Lesson 4 of 6 · Change the surroundings, change the phase
Gases and ices across environments
9 minute lesson · 3 checks along the way
Your goal for this lessonSeparate a change of phase from a reaction, and account for transfers between gas and ice reservoirs.
Follow a linked name to explore its archive card. “Back to your lesson” brings you to the same part of the reading.
Words with a dotted underline open a short definition right here in the lesson.
The word ice needs a chemical name
Ice often means frozen water in everyday conversation, but planetary science also discusses carbon dioxide ice and other frozen substances. A changes the physical state without necessarily changing the chemical species. Water vapor, liquid water, and ordinary water ice all contain H₂O. Splitting water into hydrogen and oxygen would be a chemical reaction, not another name for melting or evaporation.
The connections within a water and the interactions among neighboring molecules are different levels of organization. During an ordinary phase change, the arrangement and motion of molecules change while their chemical identities remain. This distinction prevents a common mistake: turning a picture of disappearing frost into a claim that its have vanished or transformed into another element. The material can instead move into a less visible gas reservoir.
Temperature is only part of the address
A describes equilibrium states for specified temperature and pressure. Familiar melting and boiling temperatures apply under particular conditions, rather than serving as universal rules for every atmosphere. For pure water below its pressure, a stable equilibrium liquid phase is unavailable. Mixtures add another complication: dissolved substances and the surrounding gas composition can change which phases occur and how quickly material moves between them.
Titan demonstrates why the substance must be named. At its cold surface, water ice can form solid terrain while methane and ethane participate in liquid lakes and weather. That does not make Titan’s liquids ordinary liquid water. On Mars, seasonal carbon dioxide frost can directly into gas as spring progresses. Interpreting either world requires both a chemical identity and local conditions; an Earthlike visual shape alone is insufficient.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Keep a ledger across the phase boundary
In an invented closed vessel, begin with 15 g of water ice and 5 g of water vapor. Suppose 3 g of ice sublimes, with no chemical reaction and no material entering or leaving. The final reservoirs contain 12 g of ice and 8 g of vapor. Their sum remains 20 g. The example stipulates the transfer rather than claiming these amounts are the equilibrium outcome at an unspecified temperature and volume.
The vapor mass has risen by 3/5 = 60%, while the ice mass has fallen by 3/15 = 20%. Neither percentage describes a change in the total water inventory, which is zero. The different denominators explain the unequal relative changes. Likewise, a large relative increase in a small planetary gas reservoir need not represent a similarly large change in the total amount of that chemical stored in the world.
Cassini’s May 21, 2011 view places hazy Titan in front of Saturn and its rings. Red, green, and blue filtered observations were combined into this natural-color image. The visible haze hides the surface where other measurements identified methane and ethane lakes.
Look closely
Can the visible appearance of this globe determine whether a surface lake contains water or methane?
See what to notice
The surface lakes are not visible through the haze in this image. Identifying their liquids requires other observations and chemical and environmental interpretation. A familiar globe or lake shape is insufficient to select a chemical species or a phase diagram.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
A transfer model still needs physical conditions
Our mass ledger does not determine the heat required, the final pressure, or the time elapsed. Those questions require additional information such as , temperature, available energy, vessel volume, and transfer rates. A phase diagram supplies equilibrium boundaries; it does not by itself tell you how quickly every surface reaches equilibrium. A sunlit patch and a shaded deposit can also experience different local conditions.
When examining frost or an atmospheric change, write down the proposed substance, its starting and ending reservoirs, and the evidence for the transfer. Then test alternatives: chemical production, transport from elsewhere, or a measurement bias could affect a gas record. Linking the observed change to the correct species and a conserved inventory makes the explanation stronger than simply saying that a world warmed and its ice disappeared.
A bigger gas reservoir need not mean more total water
Original mass-reservoir model with a stipulated 3 g transfer. Bar lengths encode mass rather than volume; temperature, pressure, rate, and energy requirements are not specified.
Open full-size diagram in a new tab: A bigger gas reservoir need not mean more total waterCheck 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.