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Phases of Matter


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Fig. 5-2. The planets in our solar system (not to scale).

Earth is unique in the solar system because of the amount of water on its surface, and because on earth, all three phases of water (solid, liquid, and gas) are stable. Under current conditions, water cannot exist in all three states on other planets (Fig. 5-2).  Some planets are too hot, so water vaporizes (e.g. Venus, which has a surface temperature of 480˚C); others are too cold, so water remains in a frozen state (e.g. most water on mars is frozen due to it’s average surface temperature of -55˚C). Of all the planets in our solar system, only earth is positioned at just the right distance from the sun so that its global temperatures permit water to continuously evaporate, condense, freeze and melt under normal pressure conditions.

Solid, Liquid, and Gas
In solid matter, molecules are closely packed very closely together; the molecules can vibrate but they do not move freely. A solid has a definite shape and volume. In a liquid, molecules are also closely packed together, but there is enough space for them to move freely. A liquid has a definite volume, but it takes the shape of its container. In a gas, however, the molecules are in constant, random motion through largely empty space. A gas has no definite shape or volume; it takes the shape and volume of its container. See Figure 5-3 for a relative comparison of molecules in each state of matter.
Fig. 5-3.  Comparison of molecules in a solid, liquid, and gas.
When matter changes from one state (i.e. a solid) to another state (i.e. a liquid), we call that a change of state or a phase transition. For matter to change from a solid to a liquid, or a liquid to a gas, requires additional energy (often in the form of temperature, or heat) and results in increased molecular motion (see Fig. 5-3). 
While most substances, under normal pressure conditions, will melt from a solid to a liquid, and vaporize to a gas with increasing temperature (for instance water), Figure 5-4 shows that sometimes matter can go directly from a solid to a gas (bypassing the liquid state).  We call this sublimation.  Sublimation happens when a solid, such as dry ice (which is solid carbon dioxide (CO2)) changes directly from dry ice into CO2 gas (See Fig. 5-5A).  
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Fig. 5-5.  (A) Dry Ice sublimating. (B) A dry ice maker, the cylinder stores liquid carbon dioxide under pressure, the liquid is transferred to a dry ice maker to make solid carbon dioxide.

Influence of Pressure and Temperature
In figure 5-3, phase changes are brought on by an increase in temperature.  Pressure also plays a role in the state of a substance.  Dry ice is produced from liquid carbon dioxide (Fig. 5-5B). Dry ice sublimates at normal atmospheric pressure, or 1 atm, but very cold conditions (about -78.5˚C, Fig. 5-6). In order to get liquid dry ice, you need both additional pressure (at least 5.11 atm) and temperature (at least -56.4˚C, Fig. 5-6).
Fig. 5-6.  Carbon dioxide phase diagram.
Fig. 5-7.  Water phase diagram (M. Pt. = Melting point, B. Pt. = Boiling Point, T. Pt. = Triple Point, 100kPa = 1 atm).
Water can sublimate as well (see Fig. 5-7).  When we freeze dry food; we put frozen food into a reduced pressure chamber (also called a vacuum) and the water in the food sublimes from solid ice to a gas.  The food left behind does not have water inside (Figure 5-8).  When you are camping and want to carry less weight, you can buy freeze dried food, which is light, as it has no water, and rehydrate it when you reach your destination.  Freeze dried food stays fresh for a long time, so can also be used for emergency food supplies at home.  Water also sublimates at normal atmospheric pressure, but very slowly, which we will discuss later in this unit.
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Fig. 5-8.  (A) Freeze dried ice cream and (B) Beef stew


Composition of States of Matter
In comparison to water, other common compounds on earth, such as carbon dioxide (CO2), and common elements, such as hydrogen (H2), boil at very low temperatures (see Table 5-1), and therefore exist as a gas in most of earth’s environments. Other compounds, such as the simple sugar glucose (C6H12O6), melt at a very high temperature and exist as a solid in most of earth’s environments. The element mercury (Hg), boils at very high temperatures and exists as a liquid in most of earth’s environments. 
Table 5.1. Melting/freezing and boiling points of common elements and compounds at standard atmospheric pressure (1 atm).



Melting/freezing point

(solid to liquid) °C

Boiling point
(liquid to gas) °C

Hydrogen (H2)

A colorless gas, hydrogen is the most abundant element in the universe.



Oxygen (O2)

A colorless gas, oxygen is 21% of the earth’s atmosphere.





Pale, yellow-green gas that is used in bleaching and disinfectants.



Carbon dioxide (CO2)

A colorless gas, common in the atmosphere & used by plants to conduct photosynthesis. A greenhouse gas that traps the sun’s heat near the earth’s surface.



Mercury (Hg)


Silvery liquid, the only metal that is liquid at room temperature. Used in thermometers, barometers, and electric switches. Highly poisonous.



Water (H20)


The most common substance on Earth. All living things need water to survive.



Sodium (Na)

Soft, silvery-white metal, an essential element for organisms as it allows transmission of nerve impulses



Copper Sulfate


Bright blue crystalline salt



Sulfur (S)

Bright yellow solid, used in fertilizers



Glucose (C6H12O6)

Sugar, a white crystalline solid. A product of photosynthesis and the main source of energy for most living things



Aluminum (Al)


A lightweight, silver soft metal used for many purposes. The third most common element in the Earth’s crust.



Sodium Chloride


Table salt, a white crystalline solid, used as a condiment



Gold (Au)


A soft, dense, yellow metal highly valued for its non-reactive nature (does not tarnish or corrode) and ease with which it can be shaped. Commonly used in jewelry.



Copper (Cu)

Reddish-orange soft metal used as electrical conductor, in jewelry, and plumbing



Carbon (C)


Brittle black solid (coal) OR soft gray solid (graphite) OR transparent hard solid (diamond). Diamonds are the hardest known substance. All living things are based on carbon.





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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.