This one’s really simple: take two or more chemicals, and they combine to form at least one new chemical.
A compound is made of more than one chemical. When a compound decomposes, it decays into more than one component. This usually happens when a solid is heated and a gas is almost always produced.
3. Single Replacement
One chemical replaces another when two chemicals react. The metal which is more active displaces another element. The double arrow symbols means that the process can go both ways. A and BC can combine to form AB and C; also, AB and C can combine and form A and BC.
4. Double Replacement (Precipitation)
Two compounds combine and a chemical from each one replaces the other. The compounds are aqueous solutions, meaning that they were dissolved in water. A precipitate is formed: a solid which is insoluble in water. The resulting liquid is called the supernate.
5. Acid-Base Neutralization
As the name suggests, this is when an acid and base neutralize each other. In general, an acid produces hydrogen cations (or protons, ) and a base produces hydroxide anions (). Most acids have H in the front of the compound, and most bases have OH at the end of the compound. Baking soda and vinegar are, for example, an acid-base neutralization reaction.
X and Y are unknown compound that can be anything that bonds with H and OH. The only difference between the top and bottom equations is that the bottom one is accurate. (aq) and (l) stand for aqueous (mixed in water) and liquid.
6. Oxidation-Reduction (Redox)
This occurs when atom’s oxidation numbers change by either increasing or decreasing. Oxidation numbers are given to elements or compounds based on their net electric charge. For example, oxygen typically has a -2 charge. Therefore, the oxidation number of oxygen is -2. Water is electrically neutral, so the two hydrogen atoms need to have a charge which cancels the negative charge of oxygen. So, each hydrogen atom has a +1 charge to cancel out the -2 charge. I will go into greater detail on redox reactions (an extremely important type of reaction) in a future post.
Combustion is a type of redox reaction which releases heat (an exothermic reaction). Fuel reacts with an oxidizing agent. Fuels are usually hydrocarbons and the oxidizing agent is usually oxygen. The end products are carbon dioxide and water. Looking at an equation:
The number of water and carbon dioxide molecules would depend on the X amount of carbon and Y amount of hydrogen atoms.
Fun fact: Solids and liquids themselves don’t burn – only gases. When a flammable object (the fuel) receives significant heat (thermal energy), flammable gases will be released and combust. The heat released from the flame keeps the process going until there either isn’t enough fuel, heat, or oxygen left.
In short, something heats up a material until gases come off. Then, those gases are burned. The heat from the burning gases causes more gases to come out, repeating the process. Like everything in science, there’s a name for this process: pyrolysis. For more on this, click here.