CC2abcde Methods of Separating and Purifying Substances Flashcards
The composition (make up) of a pure substances
Cannot be changed, is the same in all parts of a piece of the substance.
E.g Pure golf contains only gold atoms
State what is meant by the term ‘impure’
If a substance has more than one type of atom
Can compounds also be pure?
Yes
E.g. The sugar we use at home is a compound called sucrose. It contains carbon, hydrogen and oxygen atoms chemically bonded together to form sucrose molecules. You cannot change the composition of pure sucrose.
What is a mixture?
Contains elements and/or compounds that are chemically not joined together. You can use physical processes to separate mixtures into different substances.
Does a mixture have a fixed composition?
No
E.g. Air is a mixture of gases. When a student sits in a classroom, they use up oxygen and breathe out carbon dioxide and so the composition of the air in the room changes. We still call it ‘air’, but because air is a mixture its composition can change.
Describe how you would separate marbles from sand
Through the process of filtration.
What happens when a solid melts?
Its particles gain enough energy to overcome the weak forces of attraction between them. They move further away from one another and the solid becomes a liquid. The temperature at which this happens is called the melting point.
What is a physical property?
How a substance responds to forces and energy.
Will a pure substance melt the same way as an impure one?
No.
A pure substance has the same composition in every part of it, and so its physical properties are the same everywhere. So, all of the pure substance will melt at the same temperature until all the substance has changed state. A mixture can have different melting points.
List the ways in which pure substances are different from mixtures.
Pure - constant physical and chemical properties, sharp melting points, composition can’t be changes, made up of one element
Mixtures - impure, varied melting points, made up of more than 1 element
What is filtration?
Used to separate mixtures. They let smaller pieces or liquids through but trap bigger pieces or insoluble substances
What is a solution?
A mixture made of solutes (dissolved substances) in a liquid called the solvent.
What is crystalisation?
Solutes can be separated from a solution by evaporating the solvent to leave the solutes behind. The process forms solids crystals of various sizes. If the crystals form slowly, the particles have longer to form an ordered pattern and will make larger crystals.
Give one example of a mixture that can be separated by filtration.
Sand and water
Cereal and milk
Salt and sugar
Explain the process of filtration and crystallisation in the lab
To filter a solution, a filter funnel is lined with a filter paper that has fine holes in it. The solvent and solute pass through the fine holes to form the filtrate. Bits of insoluble substances cannot fit through the holes and so leave a residue in the filter paper. A Bunsen Burner is then used to evaporate the filtrate carefully. Care must be taken not to overheat the solution once it is saturated, because hot crystals may spit out. Further heating may also cause crystals to change chemically.
