The old becomes new
A chemical change is different from a physical change. Physical changes only modify the appearance of the matter but not the composition of the matter itself. A chemical change occurs when bonds between elements of a substance or substances are broken and then reassembled into a new substance.
Imagine a bin of 1000 LEGOS bricks. All different colors, shapes, and sizes are represented. You reach in and pick up an old model that was put together the last time you played. You pull apart those bricks and build a different model. No bricks were added or removed from the bin. You started with 1000 and ended with 1000 .
This is the same in chemical changes. We cannot create or destroy matter but it can be broken down and reassembled into new substances. We call this major idea the Law of Conservation of Matter.
There are SEVEN indicators that a CHEMICAL CHANGE has occurred. (in no particular order)
- fire/smoke
- light is given off
- heat is given off or absorbed
- there is an apparent change in mass
- a color change occurs
- a gas is produced
- a precipitate is formed
1. Fire /Smoke
When a substance burns it is being changed into something else. Carbon is left behind while Carbon dioxide and water vapor (among other gases) leave to the atmosphere. Fire causes the change while smoke is the evidence of fire. The particles of smoke used to be a part of something else. It is primarily Carbon but other elements can be found in the smoke particles as well.
2. Light is produced
When light is given off it show that energy is being released. Whether it is from fire, bioluminescence, nuclear radiation, or even a flashlight, the light is evidence that a chemical change is taking place.
3. Heat is given off or absorbed
Heat is always part of an energy transformation but sometimes a GREAT deal of heat is moved. Some chemicals can be mixed and heat is given off (exothermic) like the hand warmers you would use at a football game or campout. In other reactions, mixing the chemicals together draws heat in (endothermic) and makes the surrounding area cooler. The cold compress found in your first aid kit works this way.
4. An apparent change in mass
The match pictured below appears to have lost mass after being burned. Was some of the matter destroyed? Of course, not. That would go against the Law of Conservation of Matter. We can account for the “missing” matter mostly as Carbon dioxide and water vapor as the molecules of the wood were rearranged. 0.12 grams of the match are now in the air as other substances.
Sometimes, it appears as though matter has been created. Rusty nails have more mass than regular nails. Oxygen combines with Iron to make an Iron oxide which is heavier than the Iron by itself.
5. A color change occurs
6. A gas is produced (bubbles)
This is and Alka-seltzer tablet reacting with water. Carbon dioxide bubbles are released in this reaction. The unusual part of this gif is that it was recorded in the microgravity of space. Instead of dropping the tablets into a glass of water it is placed into a ball that is hovering in place. Cool stuff! Thanks NASA!!
Be careful not to confuse this with boiling. When we make pasta we put water on the stove and bring it to a boil. We do see bubbles but NOT because of a chemical reaction. This is simply a physical change because H2O liquid is becoming H2O in gas form.
7. A precipitate is formed
A precipitate is made when two liquids go through a chemical reaction and produce a solid. Lead iodide and Potassium nitrate react together to form a clear liquid and a YELLOW solid that does not dissolve into the liquid. That solid is called a precipitate. It will eventually settle to the bottom of the tube.
Watch: This episode of NOVA is all about chemical reactions! Learn about hot peppers, venom-inspired medicines, and Nitrogen filled compounds that go “boom”.
The FIVE basic types of chemical reactions
You’ve probably seen the Lego movie by this point. It’s fun to imagine a universe that is just a giant bin of building blocks that can be taken apart and built again in different ways. In our reality, the building blocks are particles called atoms that can bind together to form compounds.
When atoms form compounds or when those compounds are rearranged, it is called a chemical reaction. Antoine Lavoisier (as depicted in the PBS production of The Mystery of Matter) carefully measured the amount of matter BEFORE and AFTER a chemical reaction and found them to be the same. He said that “Nothing is lost, nothing is created, everything is transformed.” We call this fundamental idea the Law of Conservation of Matter. Matter cannot be created of destroyed but it can be rearranged into new substances.
While there are many types of chemical reactions, we will discuss FIVE of the most common types . They are Synthesis, Decomposition, Single Replacement, Double Replacement, and Combustion
A synthesis reaction occurs when simple substances are combined to form a more complex substance. (circles and triangles yields the compound between circle and triangle).
Let’s make some water!
Energy is released when a bit of activation energy (fire from a candle) is given to a Hydrogen balloon. The diatomic molecules (H2 and O2) are ripped apart and new bonds are formed. Less chemical energy is needed to make water so we see the excess energy as light, heat, mechanical (motion) , and sound energy.
Another common synthesis reaction demonstration is the burning of Magnesium. 
This is Magnesium metal. It is silver/grey in color and is quite malleable. It’s quite simple to rip the Magnesium ribbon with just our hands. When burned in the presence of Oxygen gas Magnesium oxide is formed. This white powder has completely different properties than Magnesium or Oxygen.
A decomposition reaction occurs when a complex substance is broken down into simpler substances. (The compound between circles and triangles has been broken into the elements circle and triangle)
Water is a very stable molecule but even it can go through decomposition with enough energy. In this demo we will add some salt to the water and run electricity through the beaker to release Hydrogen and Oxygen gas. 
We are using a power supply that adjusts to a lower voltage but a simple 9 Volt battery will do the same thing. Notice that there are TWO times more bubbles on the Hydrogen side than on the Oxygen side.
Elephant toothpaste is a rapid decomposition of Hydrogen peroxide (H2O2) into water and Oxygen gas. 
The Oxygen is released quickly because we use KI2 as a catalyst. A catalyst is a chemical that changes the rate of a reaction without changing the reaction itself.
This next decomposition is the breakdown of a carbohydrate name Sucrose (table sugar). A carbo-hydr-ate is a compound that is made of Carbon, Hydrogen, and Oxygen. If we use a strong acid like H2SO4 we can dehydrate the carbohydrate and see the Carbon that made up the sugar molecules. The water is released as a vapor along with Sulfur compounds that have a pungent odor.
BEWARE: this demonstration is for the outside only. The best way I can describe the smell is “angry brown sugar”. It has a sweetness to it UNTIL IT DOESN’T.
A single replacement reaction occurs when the bond of a compound is broken and an element within that compound has been switched out by a different element. (The compound triangles and circles has been broken by stars to yield a compound between triangles and stars which releases circles)
Silver (I) nitrate and Copper will undergo a single replacement reaction. The Copper makes a more stable compound with the nitrate ion so Silver is pushed out on its own. You can see the results of them switching places as the Silver collects on the copper wire “tree”. Every once in a while I gave it a good shake for two reasons. 1) it makes the Silver fall to the bottom like snow and 2) it allows more Copper to have surface area exposed to the Silver (I) nitrate.
A double replacement reaction occurs when two compounds have their bonds broken to have their chemical partners switched. (Triangle-circles and Hexagon-stars yield Triangle-stars and Hexagon-circles)
This is a Double replacement reaction because there are two compounds that end up with different partners at the end of the reaction. 
Combustion is the act of burning a hydrocarbon (a substance made up of Hydrogen and Carbon). A combustion reaction will always produce Carbon dioxide and Water though other compounds may be made as well.