What chemical reactions are

A chemical reaction is a process where substances change into new substances. This happens when atoms in the starting materials, which we call the reactants, rearrange to form different molecules, which we call the products. These new molecules have properties that are different from the original ones, such as new colors, smells, or states of matter.

We can display chemical reactions in words with an arrow between the reactants and products:

Reactant(s) $\to$ product(s)

How chemical reactions occur at the atomic level

Chemical reactions involve changes in the bonds between atoms. Bonds are connections that hold atoms together in molecules. During a reaction, these bonds break and reform in new ways.

The process of chemical reactions:

1. Old bonds break - The connections between atoms in the reactants come apart, freeing the atoms to move.
2. New bonds form - The freed atoms connect in different combinations, creating new molecules.
3. New properties emerge - The rearranged atoms result in products with completely different characteristics from the reactants.

How mass is conserved during chemical reactions

In a chemical reaction, the total mass of the substances stays the same before and after the reaction. This is because atoms are neither created nor destroyed—they are just rearranged. Mass is conserved, meaning it remains unchanged, as long as the reaction happens in a closed system where nothing can enter or escape.

Key principles of mass conservation

• Same number of atoms - The total count of each type of atom is identical on both sides of the reaction.
• Closed system requirement - In a setup where no matter is added or removed, the overall mass before the reaction equals the mass after.
• Why it happens - Atoms have fixed masses, and since they only rearrange without changing in number, the total mass must stay constant.

This conservation explains why you can't end up with more or less material than you started with in a sealed container.

Signs that show a chemical reaction is occurring

Chemical reactions often produce noticeable changes that indicate something new is forming. These signs help identify when a reaction is happening, even if you can't see the atoms rearranging.

Common signs of a chemical reaction

• Gas or bubbles - A gas is produced, which might appear as bubbles in a liquid or as a visible release into the air.
• Color change - The substances involved change from one color to another, showing new molecules have formed.
• Precipitate formation - A solid forms suddenly in a liquid, often appearing cloudy or as particles settling out. A precipitate is this new solid that doesn't dissolve.
• Temperature change - The mixture gets hotter or colder, indicating energy is being released or absorbed.
• Odor change - A new smell appears, different from the original substances, due to the creation of molecules with distinct scents.

These signs can occur alone or together, helping to confirm a chemical change rather than just a physical one.

The difference between exothermic and endothermic reactions

Chemical reactions involve energy changes, which determine if the reaction releases or absorbs energy from the surroundings. Surroundings refer to everything around the reaction, like the air or container.

Exothermic reactions

Exothermic reactions release energy to the surroundings. This often makes the mixture feel warmer because heat is given off.

Characteristics of exothermic reactions:

• Energy flow - Energy moves out from the reaction to the environment.
• Common sign - A rise in temperature, which can sometimes be dramatic.

Endothermic reactions

Endothermic reactions take in energy from the surroundings. This usually makes the mixture feel cooler as heat is absorbed.

Characteristics of endothermic reactions:

• Energy flow - Energy is pulled in from the environment to the reaction.
• Common sign - A drop in temperature, which might make things feel cold.

Understanding these types helps explain why some reactions heat up while others cool down.