C2.1 Flashcards
Relative atomic mass
Relative atomic mass, is the mean mass of an atom of an element compared to 1/12 the mass of a 12/6 C (carbon -12) atom
Periodic table
The periodic table shows the relative atomic masses of the elements. The top number in each box is the Ar for that element’s atoms
Chemical formula
A chemical formula tells you how many atoms of each element there are in a unit of a substance
Relative formula mass
Relative formula mass, Mr values, are the mean mass of a unit of a substance compared to 1/12 of the mass of a 12/6 C atom. You cannot find Mr values on the periodic table so you have to work them out. You add together the Ar values for all the atoms in the formula for the substance. Ar and Mr values have no units
Empirical formula
An empirical formula shows the simplest whole- number ratio of the atoms of each element in a compound. The number in some chemical formulae are already the simplest whole numbers they can be. The numbers in other chemical formulae can be simplified using their HCM.
Pure
In science a pure substance consists of just one element or compound. This means that mixtures are impure substances since they contain more than one element or compound.
Why is it difficult to obtain pure substances?
It is difficult to obtain pure substances. Just one atom or molecule of something else makes a substance impure.
Are mixtures useful?
Many useful materials are mixtures of different substances, often deliberately chosen to produce the desired properties.
Alloy
An alloy is a mixture of a metal with one or more other elements. Most metals we use are alloys.
Melting point
The melting point of a substance is the temperature at which it changes form the solid state to the liquid state. The melting point of a pure substance is a single temperature.
Melting point for impure substance
If a substance is impure it’s melting point is less than that of the pure substance. It often melts over a range of temperatures, not just one temperature. The greater the difference between the measured melting point for a substance and its accepted melting point the lower its purity is likely to be.
How do you determine a melting point?
You can determine the melting point of a substance by heating it. You can either measure the temperature at which it melts or measure its temperature at regular intervals and plot a graph. You need to heat the substance slowly as it allows the temperature of the whole sample to increase. And you need to stir it as it melts as it ensures that the entire sample is the same temperature. These two action increase the accuracy of the measurement of the melting point of a sample.
What happens when a substance dissolves?
A solution forms when one substance dissolves in another. When a substance dissolves its particles separate and become completely mixed with the particles of the solvent.
Soluble
If a substance can dissolve in a particular solvent, it is soluble in that particular solvent.
Filtration
Filtration separates an insoluble substance in the solid state from substances in the liquid state. It works because filter paper has tiny microscopic holes.
Residue and filtrate
Residue- the substance left behind on the filter paper
Filtrate- the small particles of a substance that are able to pass through the filter paper.
Crystallisation
If you heat a solution, the solvent evaporates leaving the solute behind. If you heat the solution too strongly, you get a powder but if you allow the solvent to evaporate slowly you get regularly shaped crystals. Crystallisation takes patience because you need to heat the solution gently until it becomes a saturated solution. A solution is saturated when no more solute can be dissolved at that temperature, crystals will start to form at this point so you let the solution cool slowly. As the solution cools the solubility of the solute decreases so more crystals form.
Simple distillation
This distillation separates a solvent from a solution. It relies on the solvent having a much lower boiling point than the solute. When the solution is heated the solvent boils but the solute does not. The solvent escapes from the solution in the gas state.
Fractional distillation
Separates two or more substances from a mixture in the liquid state. It relies on each substance having a different boiling point and uses a special piece of equipment called a fractioning column.
Chromatotaphy
Chromatography relies on two different chemical phases:
-a stationary phase that does not move
- a mobile phase that does move
a separation method that relies on the distribution of a substance between a mobile and secondary phase.
Paper chromatography
The stationary phase is absorbent paper and the mobile phase is a solvent in the liquid stat such as water
Thin-layer chromatography
Works in the same way as paper chromatography but the stationary phase is a thin layer of silicia or alumina powder spread over a plate of glass or plastic.
Rf values
Scientists use Rf values to compare the different spots on a chromatogram. If two spots have the same Rf value and are the same colour, they are likely to be identical.
baanced chemical equation
a model for a reaction showing formulae and number of units for all substances involved
