Common Physical ChangesPhysical PropertiesCommon ubraintv-jp.comical Changes

The difference between a physical reaction and a ubraintv-jp.comical reaction is composition. In a ubraintv-jp.comical reaction, there is a change in the composition of the substances in question; in a physical change there is a difference in the appearance, smell, or simple display of a sample of matter without a change in composition. Although we call them physical "reactions," no reaction is actually occurring. In order for a reaction to take place, there must be a change in the elemental composition of the substance in question. Thus, we shall simply refer to physical "reactions" as physical changes from now on.


Introduction

Physical changes are limited to changes that result in a difference in display without changing the composition. Some common changes (but not limited to) are:

Texture Color Temperature Shape Change of State (Boiling Point and Melting Point are significant factors in determining this change.)

Physical properties include many other aspects of a substance. The following are (but not limited to) physical properties.

Luster Malleability Ability to be drawn into a thin wire Density Viscosity Solubility Mass Volume

Any change in these physical properties is referred to as a physical change. For further information, please refer to Properties of Matter.

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ubraintv-jp.comical changes, on the other hand, are quite different. A ubraintv-jp.comical change occurs when the substance"s composition is changed. When bonds are broken and new ones are formed a ubraintv-jp.comical change occurs. The following are indicators of ubraintv-jp.comical changes:

Change in Temperature Change in Color Noticeable Odor (after reaction has begun) Formation of a Precipitate Formation of Bubbles

Note: When two or more reactants are mixed and a change in temperature, color, etc. is noticed, a ubraintv-jp.comical reaction is probably occurring. These are not definite indicators; a ubraintv-jp.comical reaction may not be occurring. A change in color is not always a ubraintv-jp.comical change. If one were to change the color of a substance in a non-ubraintv-jp.comical reaction scenario, such as painting a car, the change is physical and not ubraintv-jp.comical. This is because the composition of the car has not changed. Proceed with caution.


Common Physical Changes


Texture

The texture of a substance can differ with a physical change. For example, if a piece of wood was sanded, waxed, and polished, it would have a very different texture than it initially had as a rough piece of wood.

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Robotic arm applying paint on car parts. Image use with permission (CC BY-SA 4.0l RoboGuru).


Temperature

Although we cannot see temperature change, unless if a change of state is occurring, it is a physical change.

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Change of State

The change of state is likewise a physical change. In this scenario, one can observe a number of physical properties changing, such as viscosity and shape. As ice turns into water, it does not retain a solid shape and now becomes a viscous fluid. The physical "reaction" for the change of ice into liquid water is:

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Malleability

Malleability is also a quality of metals. Metals are said to be malleable. This means that the metals can deform under an amount of stress. For example, if you can hit a metal with a mallet and it deforms, it is malleable. Also, a paperclip can be shaped with bare hands.

Bent Paperclip

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Density

The density of an object is its mass divided by its volume (d=m/v). A substance will have a higher density if it has more mass in a fixed amount of volume. For example, take a ball of metal, roughly the size of a baseball, compressed from raw metal. Compare this to a baseball made of paper. The baseball made of metal has a much greater weight to it in the same amount of volume. Therefore the baseball made out of metal has a much higher density. The density of an object will also determine whether it will sink or float in a particular ubraintv-jp.comical. Water for example has a density of 1g/cm3. Any substance with a density lower than that will float, while any substance with a density above that will sink.

Oil Sinking in a Glass of Water

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Viscosity of Fluids

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Figure 1.6.21.6.2: Viscosity demonstration. The fluid on the left has a lower viscosity than the fluid on the right. (CC SA-BY 4.0; Synapticrelay).

Change in Temperature

A change in temperature is characteristic of a ubraintv-jp.comical change. During an experiment, one could dip a thermometer into a beaker or Erlenmeyer Flask to verify a temperature change. If temperature increases, as it does in most reactions, a ubraintv-jp.comical change is likely to be occurring. This is different from the physical temperature change. During a physical temperature change, one substance, such as water is being heated. However, in this case, one compound is mixed in with another, and these reactants produce a product. When the reactants are mixed, the temperature change caused by the reaction is an indicator of a ubraintv-jp.comical change.

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Figure: Violent Reaction (Fireworks) with heat as a product

As an example of a exothermic reaction, if \(Fe_2O_3\) is mixed with Al and ignighted (often with burning Mg), then the thermite reaciton is initiated

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This reaction generates heat as a product and is (very) exothermic.

However, physical changes can be exothermic or endothermic. The melting of an ice cube, which is endothermic, is a change in a physical property and not composition. Thus, it is a physical change.

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Change in Color

A change in color is also another characteristic of a ubraintv-jp.comical reaction taking place. For example, if one were to observe the rusting of metal over time, one would realized that the metal has changed color and turned orange. This change in color is evidence of a ubraintv-jp.comical reaction. However, one must be careful; sometimes a change in color is simply the mixing of two colors, but no real change in the composition of the substances in question.

Metal Rusting

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Formation of a Precipitate

The formation of a precipitate may be one of the most common signs of a ubraintv-jp.comical reaction taking place. A precipitate is defined to be a solid that forms inside of a solution or another solid. Precipitates should not be confused with suspensions, which are solutions that are homogeneous fluids with particles floating about in them. For instance, when a soluble carbonate reacts with Barium, a Barium Carbonate precipitate can be observed.