Science With Mr. Milstid

Just as all objects have their own set of unique physical properties, they also have chemical properties.
Chemical properties of matter describes matter’s “potential” to undergo some chemical change or reaction. Whereas physical properties are easily observable, chemical properties are only observable during a chemical reaction. 
For example, you can easily observe the length, mass, etc of a sheet of paper (its physical properties), but in order to observe its flammability (a chemical property), you must actually engage the paper in a reaction – in this case, burn it.

A very limited list of chemical properties includes:

• heat of combustion
• reactivity
• PH
• flammability

And, just as matter has chemical & physical properties, it also undergoes chemical and physical changes.

There are several differences between a physical and chemical change:

Physical Changes
Physical changes are changes of matter from one form to another, without a change in chemical properties.
Examples of physical changes include:

• Melting
• Boiling
• Change in color
• Tearing
• Bending
• Mixing/Dissolving

One important thing to remember when analyzing changes in matter is that, a physical change in a substance doesn’t change what the substance is.
For instance, if a piece of paper is cut up into small pieces it is still paper. This would be a physical change in the shape and size of the paper.
Another important idea to consider when analyzing changes in matter is this: the mass of the substance (usually) stays the same during a physical change. So for example, if you freeze 100g of water, you get 100g of ice. And if you boil 100g of water, you get 100g of steam. This means that if a change in matter results in a net change in the mass of the system, it most likely is not a physical change.

Chemical Changes
A chemical change occurs when one kind of matter changes into another kind of matter with different physical and chemical properties. In this type of change, a new substance is formed that is completely different than the original, and energy is either given off or absorbed.
A common example of this type of change is burning. When an object – say a piece of paper – is burned, it turns into smoke and ash, different types of matter that have entirely different properties than the original piece of paper: it is now no longer flammable and has entirely different physical characteristics than it began with.

Common signs of a chemical change include:

• a change in color
• the release of a gas
• the formation of a precipitate
  (a solid that will settle out of a liquid or gas)
• the absorption or release of heat

One nifty trick you can use to distinguish between the two types of changes is to determine is the change you are analyzing can be reversed. In general (though not always), physical changes can be reversed in some way, while chemical changes are incredibly difficult to reverse.

Energetic Chemical Changes
When chemical changes/reactions occur, new products are formed, and there is also a heat energy change which can often be detected as a temperature change.
This means the products of the reaction (what was made) have a different energy content than the original reactants (what was combined).
There are two main types of energetic chemical reactions:
- Endothermic reactions
- Exothermic reactions

If the new products contain less energy than the reactants, heat is released or  given out to the surroundings and the change is called exothermic.
The temperature of the system rises in an exothermic change.

Examples of exothermic reactions

• The burning or combustion of hydrocarbon fuels (e.g. gas).
• The burning of magnesium, reaction of magnesium with acids, or the reaction of sodium with water
• The neutralization of acids and alkalis (e.g. adding baking soda to water)
• Using hydrogen as a fuel in fuel cells

If the products contain more energy than the reactants, heat is taken in or absorbed from the surroundings and the change is called endothermic.

Examples of Endothermic Reactions

• the thermal decomposition of limestone (used in industrial applications)
• the cracking of oil fractions (used to create oil products)

Tags: Chemical Properties, Chemical Reaction, Endothermic Change, Exothermic Change, Physical Reaction
Posted in Matter & Properties of Matter, Physics & Physical Science