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✅ Definitions :
• Elements : Pure substances that cannot be broken down into simpler substances by chemical methods.
• Compounds : are defined as pure substances made up of atoms of two or more elements chemically combined together.(can only be separated by chemical methods)
• Mixtures : Consist of different substances that are not chemically combined together.(can be separated with physical methods)
• Monatomic : consist of single atom
• Molecules of elements : are pure substances made up of two or more atoms of the same type chemically joined together.
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Key important points to take note of ..
• All substances which are composed of only one type of atom are called elements .
• Elements can exist as MONATOMIC elements or as MOLECULES OF ELEMENTS. (find the meaning above)
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• Each element/Compound has its own unique physical properties; such as MELTING POINT and BOILING POINT.

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The formation of a compound requires a chemical reaction. In the process , energy change is usually involved ;
• Exothermic : when energy is given out TO the surrounding .(remember the word 'exit' which is somewhat like EXothermic)
• Endothermic : when energy is taken in from the surrounding .(remember the word 'enter' which is somewhat like ENdothermic)
• A COMPOUND has its own physical and chemical properties . But thy are different from its constituent elements.
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••• A compound is made up of atoms of different elements chemically combined in a FIXED ratio.

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• A mixture does not have a fixed composition by mass.
•• When a pure substance is contaminated with impurities, a mixture is formed. The impurities can affect the MELTING and BOILING point of the pure substance.
•• The greater the amount of impurities, the lower the impure substance melts below its melting point.

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• Matter takes up space and has mass.
• Matter can exist in 3 different physical states ; which are SOLID, LIQUID and GAS.
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• Kinetic particle theory states that :
Matter is made up of tiny particles. These tiny particles can be atoms, ions or molecules.
•The particles have kinetic energy and hence, are in constant and random motion .