Chemical Reactions

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Chemical Reaction Noun

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[kem-i-kuhl ree-ak-shun]

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Chemical Reaction

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A process where a set of substances undergo a chemical change to form a different substance.

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Example: This image shows a chemical reaction where starting substances, called reactants (left), are transformed into new substances, called products (right), by rearranging their atoms.

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Reactant Noun

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[ree-ak-tant]

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Reactant

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A substance that is consumed or used up during a chemical reaction, written on the left side of a chemical equation.

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Example: The image shows that reactants, like methane (CH₄) and oxygen (O₂), are the starting substances on the left side of a chemical reaction arrow.

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Product Noun

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[prod-ukt]

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Product

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A new substance that is formed as a result of a chemical reaction, written on the right side of a chemical equation.

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Example: In a chemical reaction, the starting materials (reactants) are transformed into new substances called products, like the carbon dioxide and water shown here.

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Coefficient Noun

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[koh-uh-fish-uhnt]

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Coefficient

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A number placed in front of a chemical formula that indicates the number of molecules of that substance in the reaction.

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Example: The large number in front of a chemical formula (the green '2') is the coefficient, indicating the number of molecules in a chemical reaction.

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Subscript Noun

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[sub-skript]

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Subscript

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A number written after an element's symbol in a formula that indicates the number of atoms of that element.

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Example: The chemical formula for water, H₂O, shows that the small number '2' is a subscript, indicating there are two hydrogen (H) atoms in one molecule.

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Aqueous Adjective

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[ay-kwee-uhs]

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Aqueous

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A state of matter designation indicating that a substance has been dissolved in a water-based solution.

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Example: This diagram shows a solute (particles) dissolved in a solvent (liquid) to form a solution. An aqueous solution is a specific type where water is the solvent.

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Precipitate Noun

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[pri-sip-i-teyt]

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Precipitate

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An insoluble solid that emerges from a liquid solution as the result of a chemical reaction.

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Example: A chemical reaction in a liquid solution can form an insoluble solid, called a precipitate, which then settles at the bottom.

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Catalyst Noun

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[kat-l-ist]

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Catalyst

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A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

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Example: A catalytic converter uses a catalyst (the material inside) to change harmful car exhaust gases into safer substances like carbon dioxide and water.

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Exothermic Adjective

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[ek-soh-thur-mik]

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Exothermic

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A type of reaction or process that releases energy from the system, usually in the form of heat or light.

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Example: A combustion reaction, like a burning campfire, is exothermic because it releases energy into the surroundings as heat and light.

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Endothermic Adjective

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[en-doh-thur-mik]

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Endothermic

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A type of reaction or process that absorbs energy from its surroundings, usually in the form of heat.

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Example: An instant cold pack contains chemicals that, when mixed, start an endothermic reaction, absorbing heat from the surroundings and making the pack feel cold.