A Change in ColorMany chemical reactions produce substances with a different color than the original substances. An unexpected color change can provide evidence that a chemical change has occurred.
Production of GasSome chemical reactions produce a gas. The formation of a new gas may be observed as bubbles or fizzing.
Heads up: Bubbles do not always mean that a chemical reaction occurred. Boiling also produces gas bubbles, but boiling is a physical change because no new substance is formed.
Formation of a Precipitate
When two solutions are mixed, a chemical reaction may produce a new substance that is insoluble, meaning that it does not remain dissolved in the liquid. The solid that forms is called a precipitate.
This process is called formation of a precipitate.The solid that forms from the reaction in a liquid mixture is called a precipitate. The precipitate may make the mixture appear cloudy, remain suspended in the liquid, or settle to the bottom.
Temperature ChangeSome chemical reactions involve an energy change that can cause the temperature of the surroundings to increase or decrease without external heating or cooling.
When a chemical reaction releases thermal energy to its surroundings, it is exothermic, and the surroundings may become warmer.
When a chemical reaction absorbs thermal energy from its surroundings, it is endothermic, and the surroundings may become cooler.
Production of Light
Some chemical reactions release energy in the form of light. If light is produced without an outside light source, it may provide evidence that a chemical change has occurred.
A Change in Odor
Some chemical reactions produce new substances that have a different odor from the substances with which you started. An unexpected change in odor can therefore provide evidence of a chemical change.
But wait... if you can smell something, does that mean a gas was produced? Not necessarily.
To smell a substance, particles of that substance must be present in the air and reach your nose. Those particles may be part of a gas, or they may be a vapor.
A gas is a substance that normally exists in the gaseous state under the conditions being considered. A vapor is the gaseous form of a substance that would normally be a liquid or solid under those same conditions. For example, oxygen (O₂) in the air is a gas, while gaseous H₂O above liquid water is water vapor.
A liquid does not have to reach its boiling point to form a vapor. Evaporation can occur at temperatures below the boiling point, as particles at the surface of a liquid escape into the gas phase. That is why a puddle of water can gradually disappear even though the water never boils.
Volatility describes how easily a substance evaporates and enters the gas phase. A volatile substance evaporates relatively easily. For example, many of the substances responsible for the smell of perfume are volatile. They evaporate into the air and can then reach your nose.
This is an important distinction: evaporation is a physical change. Smelling a substance does not automatically mean that a chemical reaction has occurred or that a reaction has produced a gas.
During a chemical reaction, however, a new substance with a new odor may be formed. An unexpected change in odor can therefore be evidence that a chemical change has occurred.
Bottom line: A new odor can be evidence of a chemical change, but odor does not automatically mean that a gas-producing chemical reaction has occurred. The substance you smell may simply be a vapor produced by evaporation.
Evidence of a Chemical Change — Summary
Chemical Change
- A chemical change occurs when one or more substances change at the atomic or molecular level and form new substances.
- Observable changes can provide evidence that a chemical reaction occurred.
- A single observation does not necessarily prove that a chemical change occurred.
- The key question is: Was a new substance formed?
Evidence of Chemical Change
Change in Color
- A chemical reaction may produce a substance with a different color from the original substances.
- An unexpected color change can be evidence of a chemical change.
- A color change alone does not prove that a chemical reaction occurred.
Production of Gas
- Some chemical reactions produce a gas as a new substance.
- Gas production may be observed as bubbles or fizzing.
- Bubbles do not always indicate a chemical change.
- Boiling also produces bubbles but is a physical change because no new substance is formed.
Formation of a Precipitate
- A reaction between two solutions may produce an insoluble solid.
- This newly formed solid is called a precipitate.
- A precipitate may make a mixture appear cloudy, remain suspended in the liquid, or settle to the bottom.
- The formation of a precipitate can provide evidence that a new substance has formed.
Temperature Change
- Some chemical reactions involve an energy change that causes the surroundings to become warmer or cooler without outside heating or cooling.
- An exothermic reaction releases thermal energy to the surroundings, which may make the surroundings warmer.
- An endothermic reaction absorbs thermal energy from the surroundings, which may make the surroundings cooler.
Production of Light
- Some chemical reactions release energy as light.
- Light produced without an outside light source can provide evidence of a chemical change.
Change in Odor
- A chemical reaction may produce a new substance with a new or different odor.
- A new odor can provide evidence of a chemical change.
- Smelling something does not automatically mean that a reaction produced a gas.
- The substance being smelled could be a vapor produced by evaporation.
- Evaporation is a physical change, so odor alone does not prove that a chemical reaction occurred.
Critical Vocabulary
- Chemical Change — A change in which one or more new substances are formed.
- Chemical Reaction — A process in which atoms are rearranged to form new substances.
- Evidence — An observation that supports the conclusion that something has occurred.
- Physical Change — A change in form, state, or appearance that does not produce a new substance.
- State of Matter — The physical form in which matter exists, such as solid, liquid, or gas.
- Solid — A state of matter with a definite shape and definite volume.
- Liquid — A state of matter with a definite volume but no definite shape; it takes the shape of its container.
- Gas — A state of matter with no definite shape or volume; a gas expands to fill its container.
- Vapor — The gaseous form of a substance that would normally be a liquid or solid under the conditions being considered.
- Evaporation — The process in which particles at the surface of a liquid enter the gas phase; evaporation can occur below the boiling point.
- Boiling — The change from liquid to gas occurring throughout a liquid when it reaches its boiling point under the given pressure.
- Volatility — A measure of how easily a substance evaporates and enters the gas phase.
- Volatile — Describes a substance that evaporates relatively easily.
- Precipitate — An insoluble solid that forms from a reaction occurring in a liquid mixture.
- Insoluble — Unable, or only slightly able, to dissolve in a particular solvent.
- Thermal Energy — Energy associated with the motion of particles in matter and transferred as heat because of a temperature difference.
- Exothermic — Describes a reaction that releases thermal energy to its surroundings.
- Endothermic — Describes a reaction that absorbs thermal energy from its surroundings.
- Odor — A smell caused when particles of a substance in the air interact with receptors in the nose.
- Reactant — A starting substance in a chemical reaction.
- Product — A new substance formed by a chemical reaction.
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