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: If a '''chemical reaction''' gives off more energy than it needs to start then it is called an [[exothermic]] '''reaction'''. In these '''reactions''' the [[temperature]] increases.
: If a '''chemical reaction''' takes in more energy than it gives out then it is called an [[endothermic]] '''reaction'''. In these '''reactions''' the [[temperature]] decreases.
===Signs of Chemical Reactions===
There are three signs that a '''chemical reaction''' is taking place:
*A colour change.
*A temperature change.
*[[Effervescence]] (bubbles of [[gas]] are given off.
===Examples===
*[[Displacement Reaction]] - One [[element]] replaces another in a [[compound]].
*[[Neutralisation]] - An [[acid]] and a [[base]] '''react''' to make a [[salt]].
==Key Stage 4==
===Meaning===
A [[Chemical Reaction]] is when the [[atom]]s in [[molecule]]s break their [[Chemical Bond|bonds]] and form new [[Chemical Bond|bonds]], usually with other [[atom]]s.
===About Chemical Reactions===
: In a '''chemical reaction''' the [[atom]]s rearrange to form a new [[substance]].
: '''Chemical reactions''' often need some [[energy]] to start, like [[combustion]] where the [[fuel]] needs to be [[Heat|heated]] to start.
: If a '''chemical reaction''' gives off more energy than it needs to start then it is called an [[exothermic]] '''reaction'''. In these '''reactions''' the [[temperature]] increases.
: If a '''chemical reaction''' takes in more energy than it gives out then it is called an [[endothermic]] '''reaction'''. In these '''reactions''' the [[temperature]] decreases.
There are three ways that '''chemical reactions''' can happen:
*[[Electron Transfer]] - [[Electron]]s from the [[Outer Shell]] of one [[atom]] move to the [[Outer Shell]] of another [[atom]] to form an [[Ionic Bond]].
*[[Proton Transfer]] - [[Hydrogen#Ion|Hydrogen Ions]] are removed from an [[acid]] and added to a [[base]].
*[[Electron Sharing]] - [[Electron]]s in the [[Outer Shell]] of one [[atom]] are shared with the [[Outer Shell]] of another [[atom]] forming a [[Covalent Bond|covalent bond]].
===References===
====AQA====
:[https://www.amazon.co.uk/gp/product/0008158770/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=0008158770&linkCode=as2&tag=nrjc-21&linkId=ec31595e720e1529e49876c3866fff6e ''Chemical reaction, page 108, GCSE Physics; Student Book, Collins, AQA '']
====OCR====
:[https://www.amazon.co.uk/gp/product/0198359837/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=0198359837&linkCode=as2&tag=nrjc-21&linkId=3c4229e8b023b2b60768e7ea2307cc6f ''Chemical reactions, pages 27, Gateway GCSE Physics, Oxford, OCR '']
==Beyond the Curriculum==
{{#ev:youtube|https://www.youtube.com/watch?v=8m6RtOpqvtU}}