C8 Flashcards
What is a pure substance in chemistry?
In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
What are the characteristics of pure elements and compounds
Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.
What can a pure substance mean?
In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
Note:
AQA says that students should be able to use melting point and boiling point data to distinguish pure from impure substances.
What is a formulation?
A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose.
How are formulations made?
Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties.
What are examples of formulations?
Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
Note:
AQA says that students should be able to identify formulations given appropriate information.
Note:
AQA says that students do not need to know the names of components in proprietary products.
What can chromatography be used for?
Chromatography can be used to separate mixtures and can give information to help identify substances.
What does chromatography involve?
Chromatography involves a stationary phase and a mobile phase.
What does separation depend on?
Separation depends on the distribution of substances between the phases.
What is the Rf value?
The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent
How do you find the ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent
Rf = distance moved by substance / distance moved by solvent
What helps us identify different compounds?
Different compounds have different R values in different solvents, which can be used to help identify the compounds.
What may happen with compounds in a mixture?
The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.
Explain how paper chromatography separates mixtures
The paper is then placed upright in a suitable solvent , such as water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.
Suggest how chromatographic methods can be used for distinguishing pure substances from impure substances
A chromatogram produced by paper chromatography or thin layer chromatography (TLC) can be used to distinguish between pure and impure substances: a pure substance produces one spot on the chromatogram. an impure substance produces two or more spots.
Note:
AQA says that students should be able to:
• interpret chromatograms and determine R values from chromatograms
• provide answers to an appropriate number of significant figures.
Practical 12:
AAAA
What is used for the test of hydrogen?
The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
What is used for the test of oxygen?
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
What is used for the test of carbon dioxide?
The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
What is used for the test of chlorine?
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.
