Chemical Reactions

• Examples of exothermic changes include condensing and freezing.
• A chemical reaction is a change in which atoms are rearranged to create new substances, the atoms are joined together in one way before the reaction and in a different way after the reaction.
• All chemical reactions make new substances, transfer energy to or from the surroundings, and most are irreversible.
• An example of a chemical reaction is the change from a raw egg to a fried egg.
• Signs of a chemical reaction include seeing huge flames or tiny sparks, noticing a sweet smell or a foul stink, feeling the chemicals getting hotter or colder, and hearing a loud bang or gentle fizzing.
• Useful substances made by chemical reactions include medicines like paracetamol and fabrics like polyester.
• Chemical reactions also transfer energy, for example, burning petrol makes vehicles go and burning coal heats water to produce steam to generate electricity.
• Some chemical reactions aren't useful, for example, rusting can damage cars, boats, and bridges.
• Chemical reactions can also make food rot.
• Chemists may use catalysts to speed up slow reactions, different reactions need different catalysts, and catalysts are not used up during a reaction.
• Reversible changes such as changes of state and dissolving are not chemical reactions, they are physical changes.
• Word equations represent reactions in a simple way, for example, Magnesium burns in air, it reacts with oxygen, the product is magnesium oxide.
• In a chemical reaction, the atoms get rearranged.
• Many central-heating systems burn methane gas.
• Fuel is a material that burns to transfer energy by heating, examples of fuels include petrol, diesel, coal, and hydrogen.
• Rusting is when iron reacts with oxygen and water.
• A discrete variable is described by words or by numbers that can only have certain values such as shoe sizes.
• Hydrogen peroxide decomposes into water and oxygen, this is a decomposition reaction.
• Some cars burn hydrogen in their engines, in this reaction, hydrogen joins with oxygen, producing only one product which is water.
• Fuels burn in chemical reactions, burning is also called combustion.
• Petrol, diesel, coal and methane from under the ground or sea are fossil fuels, meaning they are non-renewable and cannot be replaced once they've been used.
• Petrol is a mixture of compounds, most of its compounds consist of atoms of hydrogen and carbon, petrol makes mainly carbon dioxide and water when it burns in car engines.
• Burning reactions are oxidation reactions.
• Discrete variables should be presented in a table.
• The hydrogen used to fuel cars comes from methane or water and the processes companies use to make hydrogen also produce harmful gases.
• Methane burns with oxygen, producing carbon dioxide and water.
• Lead carbonate, zinc carbonate and other carbonates decompose on heating.
• Hydrogen should be used to fuel cars as it produces only one product after burning, which is harmless water, other fuels produce carbon dioxide, making hydrogen better for the planet.
• Copper carbonate is a green compound made up of atoms of three elements: copper, carbon and oxygen, when heated, it decomposes into copper oxide and carbon dioxide(g).
• Hydrogen peroxide (H2O2) is a compound made up of hydrogen and oxygen that's used to bleach hair, it cannot be used to bleach hair with old H2O2 as H2O2 molecules break up.
• Methane is a compound of carbon and hydrogen, its chemical formula is CH⁴.
• Balanced symbol equation show the formulae of reactants and products, how the atoms are rearranged, and the relative amounts of reactants and products.
• The reaction of thionyl chloride with cobalt sulphate heptahydrate is an example of an endothermic reaction.
• During an endothermic reaction, the reacting mixture’s temperature decreases, however, it slowly returns to the temperature of its surroundings once the reaction is complete.
• Combustion or burning substances reacts with oxygen and produces oxidized, often gaseous products.
• When the inner bag of a cold pack is broken, the water and ammonium nitrate mix, the solid dissolves in the water and the mixture cools.
• During an exothermic reaction, the reacting mixture’s temperature increases, however, it slowly returns to the temperature of its surroundings once the reaction is complete.
• Some changes transfer energy TO THE SURROUNDINGS, these are exothermic changes.
• In any chemical reaction, the total mass of reactants is equal to the total mass of products, this is called conservation of mass, mass is also conserved in physical changes.
• Burning magnesium results in the chemical equation 2Mg + O2 -> 2MgO.