Chem-8 Flashcards
what is a pure substance
The word pure is used in chemistry in a different way from its everyday meaning. For example, shops sell cartons labelled as ‘pure’ orange juice. The label means that the contents are just orange juice, with no other substances added. However, the juice is not pure in the chemical sense, because it contains different substances mixed together. In chemistry:
a pure substance consists only of one element or one compound
a mixture consists of two or more different substances, not chemically joined together
What are the different types of chemical substances
an element contains just one type of atom
a compound contains two or more types of atom joined together
a mixture contains two or more different substances that are not joined together
the different substances in a mixture can be elements or compounds.
How can you distinguish between pure substances and mixtures
Pure substances have a sharp melting point but mixtures melt over a range of temperatures. This difference is most easily seen when the temperature of a liquid is measured as it cools and freezes.
What is a formulation and some examples
A formulation is a mixture which has been designed as a useful product. Formulations are all around us, for example:
fuels
cleaning products
paints
medicines
alloys
fertilisers
foods
In a formulation, every chemical has been added in a carefully measured amount. Each chemical has a specific purpose in the formulation.
What is chromatography
Paper chromatography is used to separate mixtures of soluble substances and to provide information on the possible identity of the substances present in the mixture. These are often coloured substances such as food colourings, inks, dyes or plant pigments.
What are the phases of chromatography
Chromatography relies on two different ‘phases’:
the mobile phase is the solvent that moves through the paper, carrying different substances with it
the stationary phase is contained on the paper and does not move through it
The different dissolved substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.
Hoe can you interpret a chromatogram
Separation by chromatography produces a chromatogram. A paper chromatogram 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
A paper chromatogram can also be used to identify substances by comparing them with known substances. Two substances are likely to be the same if:
they produce the same number of spots, and these match in colour
the spots travel the same distance up the paper (have the same Rf value)
Explain everything you know on Rf values
Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value is always the same for a particular substance if run in the same solvent system.
Rf values vary from 0 (the substance is not attracted to the mobile phase) to 1 (the substance is not attracted to the stationary phase).
What’s the equation for a Rf value
distance travelled by substance divided by distance travelled by solvent.
Explain what testing for gases means
There are several different tests to detect and identify gases and the ions in compounds. It is important that the test for any gas or ion is unique. The results of a test must let you determine which gas or ion is present, rather being uncertain about which one it is.
What’s the oxygen test
Oxygen supports combustion. If oxygen is present in a test tube, a glowing splint relights when it is held inside.
What’s the hydrogen test
Hydrogen ignites in air. If hydrogen is present in a test tube, a lighted splint held near its mouth ignites with a squeaky pop.
What’s the test for carbon dioxide
Carbon dioxide reacts with calcium hydroxide solution to produce a white precipitate of calcium carbonate. Limewater is a solution of calcium hydroxide. If carbon dioxide is bubbled through limewater, the limewater turns milky or cloudy white.
What’s the test for chlorine
Chlorine is an acidic gas that also acts as a bleach. Damp litmus paper is bleached white when it is placed in chlorine.
If damp blue litmus paper is used, the paper turns red then white.
