds have different Rf values in various solvents, which can help identify them. A pure compound will always produce a single spot in all solvents.
How do you test for hydrogen gas?
-Hydrogen burns rapidly with a pop sound when a burning splint is held at the open end of a test tube containing the gas.
How do you test for oxygen gas?
- Oxygen will relight a glowing splint when it is inserted into a test tube of the gas.
How do you test for carbon dioxide gas?
- Carbon dioxide turns limewater cloudy when it is bubbled through or shaken with an aqueous solution of calcium hydroxide.
How do you test for chlorine gas?
- Chlorine bleaches and turns litmus paper white when damp litmus paper is exposed to the gas.
What flame color does lithium produce in a flame test?
- Lithium produces a crimson flame.
What flame color does sodium produce in a flame test?
- Sodium produces a yellow flame.
What flame color does potassium produce in a flame test?
-Potassium produces a lilac flame.
What flame color does calcium produce in a flame test?
-Calcium produces an orange-red flame.
What flame color does copper produce in a flame test?
-Copper produces a green flame.
How do sodium hydroxide tests help identify metal ions?
- Some metal ions form precipitates with sodium hydroxide. For example, aluminium, calcium, and magnesium form white precipitates, while copper(II), iron(II), and iron(III) form coloured precipitates.
What happens when aluminium hydroxide is added to excess sodium hydroxide?
- Aluminium hydroxide dissolves in excess sodium hydroxide.
What colors do copper(II), iron(II), and iron(III) precipitates form with sodium hydroxide?
- Copper(II) forms a blue precipitate, iron(II) forms a green precipitate, and iron(III) forms a brown precipitate.
What reaction occurs when carbonates react with dilute acids?
-Carbonates react with dilute acids to produce carbon dioxide gas, which can be identified by turning limewater milky.
What do halide ions form when reacted with silver nitrate solution?
-: Halide ions form precipitates with silver nitrate in the presence of dilute nitric acid: silver chloride is white, silver bromide is cream, and silver iodide is yellow.
What happens when sulfate ions react with barium chloride solution?
-Sulfate ions produce a white precipitate with barium chloride in the presence of dilute hydrochloric acid.
What is flame emission spectroscopy used for?
-to analyze metal ions in solutions by observing the light emitted by the sample when it is placed in a flame.
How does flame emission spectroscopy work?
- The sample is placed in a flame, and the light emitted is passed through a spectroscope. The resulting line spectrum can identify the metal ions and measure their concentration.
describe the required practical for paper chromatography
- first we draw a starting line on the chromatography paper using a pencil
- place spot of food colouring on start line
- place solvent in beaker
- place chromatography paper in beaker
- so chromatography paper is in solvent
- make sure solvent is below start line
- use a lid
- wait for solvent to travel up the chromatography paper until close to top
- mark solvent front
- dry the chromatography paper
- measure distance between start line and centre of spot
- measure distance between start line and solvent front
-use these measurements to calculate Rf value
what is the stationary phase?
-the paper because it does not move
what is the mobile phase?
-the solvent as it does move
why do we draw our starting line in pencil?
-if we drew the line in pen, the pen ink would move up the paper, with the solvent
why might we need to repeat the experiment using a different solvent?
-several different substances could have the same Rf value
what happens if the substance has never been analysed before?
-there will not be an Rf value on the database so further investigation needs to be carried out
what are disadvantages of flame tests?
-the colour of a flame test can be difficult to distinguish especially if there is only a low concentration of the metal compound
-sometimes a sample contains a mixture of metal ions which can mask the colour of the flame
what is flame emission spectroscopy an example of?
-an instrumental method
state the three advantages of flame emission spectroscopy
-instrumental methods are rapid as flame emission spectroscopy analyses samples more rapidly than flame tests
-instrumental methods are sensitive - flame emission spectroscopy will work even on a tiny sample of metal compound
-instrumental methods are accurate.Flame emission spectroscopy is more likely to identify a metal ion correctly than using a flame test
