A chemical reaction happens when substances change to form new substances., Reactants are the starting substances in a chemical reaction., Products are the new substances formed during a reaction., In a chemical change, new substances are made and the change is usually difficult to reverse., A physical change does not create a new substance and is often reversible., Atoms are the smallest particles that make up elements., Molecules are formed when two or more atoms join together chemically., A compound contains two or more different elements chemically bonded., A mixture contains substances that are not chemically bonded., Elements contain only one type of atom., During combustion, a substance reacts quickly with oxygen and releases energy., Oxidation happens when a substance gains oxygen., Neutralisation occurs when an acid reacts with a base to form salt and water., Acids have a pH lower than seven., Alkalis are soluble bases with a pH higher than seven., Indicators can be used to test whether a substance is acidic or alkaline., Temperature change can be evidence that a chemical reaction has taken place., A colour change can show that a new substance has formed., Gas bubbles may form during a chemical reaction., A precipitate is a solid formed from two solutions reacting., Mass is conserved in a chemical reaction., In a closed system, the total mass before and after a reaction stays the same., Particles rearrange during a chemical reaction., No atoms are created or destroyed in a chemical reaction., Burning magnesium produces magnesium oxide., When vinegar reacts with baking soda, carbon dioxide gas is produced., Rusting is an example of oxidation., Iron reacts with oxygen and water to form rust., Energy changes occur during chemical reactions., Exothermic reactions release energy to the surroundings., Endothermic reactions take in energy from the surroundings., Heating can speed up many chemical reactions., Increasing the surface area of a solid can increase the rate of reaction., Stirring can help reactants mix and react faster., A catalyst speeds up a reaction without being used up., Enzymes are biological catalysts., The pH scale measures how acidic or alkaline a solution is., Strong acids have very low pH values., Strong alkalis have very high pH values., Water is neutral and has a pH of seven., Acids can react with metals to produce a salt and hydrogen gas., Acids can react with carbonates to produce a salt, water and carbon dioxide., Chemical equations show reactants turning into products., Word equations use words to describe a reaction., Symbol equations use chemical formulas to describe a reaction., Balanced equations show equal numbers of each type of atom on both sides., The arrow in a chemical equation means “reacts to form.”, Some reactions are reversible., In a reversible reaction, products can change back into reactants., Understanding key terms helps explain and predict chemical reactions.

Changing Substances

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