3. Endothermic/Exothermic, Heat, Energy

1.  Phase changes (sublimation)

2.  Graph

3.  Endothermic/Exothermic, heat, energy

4.  States of matter

5.  Chemical changes – indicators

6.  Chemical changes – process

7.  Conservation

8.  Solutions

9.  Physical changes

10.  Intensive/Extensive

11.  Density

1.  A phase change is a PHYSICAL change in which a substance moves from one of the following states to another:

1. Solid
2. Liquid
3. Gas

The processes that can occur are:

a.  Melting

b.  Freezing

c.  Boiling

d.  Evaporating

e.  Condensing

f.  Subliming

These processes can occur in either direction and are not permanent. They are dependent on temperature.

2.  Phase change graph indicates the temperature of a heating or cooling substance over time:

When a substance is heated or cooled, it changes phases. These changes occur at the freezing/melting point and the boiling/condensation point. For a substance to change phases, energy is either absorbed or released by the substance. Because of this, the temperature will remain constant as the substance changes phase. Melting and boiling require the addition of heat to occur. Thus, these changes are ENDOthermic processes. Freezing and condensing, on the other hand, require energy to leave the substance. Thus, these changes are EXOthermic processes.

3.  During a chemical reaction, energy may be absorbed or released. This will cause the liquid or air in which the reaction occurs to feel warmer or cooler. If the chemical reaction absorbs energy from the liquid or air, the process is called ENDOTHERMIC. If the chemical reaction releases energy into the liquid or the air, the process is called EXOTHERMIC. Phase changes are not chemical reactions. They are physical changes. However, they also involve processes that require or release energy. Because you must ADD HEAT to cause a substance to melt or boil, these are ENDOthermic processes. Because freezing and condensing require heat to be removed FROM the substance, these processes are EXOthermic.

4.  There are three states of matter: solid, gas, and liquid. A solid contains atoms or molecules that are already compressed tightly together, so the solid is not compressible. Because the molecules are so close together, they are stuck in position in a regular pattern. Therefore, solids maintain their own shapes regardless of their container. Because they are compressed, the atoms or molecules cannot move freely and are less energetic. Solids are the most dense state of matter.

Liquids contain atoms or molecules that are not as close together, so they have some mobility and are thus more energetic than in solids. However, a liquid is still incompressible. Liquids do not maintain their own shape; rather, they take the shape of the container in which they are held. Liquids do not expand to fill all available space; instead, they flow and remain in the bottom of the container.

Gases contain atoms or molecules that are spaced far apart. They move freely and are thus highly energetic. Gases do not maintain any shape or form. They fill a container entirely and take advantage of all available space. Unlike solids and liquids, gases are very compressible.

5.  A chemical change, also known as a chemical reaction, involves transforming a substance into a completely different substance. The actual molecules break down and reform into something else. There are several indicators that a chemical change has occurred:

1. Emission of light
2. Change in temperature without an external heat/cooling source
3. New color
4. Formation of a precipitate (solid forms when two liquids are combined)
5. Change in odor
6. Formation of a gas (bubbles)

6.  A chemical change, also known as a chemical reaction, involves transforming a substance into a completely different substance. The actual molecules break down and reform into something else. Chemical changes are usually very difficult, if not impossible, to reverse. Some types of chemical reactions are:

1. Combustion (burning)
2. Decomposition (rotting meat, digestion)
3. Acid-Base reaction (forms water)
4. Oxidation (rust)

Two things that are often confused with chemical changes but are actually PHYSICAL CHANGES include dissolving (salt or sugar in water) and phase changes (melting, boiling, condensing, freezing).

7. There are two laws of conservation often used in chemistry. The first, the Law of Conservation of Mass (also called the Law of Conservation of Matter) states that mass or matter cannot be created or destroyed. There is a set amount of matter in the universe, and that amount remains constant. In chemical reactions, molecules are changed, but the matter within them is not destroyed; rather it is used to form a new substance. The matter within the new substance is not created from nothing. Thus, when a precipitate is “created” in a reaction, it is actually just transformed from the atoms in the original material.

Likewise, no material is lost in a reaction, even when a gas is formed. The total weight of the final products of a reaction is always the same as the total weight of the original reactants. In a reaction forming a gas, you may find that the weight of your remaining products is lower than your original mass. However, the escaped gas also had mass! This accounts for the “lost” weight.

The second law, the Law of Conservation of Energy states that energy cannot be created or destroyed. Like matter, there is a set, unchanging, amount of energy in the universe. In a chemical reaction, you may notice a change in temperature. The container in which a reaction took place may feel warm or cool. This may seem like energy was lost or gained during the reaction. However, like mass, the energy was simply transferred, not gained or lost. Chemical reactions are very energetic! Sometimes in order for a reaction to take place, energy will be absorbed from the air or liquid in which the reaction takes place, thus making the container feel cool. This is an ENDOthermic reaction. Other times, energy is already stored within the reactants, so when the reaction takes place, this stored energy is released. The surrounding air or liquid becomes warm as the energy that had been stored in the reactants is released. This is an EXOthermic reaction. Some of these are very spectacular (fireworks are an exothermic chemical change). Remember, even the sun does not produce energy. It simply releases the energy stored within it.

8. A solution is a homogeneous (uniform) mixture of solids, liquids, or gases. Most often, the solutions you will work with are called aqueous solutions (a liquid solution involving water as the solvent). In a solution, a solute (such as salt) is dissolved in a solvent (such as water). You should be unable to see any remaining solute (salt) without special equipment. Solutions do not scatter light. They are stable, and they cannot be separated through filtration. Liquid solutions usually CAN be separated through processes such as distillation. Solutions do not have to be liquids! Bronze, steel, and clear plastic are examples of solid solutions.

A colloid is a mixture in which the suspended material does not settle out. Examples include milk, butter, fog, ink, and paint. The particles in colloids are larger than those in solutions and are evenly distributed.

A suspension is a heterogeneous mixture in which a suspended material WILL settle out. For instance, if you shake oil and water, they will mix, but eventually the densities of the two substances will cause them to settle out (separate).

9.  A physical change is any change that occurs to a substance that does not actually alter the identity of the substance. In other words, the chemical properties of the substance remain the same. Examples include cutting, dyeing (color), dissolving, melting, and boiling. Physical changes can often, but not always, be reversed (for instance, cut hair can never be returned, but it is still hair).

10.  An intensive property is a physical property of a substance that can not be changed by adding to or decreasing the AMOUNT of the substance. For instance, color, density, melting point, malleability, and hardness are intensive properties.

An extensive property is dependent on the amount of a substance. For example, volume, weight, length, energy, and mass are extensive properties.

11. Density measures how compacted the molecules in a substance are. Different densities will cause materials to “settle” over time into different layers. Density is measured as mass over volume. Density is dependent on temperature, but it is not dependent on amount.