Which Statements Describe Phase Changes Check All That Apply

Matter tin can exist in ane of several different states, including a gas, liquid, or solid state. The corporeality of energy in molecules of matter determines the
land of thing.

• A
  gas
  is a state of matter in which atoms or molecules have enough energy to movement freely. The molecules come into contact with one another only when they randomly collide.
• A
  liquid
  is a country of matter in which atoms or molecules are constantly in contact but take enough energy to keep changing positions relative to one some other.
• A
  solid
  is a state of matter in which atoms or molecules do not take enough energy to move. They are constantly in contact and in fixed positions relative to one another.

The following are the changes of state:

• If heat is added to a substance, such every bit in melting, vaporization, and sublimation, the process is
  endothermic. In this case, rut is increasing the speed of the molecules causing them move faster (examples: solid to liquid; liquid to gas; solid to gas).
• If rut is removed from a substance, such as in freezing and condensation, then the process is
  exothermic. In this instance, heat is decreasing the speed of the molecules causing them motility slower (examples: liquid to solid; gas to liquid). These changes
  release heat
  to the surroundings.
• The amount of heat needed to modify a sample from solid to liquid would exist the same to opposite from liquid to solid. The but difference is the direction of oestrus transfer.

A phase change is a physical process in which a substance goes from one phase to another. Usually the change occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point of the substance. The melting point is the temperature at which the substance goes from a solid to a liquid (or from a liquid to a solid). The boiling point is the temperature at which a substance goes from a liquid to a gas (or from a gas to a liquid). The nature of the phase change depends on the direction of the heat transfer. Heat going
into
a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat
from
a substance changes a gas to a liquid or a liquid to a solid.

Two central points are worth emphasizing. Outset, at a substance’s melting point or boiling point, ii phases can exist simultaneously. Take h2o (H_iiO) as an case. On the Celsius scale, H₂O has a melting point of 0°C and a boiling point of 100°C. At 0°C, both the solid and liquid phases of H_twoO can coexist. However, if heat is added, some of the solid H₂O will cook and plow into liquid H_iiO. If heat is removed, the opposite happens: some of the liquid H₂O turns into solid H₂O. A similar process can occur at 100°C: adding heat increases the amount of gaseous H₂O, while removing oestrus increases the amount of liquid H₂O (Figure \(\PageIndex{1}\)).

2d, equally shown in Figure \(\PageIndex{1}\),
the temperature of a substance does not change
every bit the substance goes
from one phase to another. In other words, stage changes are isothermal (isothermal means “constant temperature”). Over again, consider H₂O every bit an case. Solid water (ice) tin exist at 0°C. If estrus is added to ice at 0°C, some of the solid changes stage to brand liquid, which is also at 0°C. Retrieve, the solid and liquid phases of H₂O can coexist at 0°C. Only subsequently all of the solid has melted into liquid does the addition of heat change the temperature of the substance.

For each phase change of a substance, there is a characteristic quantity of heat needed to perform the phase modify per gram (or per mole) of textile. The heat of fusion (ΔH
_fus) is the amount of rut per gram (or per mole) required for a stage alter that occurs at the melting point. The heat of vaporization (ΔH
_vap) is the amount of heat per gram (or per mole) required for a stage change that occurs at the boiling point. If yous know the total number of grams or moles of material, you lot tin can use the ΔH
_fus
or the ΔH
_vap
to determine the total heat being transferred for melting or solidification using these expressions:

\[\text{heat} = n \times ΔH_{fus} \label{Eq1a} \]

wher
e \(n\) is thursday
due east number of moles

and \(ΔH_{fus}\) is expressed in energy/mole

or

\[\text{rut} = chiliad \times ΔH_{fus} \label{Eq1b} \]

where

\(chiliad\) is

the mass in grams

and \(ΔH_{fus}\) is expressed in free energy/gram.

For the boiling or condensation, use these expressions:

\[\text{oestrus} = n \times ΔH_{vap} \label{Eq2a} \]

wher
e \(n\) is

the number of moles)

and \(ΔH_{vap}\) is expressed in energy/mole

or

\[\text{estrus} = one thousand \times ΔH_{vap} \label{Eq2b} \]

wh
ere \(m\) i
s the mass in grams

and \(ΔH_{vap}\) is expressed in energy/gram.

Call back that a stage change depends on the management of the heat transfer. If oestrus transfers in, solids get liquids, and liquids get solids at the melting and boiling points, respectively. If heat transfers out, liquids solidify, and gases condense into liquids. At these points, there are no changes in temperature as reflected in the above equations.

Tabular array \(\PageIndex{1}\) lists the heats of fusion and vaporization for some common substances. Note the units on these quantities; when you use these values in trouble solving, brand certain that the other variables in your calculation are expressed in units consistent with the units in the specific heats or the heats of fusion and vaporization.