Cambridge IGCSETM Co-ordinated Sciences Chemistry Student\'s Book (Collins Cambridge IGCSETM)
This document provides all of the content you need to know if you are taking the IGCSE coordinated science exam. It is a very big document as it is a very difficult and wide subject although it is summarised in order to understand it. This only includes the coordinated content and not the combined ...
Scientist have confidence in particle theory because of the evidence from simple experiments . The random mixing and moving of particles in liquid and gases is known as diffusion. The
examples given below the effects of diffusion..
• Dissolving crystals in water. there are no water so only particle theory can explain this. The particles of the crystals gradually move into the water and mix with
the water particles.
• Mixing gases. These shows a jar of air and a jar of bromine gas bromine gas is red brown and heavier than air. The jar of air has been placed on top of the jar
of bromine and the lids removed so the gases can mix after about 24 hours the bromine gas and the air has spread through both jars.. particle theory says that the particles of bromane
gas can move around randomly so that they can fill both jars, which also occurs with hydrogen and air.
,The purity of solids and liquids .
It is very important that manufactured foods and drugs contain only the substances the manufacturer want in them. They must not contain any contaminants.
The simplest way of checking, the purity of solids and liquids is using heat to find the temperature at which they melt or boil . An impure solid will have a lower melting point than the
pure solid liquid containing and solid solid as salute will have a higher boiling point and the pure solvent. the best examples to use to remember these facts are water and ice.
• Pure water boils at 100°C - salted water for cooking vegetables boil at about 102°C
• Pure ice melts at 0°C - ice with salt added to it melts at about -4°C
Some chemical reactions can be reversed by changing the reaction conditions.
When hydrated blue copper(Il) sulfate crystals are heated, a white powder is formed (anhydrous copper(Il) sulfate) and water is lost as steam. If water is added to this white powder,
hydrated blue copper (II) sulfate is formed again. The reaction is reversible:
hydrated copperIl) sulfate crystals (CuSO4.5H2O (s)) —> anhydrous copper(Il) sulfate + water (CuSO4 (s) + 5H2O(l))
,A reversible reaction can go from left to right or from right to left - notice the double-headed - arrow used when writing these equations.
A similar reaction occurs when hydrated cobaltII) chloride crystals are heated.
The pink hydrated cobalt(II) chloride crystals turn to blue anhydrous cobaltII) chloride on strong heating. If water is added to the blue anhydrous cobalt II) chloride, the pink hydrated
cobaltII) chloride is reformed.
Isotopes
Atoms of the same element with the same number of protons and electrons but
different numbers of neutrons are called isotopes. For example, there are two isotopes of chlorine:
IONS AND IONIC BONDS—>
When the atoms of elements react and join together, they form compounds. When one of the reacting atoms is a metal, the compound formed is called an ionic compound. They do not
contain molecules; instead they are made of particles called ions. Ionic compounds have similar physical properties, many of which are quite different from the properties of substances
made up of atoms or molecules.
MOLECULES AND COVALENT BONDS—>
Unlike ionic compounds, covalent substances are formed when atoms of non-metals combine. Although covalent substances all contain the same type of bond, their properties can be
quite different - some are gases, others are very hard solids with high melting points. Plastics are a common type of covalent substance. Because chemists now understand how the
, molecules form and link together, they can produce plastics with almost the perfect properties for a particular use, from soft and flexible (as in contact lenses) to hard and rigid (as in
electrical sockets).
MACROMOLECULES—>
• Diamond and graphite:
Some covalently bonded compounds do not exist as simple molecular structures in the way that hydrogen does. Diamond, for example, has a giant structure with each carbon atom
covalently bonded to four others (Fig. 1.64). Another form of carbon is graphite. Graphite has a different giant structure, as seen in Fig. 1.65. Different forms of the same element, like
these, are called allotropes. In diamond, each carbon atom forms four strong covalent bonds. In graphite, each carbon atom forms three strong covalent bonds. There are weak forces of
attraction between the layers in graphite (Fig. 1.65).
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