Physical and Chemical changes

Physical Changes
How would you describe water? If you think about water in a stream, you might say that it is a cool, clear liquid. If you think about water as ice, you might describe it as a cold solid. How would you describe the change from ice to water? As ice melts, its state of matter and shape change. You might recall that physical state and shape are physical properties. A physical property is a characteristic of matter that can be observed or measured without changing the identity of the matter. Additional physical properties include size, mass, volume, magnetism, solubility, density, boiling point, and melting point. 

When ice melts, its physical properties change, but its composition stays the same. That's why melting is an example of a physical change. A physical change is a change in the size, shape, form, or state of matter that does not change the matter’s identity. Matter can experience different physical changes including changing shape and size, dissolving, and changing physical state.

Change in Shape and Size
Imagine you are folding clothes. When you fold clothes, their shape and size change, but they are still clothes. Because the identity of the matter did not change, these are physical changes.
Often when matter changes shape or size, the physical change is reversible. However, some changes in shape or size cannot be reversed. Imagine a glass that breaks into small pieces. The glass cannot be put back together, so this physical change is not reversible.

Dissolving

Have you been to an aquarium that has ocean wildlife? To create the correct environment for these organisms, salts are added to freshwater. Can you see the salt in the water? Probably not, but it is still there. It is not visible because it is dissolved in the water. To dissolve means to form a solution by mixing evenly. Because the identities of the substances—water and salt—are not changed, dissolving is a physical change. Like many physical changes, dissolving is usually easy to reverse. If you boil salt water, the liquid water will change to water vapor, leaving the salt behind. You once again can observe the salt because the particles that make up the substances do not change identity during a physical change.

Change in State
Recall that matter can exist in different states, and it can change from one state to another. Matter is made up of particles that are constantly moving. These particles can be atoms or molecules depending on the substance. In a solid, the particles of the substance typically move slowly and are close together. The particles in a liquid of the same substance move faster and are farther apart than in a solid. The particles in a gas of the same substance move faster and are farther apart than in a liquid. 

When matter changes state, the spaces between the particles and their motion change. However, the identity of the particles does not change. That is why a change in state is a physical change. 

A change in state is a reversible physical change. For example, when a liquid freezes into a solid, the removal of thermal energy causes the particles of the substance to slow down and move closer together. If thermal energy is added to the solid, the particles will speed up and move apart. The substance melts back into a liquid. Freezing and melting are reverse processes that occur at the same temperature. 

Boiling and condensation are also reverse processes. A liquid boils when it becomes a gas, and a gas condenses when it becomes a liquid. As with other state changes, only the spacing and the speed of the particles change when energy is added or removed. The identity of the substance remains the same.

Chemical Changes
Think back to the Quick Launch Making Changes and the different ways a piece of paper could be changed. Whether you folded the paper, crumpled the paper, or ripped it into two pieces, it was still paper. These changes were all physical changes because even though the size or shape changed, the paper maintained its identity. Now imagine that the paper is lit on fire. The paper will become dark brown and then transform into ash and smoke. Is the paper still paper? No, because the paper experienced a chemical change. A chemical change is a change in matter in which the substances that make up the matter change into other substances with different chemical and physical properties. 

Sometimes it can be difficult to distinguish between a physical change and a chemical change. When you put liquid water in a freezer, it changes to solid ice. When you pour brownie batter into a pan and bake it, the liquid batter changes to a solid too. In both  cases, a liquid changed to a solid. What makes these changes in matter different?

Evidence of Chemical Changes
The main difference between a physical change and a chemical change is that in a chemical change new substances form. These new substances have different physical and chemical properties than the original substances. For example, when bread dough bakes, a chemical change occurs. Many of the substances in the baked bread are different from the substances in the dough. As a result, the baked bread is a different color and has a different
smell than the dough.

Signs of a Chemical Change
There are four pieces of evidence that indicate a chemical change may have occurred. They include color change, production of a precipitate, formation of a gas, and a change in energy.

Color Change
During a chemical change, new substances can form that cause a color change. For example, the leaves of deciduous trees change color when chlorophyll breaks down. When the chlorophyll changes to a new substance, the characteristic green of the leaves disappears. This allows the orange, yellow, and red pigments of the leaves to become visible.

Production of a Precipitate
A precipitate is a solid that sometimes forms when two liquids combine. This solid is not soluble, or does not dissolve, so it settles to the bottom of the container. Since the original liquids contained substances that were soluble, and the precipitate isn't soluble, it has different properties. That means it must be a new substance.

Formation of a Gas
The formation of a gas can be observed when bubbles form after two or more substances are mixed. When yeast is added to water and sugar, bubbles of carbon dioxide form. Another indication that a gas has formed is an odor. When an apple rots, it produces a smell that is different from the smell of the original apple.

Change in Energy
A change in energy is often perceived as a change in temperature. In many chemical changes, the resulting mixture gets warmer or cooler. However, some chemical changes give off
light or sound, which are also forms of energy. When a change is accompanied by light, sound, or a temperature change, it's likely that a chemical change has occurred.