Chemical changes

• physical change is when there is no new substances formed after the change and it is reversible
• chemical change is when there are 2 or more new substances formed after the change and it is irreversible
• an example of a physical change is ice melting and water evaporating and salt dissolving in water
• an example of chemical change is rusting of an iron nail and burning of petrol
• physical change has no new substances formed, is reversible and substances have the same properties before and after change
• chemical change has new substances formed, is irreversible and has different properties before and after change
• in a chemical reaction,there will be reactant, product and condition
• reaction is the substance that reacts and is present at the start of the chemical reaction
• product is the new substance formed after the chemical reaction
• condition is the factor required for a reaction to take place, such as heat and light
• in any chemical reactions, atoms are rearranged to form different substances,they are not created or destroyed. Thus mass is always conserved during chemical reaction
• mass of reactants = mass of products
• thermal decomposition is a reaction in which a substance is broken down into 2 or more simpler substances when heated
• when sugar is heated strongly, it decomposes to form carbon and water vapour
• sugar----->carbon + water vapour
• sugar is thermally unstable so it decomposes strongly upon strong heating
• combustion is a reaction in which a substance combines with oxygen to produce 1 or more new substances and a lot of heat
• a large amount of heat is released through combustion
• incomplete combustion occurs when there is an insufficient supply of oxygen
• examples of incomplete combustion is carbon monoxide and soot
• oxidation is when a substance gains oxygen naturally and usually no energy is released
• combustion is a type of oxidation in which substances react with oxygen in the air and produce a lot of energy in the form of heat and light
• in combustion,carbon burns in oxygen
• in oxidation,carbon gains oxygen for carbon dioxide
• cellular respiration is a very important oxidation that takes place in most living cells
• glucose + oxygen --> carbon dioxide + water
• photosynthesis is an important process that ensures the survival of living things
• carbon dioxide + water --> glucose + oxygen
• electrolysis is the chemical decomposition of substances with the passage of an electric current
• electrolysis of water is when electricity is passed through water, it is broken down into its constituent elements, hydrogen and oxygen
• water --> hydrogen + oxygen
• cellular respiration is when glucose reacts with oxygen to produce carbon dioxide, water and energy
• acid is a corrosive substance with particular chemical properties including a pH value lesser than 7 ,turning blue litmus paper red,neutralising alkalis and has a sour taste
• acids taste sour, conduct electricity in aqueous solutions and turns blue litmus paper red
• hydrochloric acids(HCl) produces "chloride salts"
• nitric acid(HNO3) produces "sulfate" salts
• sulfuric acid(H2SO4) produces "nitrate" salts
• acid reacts with some metals to produce salt and hydrogen but not all metals are reactive
• acid + metal --> salt + hydrogen
• hydrochloric acid + zinc --> zinc chloride + hydrogen