GCSE CHEMISTRY Flashcards
Define an element.
Give an example
In an element, all of the atoms are the same and it is a pure substance.
Eg: magnesium or sulfur.
Define a compound.
Give an example.
Compounds contain two or more different elements that are chemically combined in fixed proportions.
Eg: magnesium sulfide.
Define a mixture
A mixture contains two or more elements or compounds that are not chemically combined.
Give 4 ways we can separate a mixture using physical means.
Filtration , crystallisation , distillation and chromatography.
Definition of a molecule.
Give examples
A molecule has any elements chemically combined , even if their the same element.
- O2, Cl2
Explain why when you mix two elements together you can often separate them easily by physical means, yet when 2 elements are chemically combined in a compound, they are difficult to separate . (2marks)
-You can easily separate the elements in a mixture as they are not chemically bonded and have not changed properties so you can use physical methods such as filtration to separate them.
- However to separate the elements from a compound, you must break the chemical bonds that hold them together , which requires chemical reactions and not physical methods.
Describe the basic structure of an atom (2marks)
Atoms consist of a central nucleus containing protons and neutrons, surrounded by electrons that orbit in shells.
What can physical techniques only be used to separate?
Mixtures
What is filtration used for?
Used to separate an insoluble solid from a liquid.
What 3 things do we need for filtration?
Filter funnel , filter paper, conical flask.
Explain step by step the method for filtration.
1- Once everything is set up ( filter funnel in conical flask and filter paper in funnel) you pour your mixture into the filter paper.
2 - The liquid/filtrate passes through the tiny pores in the filter paper however the solid material cannot pass through the filter paper so it’s trapped.
3- At the end we have our liquid separated from our solid.
What is crystallisation used for?
Used to separate a soluble solid from a liquid.
What does the state symbol aq mean?
It means that the element has dissolved in water.
Describe a method of crystallisation to separate sodium chloride from water.
- If we leave the solution for a few days then the water will evaporate, which will leave behind crystals of solid sodium chloride, changing the state symbol.
- But we can make crystallisation happen faster by gently heating the solution in an evaporating dish on a water bath to evaporate the water.
- Heating should be stopped when the solution is at the point of crystallisation.
- The rest of the water is then left to evaporate off the satured solution at room temp.
- A flat- bottomed crystallisation dish or Petri dish can be used for that last step as it gives a larger surface area for the water to evaporate from.
What is a risk of using a Bunsen burner to make crystallisation happen faster?
Some chemicals may break down if we heat them.
