What are chemical changes?

Matter is constantly changing around you.

Water vapor forms clouds.

Tree leaves fall and rot.

Foods in the kitchen are

cooked.

A starting substance in a chemical reaction is called a reactant.

The new substance is called a product.

We say that “reactants yield products” and we write it as:

reactants products

Chemical versus Physical Change

When matter undergoes physical

changes, no new substances are

formed.

PHYSICAL

When matter undergoes chemical changes, a

chemical reaction occurs. In a chemical reaction, substances change into

new substances.

CHEMICAL

1. Star the reactants. 2. Box the product.

Chemical reactions may occur between atoms, molecules, or compounds. A common example of a chemical reaction is the formation of carbon dioxide (CO2). The following diagram shows fire causing a chemical reaction between carbon (C) atoms and oxygen (O) molecules.

The reactants, carbon and oxygen, are chemically changing into a product, carbon dioxide. Fire provides the energy needed for molecules to rearrange.

Interactive

(vinegar) (baking soda)

The reaction between vinegar and baking soda is an example of a chemical change between compounds. When the two reactants are mixed, their atoms rearrange into new patterns that form three new compounds.

The signs of chemical change are the many gas bubbles. These are filled with carbon dioxide, one product of the reaction.

A second product is water.

The last product is sodium acetate. Sodium acetate is commonly used in the textile, rubber and paper industries.

(vinegar) (baking soda)

The total mass of the reactants always equals the total mass of the products.

The total number of atoms remains the same. Atoms are neither created nor destroyed. They are simply rearranged due to a chemical reaction.

For example, the same number of atoms are in the reactants and products shown above. Count them.

Chemical Reactions

Atoms simply rearrange into new combinations. The change in combinations of atoms is what gives the products new and different properties.

Atoms are neither gained nor lost during chemical changes. They are always conserved. Molecular models (like the one you just saw) show you how the atoms are rearranged in the products.

What are signs of a chemical change?

Signs of a Chemical Change.

There are several common signs that show that substances have undergone a chemical change. One of these is the formation of a solid when solutions are mixed. The formation of such a solid in the liquid is an example of precipitate. A precipitate is a solid that forms in solution due to a chemical reaction.

Video

- Forms a precipitate -

Signs of a Chemical Change.

A second sign of a chemical change is when a gas is produced. If you put an antacid tablet containing a mixture of baking soda and citric acid in a glass of water, bubbles of carbon dioxide gas are produced. The formation of gas upon the mixing of chemicals likely means a chemical change has happened.

- Forms a gas -

Signs of a Chemical Change.

Temperature increases are a sign of a chemical change that releases energy. Similarly, a release of energy as light indicates a chemical change. The light from a candle flame is an example. A flame is a burning gas. Heat first melts the candle wax, the turns it to gas, and finally ignites it.

- Temperature changes -

- Energy is released -

Signs of a Chemical Change.

A change in Color is another sign of a chemical change. For example, suppose you’ve spilled food coloring on a white T-shirt. How would you clean it? You would add some bleach to the wash to remove the color. The bleach takes away the color by chemically changing the molecules of the dye. When a drop of red-colored iodine dissolved in alcohol falls on starch, the reddish color turns black. Baking soda turns red cabbage juice to brown or yellow.

- Color changes-