Physical vs. Chemical Changes
Before we study the different types of chemical reactions it's important to understand what chemical reactions are and how to recognize them.
During the Periodic Table unit we studied matter and some of its physical and chemical properties. Now it's time to talk about the changes matter undergoes. Not all changes are chemical so before we look at chemical reactions in more detail let's make sure we know when we are actually seeing a chemical change or reaction.
Matter can change state, temperature, size, shape and even color without changing chemically. When a substance keeps its core chemical make up after a change that change is NOT a chemical reaction, it's merely a physical change. No chemical bonds are broken or formed. Physical changes are very common and are happening around you all the time!
Examples of physical changes include:
Chemical Changes (Reactions)
When a substance changes chemically existing bonds are broken and new bonds are formed. The substance you have after the reaction is chemically different (new!) than the substance you started with.
Examples of chemical changes include:
Many textbooks tell you "chemical changes are not reversible while physical changes are," but that's not a great way to think about things. After all it's hard to untear a piece of paper and some chemical reactions are actually easy to reverse. Instead keep reminding yourself if nothing chemically new is formed it's a physical change, but if something chemically new is formed then it's a chemical change.
These resources will help you understand the basics physical vs. chemical changes:
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What are Chemical Reactions?
What makes it possible for just over 116 elements make up millions of different things in our world? Chemical reactions! Chemical reactions, simply put, are the ways in which elements and compounds combine and recombine. Millions of chemical reactions are happening around you right now! Breathing, digesting, oxidizing, fermenting; all are examples of every day chemical reactions where existing chemical bonds get broken and new bonds get formed.
Evidence of Chemical Changes (Reactions)
If you light a candle you’ve set of a chemical reaction but if that same candle melts in the summer sun it’s just a physical change of state. What’s the difference between a chemical reaction and a physical change? Chemical reactions always create products chemically unique from their reactants (the starting materials). In our example, the burning candle is actually using up the wax to fuel a combustion reaction while the melted candle never changes chemically, it only changes shape.
There are some clear indicators that a chemical reaction is taking place. These include:
Types of Chemical Reactions
3. Charge-balance the new compounds. K2SO4(aq) + AgNO3(aq) --> KNO3 + Ag2SO4
4. Check the solubility of each of the new compounds. While KNO3 is soluble (aq), Ag2SO4 forms a solid precipitate (s).
5. Now balance the equation and include all states of matter. K2SO4(aq) + 2AgNO3(aq) --> 2KNO3(aq) + Ag2SO4(s)
When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt. The H(+) cation of the acid combines with the OH(-) anion of the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt.
Redox is an abbreviation of reduction/oxidation reactions. This is exactly what happens in a redox reaction, one species is reduced and another is oxidized. Reduction involves a gain of electrons and oxidation involves a loss, so a redox reaction is one in which electrons are transferred between species. Reactions where something is "burnt" (burning means being oxidised) are examples of redox reactions, however, oxidation reactions also occur in solution, which is very useful and forms the basis of electrochemistry.