introduction to chemical reactions

evidence of chemical reactions

Not all changes in matter involve chemical reactions. For example, there are no chemical reactions involved in changes of state. When liquid water freezes or evaporates, it is still water. No bonds are broken and no new products are formed. How can you tell whether a change in matter involves a chemical reaction? Often, there is evidence. Four common signs that a chemical reaction has occurred are: Change in color: the products are a different color than the reactants. 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. You can see examples of each type of evidence in Figure 8.3 and at this URL: MEDIA Click image to the left or use the URL below. URL:

what is a chemical reaction

A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. A chemical reaction can be represented by this general equation: Reactants ! Products The arrow (!) shows the direction in which the reaction occurs. The reaction may occur quickly or slowly. For example, foam shoots out of a fire extinguisher as soon as the lever is pressed. But it might take years for metal to rust.

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breaking and reforming chemical bonds

In chemical reactions, bonds break in the reactants and new bonds form in the products. The reactants and prod- ucts contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. Look at the example in Figure 8.2. It shows how water forms. Bonds break in molecules of hydrogen and oxygen. Then new bonds form in molecules of water. In both reactants and products, there are four hydrogen atoms and two oxygen atoms. But the atoms are combined differently in water. You can see another example at this URL: http://w

reaction direction and equilibrium

The arrow in Figure 8.2 shows that the reaction goes from left to right, from hydrogen and oxygen to water. The reaction can also go in the reverse direction. If an electric current passes through water, water molecules break down into molecules of hydrogen and oxygen. This reaction would be represented by a right-to-left arrow ( ) in Figure Many other reactions can also go in both forward and reverse directions. Often, a point is reached at which the forward and reverse reactions occur at the same rate. When this happens, there is no overall change in the amount of reactants and products. This point is called equilibrium, which refers to a balance between any opposing changes. You can see an animation of a chemical reaction reaching equilibrium at this URL:

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instructional diagrams

No diagram descriptions associated with this lesson

questions

Which of the following changes is a chemical change?

a. ice melting

b. wax melting

c. water boiling

-->  d. wax burning

Most chemical reactions take place in labs.

a. true

-->  b. false

All changes in matter involve chemical reactions.

a. true

-->  b. false

Which statement is true of all chemical reactions?

a. They can go in just one direction.

b. They occur only in science labs.

-->  c. They break and reform bonds.

d. They create new elements.

Evaporation is an example of a chemical change.

a. true

-->  b. false

Reactants and products in chemical reactions may be

a. elements.

b. compounds.

c. exactly the same compounds.

-->  d. two of the above

All chemical changes in matter involve

a. changes of state.

-->  b. chemical reactions.

c. changes in color.

d. two of the above

Reactants and products can be elements or compounds.

-->  a. true

b. false

Chemical reactions may occur quickly or slowly.

-->  a. true

b. false

Which statement is true about a precipitate?

a. It is a solid.

b. It settles out of a liquid solution.

c. It is evidence of a chemical reaction.

-->  d. all of the above

Some chemical reactions can proceed in just one direction.

-->  a. true

b. false

An example of a chemical change is water boiling.

a. true

-->  b. false

Freezing involves a chemical reaction.

a. true

-->  b. false

A banana turning brown is a chemical change.

-->  a. true

b. false

Wax melting is an example of a chemical reaction.

a. true

-->  b. false

The products and reactants of a chemical reaction have different atoms.

a. true

-->  b. false

All chemical reactions are reversible.

a. true

-->  b. false

Rusting is an example of a chemical change.

-->  a. true

b. false

Condensation is a type of chemical reaction.

a. true

-->  b. false

A change in temperature is a common sign of a chemical reaction.

-->  a. true

b. false

substance produced in a chemical reaction

a. chemical reaction

b. melting

c. equilibrium

d. chemical bond

-->  e. product

f. rusting

g. reactant

force of attraction that breaks and reforms in a chemical reaction

a. chemical reaction

b. melting

c. equilibrium

-->  d. chemical bond

e. product

f. rusting

g. reactant

substance that starts a chemical reaction

a. chemical reaction

b. melting

c. equilibrium

d. chemical bond

e. product

f. rusting

-->  g. reactant

example of chemical change

a. chemical reaction

b. melting

c. equilibrium

d. chemical bond

e. product

-->  f. rusting

g. reactant

balance between opposing changes

a. chemical reaction

b. melting

-->  c. equilibrium

d. chemical bond

e. product

f. rusting

g. reactant

process in which some substances become different substances

-->  a. chemical reaction

b. melting

c. equilibrium

d. chemical bond

e. product

f. rusting

g. reactant

example of a physical change

a. chemical reaction

-->  b. melting

c. equilibrium

d. chemical bond

e. product

f. rusting

g. reactant

What do the formation of rust and the formation of cottage cheese have in common?

a. Both occur very quickly.

-->  b. Both involve chemical reactions.

c. Both are changes of state.

d. Both are physical processes.

Which of the following changes does not involve chemical reactions?

-->  a. clouds forming

b. candles burning

c. leaves turning color

d. fire extinguishers foaming

During chemical reactions, atoms are

-->  a. rearranged.

b. created or destroyed.

c. changed to new elements.

d. two of the above

There is no overall change in reactants and products whenever a chemical reaction

a. goes in just one direction.

b. goes in two directions.

-->  c. reaches equilibrium.

d. proceeds slowly.

Evidence of chemical reactions include changes in

a. state.

b. color.

c. temperature.

-->  d. two of the above

diagram questions

No diagram questions associated with this lesson