Chemical reactions

• Chemical reactions make new chemicals.
• Atoms are rearranged during a chemical reaction, but the number of atoms does not change.
• Evidence of chemical reactions includes a large temperature change, bubbles or a colour change.
• Chemical reactions can be represented using equations.
• A new substance must be produced for a chemical reaction to take place.
• Physical changes, such as melting, boiling and dissolving, do not make new chemicals. They are usually easy to reverse.
• In a chemical reaction, chemical bonds between atoms are broken and make, so the atoms get rearranged into new substances.
• The simplest kind of chemical reactions involve two elements reacting together to make a compound.
• When energy is transferred to the surroundings, this is called an exothermic reaction and usually feels hot.
• When energy is taken from the surroundings, this is called an endothermic reaction and usually feel cold.
• Exothermic reactions are chemical reactions which release energy from the chemicals into the surroundings. This energy is usually released as heat, so the surrounding get hotter.
• Endothermic reactions absorb energy from the surroundings. This energy is usually absorbed as heat, so the surrounding get colder.
• Photosynthesis is an endothermic reaction because plant leaves absorb light energy.
• Thermal decomposition reactions are endothermic because they absorb energy when the chemicals are heated.
• A catalyst is a substance that speeds up a chemical reaction without being used up.
• Enzymes are catalysts that work inside the cells of living organisms.
• Thermal decomposition is a chemical reaction that happens when a compound breaks down when heated.
• Thermal decomposition reactions happen at high temperatures. The reactants absorb lots of energy as they break down into the products.
• Thermal decomposition reactions are examples of endothermic reactions, and are useful when cooking and baking cakes.
• In a combustion reaction, fuel is burned and reacts with oxygen to release energy.
• Combustion is another word for burning.
• The fire triangle summarises the three components needed for combustion - a fuel, heat and oxygen. If one of these things are removed from a fire, the fire goes out.
• When fuels burn in combustion reactions, they release useful thermal energy (heat).