The difference between leavened and unleavened bread is found in the word itself – leaven. Leaven is what is placed in bread dough to make the bread rise. It is placed in the dough and spreads throughout the entire lump.
We normally think of yeast as a leavening agent, but there are many such ingredients that can be used such as baking powder or baking soda.
A search on Wikipedia gave two lists of leavening agents. One was chemical and the other natural.
The list of chemical leavens contained baking soda, baking powder, horn salt (a.k.a. baker’s ammonia), potash, etc.
The list of natural leavens contained yeast, beer, yogurt and buttermilk.
The Jews were commanded to eat unleavened bread in the observation of the Passover and feast of unleavened bread (Exodus 12:1-15). In fact, no leavening agents were to be found anywhere in their houses during this time (Exodus 12:15).
Jesus and the apostles described sin as leaven. This is because just a little sin into our lives or into the church allows it to spread throughout our whole life or the whole church (Mark 8:15; 1 Corinthians 5:6-8).