READ: Reactions
READ: Reactions
What is a Chemical Reaction?
Yummy! S’mores are on the way! Did you ever toast marshmallows over a campfire? The sweet treats singe on the outside and melt on the inside. Both the fire and the toasted marshmallows are evidence of chemical changes. In the process of burning, the wood changes to ashes and gases, and the outside of the marshmallow turns brown and crispy. Neither the wood nor the marshmallows can change back to their original form easily. That’s because burning is a chemical change and chemical changes are often impossible to undo. In this unit, you’ll learn about many types of chemical changes, including how they occur and why you can’t live without them. A chemical reaction (a process in which some substances change into different substances) occur around us and in us all the time. Whenever substances go through a change in which different types of molecules or particles are present at the end of the change a chemical reaction has occurred. Reactants(the starting materials in a chemical reaction) are converted into products (the ending materials in a chemical reaction). Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right).
Chemical reactions may occur quickly or slowly. Look at the two pictures in the figure below. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust.
Breaking and Making Chemical Bonds
The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms end up in different combinations in the products. This makes the products new substances that are chemically different from the reactants.
Consider the example of the reaction between sodium and chlorine to made sodium chloride (table salt). Sodium is a metal that is shiny, an excellent conductor of electric current, and reacts violently with water. Chlorine is a poisonous green gas. When sodium and chlorine are chemically combined to form sodium chloride, the product has an entirely new set of properties. Sometimes we sprinkle salt (sodium chloride) on our food. The properties of the products are different than the properties of the reactants. We would not put salt on our potato chips if it had the same explosive and deadly properties as the original reactants.
Q: Watch the animation of a similar chemical reaction at the following URL. Can you identify the reactants and the product in the reaction?
Look at the girl’s hair in the photo above. It has obviously changed color. The process in which this occurred involved chemical reactions. How do you know that chemical reactions have occurred? The change in color is the most obvious clue.
Chemical reactions occur when bonds are broken and/or formed as the reactants are changed into the products. However, we can’t directly observe the breaking and forming of bonds. We have to look for other evidence of chemical change. Because the products of a chemical reaction have different properties than the reactants, there are several observations that we can make to help us know when a chemical change has occurred.
- Change of color.
- Light is given off.
- Change in temperature – Heat is released or absorbed during the reaction.
- Production of a gas – Gas bubbles are released during the reaction.
- Production of a solid – A solid settles out of a liquid solution. The solid is called a precipitate.
Examples of Chemical Reactions
Look carefully at the figures below. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place.
A burning campfire can warm you up on a cold day.
Dissolving an antacid tablet in water produces a fizzy drink.
Adding acid to milk produces solid curds of cottage cheese.
These vividly colored maple leaves were all bright green during the summer. Every fall, leaves of maple trees change to brilliant red, orange, and yellow colors. A change of color is a sign that a chemical change has taken place. Maple leaves change color because of chemical reactions.
Summary
- All chemical reactions involve both reactants and products. Reactants are substances that start a chemical reaction, and products are substances that are produced in the reaction.
- A chemical reaction can be represented by the general chemical formula:
- Bonds break and reform during chemical reactions. Reactants and products contain the same atoms, but they are rearranged during the reaction, so reactants and products are different substances.
- Potential signs that chemical reactions have occurred include a change in color, change in temperature, production of a gas, and production of a solid precipitate.
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