P-T Graph of Water: Understanding Density & Expansion

In summary, the phase diagram for water shows that the liquid phase is more dense than the solid phase due to the crystal structure of the solid. This is not the case for all substances, as shown by the example of carbon dioxide where higher pressure can result in a denser solid. Le Chatelier's principle explains how substances will try to resist changes in pressure, leading to the formation of a denser state. The freezing point on a phase diagram indicates the temperature at which a substance will transition from a liquid to a solid, and higher pressure can make it easier to form a solid.
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  • #2
It explains it in the link a bit further down.

Normally the solid/liquid phase line slopes positively to the right (as in the diagram for carbon dioxide above). However for other substances, notably water, the line slopes to the left as the diagram for water shows. This indicates that the liquid phase is more dense than the solid phase. This phenomenon is caused by the crystal structure of the solid phase. In the solid forms of water and some other substances, the molecules crystalize in a lattice with greater average space between molecules, thus resulting in a solid with a lower density than the liquid. Because of this phenomenon, one is able to melt ice simply by applying pressure and not by adding heat.
 
  • #3
So can I say at higher pressure what is more likely to form will be denser?
One thing I don't understand is let's say carbon dioxide, when we apply higher pressure, the freezing curve of CO2 slopes positively to the right , that means higher the pressure ,higher the freezing point, more difficult to form solid. Is this correct?
But I always think that we need high pressure so that we can easily change from gas to liquid then to solid.
 
  • #4
Not exactly. Try to think of it in terms of Le Chatelier's principle. Imagine I take a block of ice just bellow melting point. Then I compress it. In other words, I raise pressure. Le Chatelier's principle tells me that system will try to resist the change. In other words, it will try to do something to reduce pressure. For example, it can go into a denser state if one is available. You can also see what happens based on PV diagram. If I increase pressure, temperature just bellow freezing becomes just above freezing. Ice melts. Combined with above it tells me that substance found a denser state, which happens to be liquid.

Higher freezing point means solid will form at higher temperature. It will be easier to make solid CO2 at higher pressure. What's more important is that if you start with a block of frozen CO2 at just bellow the freezing point and compress it, increasing pressure, you stay bellow freezing point. On the other hand, if you start with liquid CO2 just above freezing point, compressing it will put you bellow freezing point, meaning that in this case the solid is denser.
 
  • #5
for your inquiry. The P-T graph of water shows the relationship between pressure and temperature at which water exists in different phases (solid, liquid, and gas). The density of a substance is directly related to its phase and temperature. From the graph, we can see that as water freezes from a liquid to a solid, the density increases, indicating that the solid phase is more compact and therefore more dense than the liquid phase. This is because the molecules in a solid are arranged in a more organized and tightly packed structure compared to the more spread out and less organized molecules in a liquid. Additionally, as water freezes, it expands due to the formation of hydrogen bonds between the molecules, which creates a lattice-like structure. This expansion is also reflected in the graph as the solid-liquid boundary line curves upwards, indicating an increase in volume as pressure and temperature decrease. Overall, the P-T graph of water provides valuable information about the density and expansion of water during phase transitions, which is important for understanding its physical properties and behavior.
 

What is a P-T graph of water?

A P-T graph of water is a graph that shows the relationship between pressure and temperature for water. It is used to understand the behavior of water under different conditions.

Why is understanding density and expansion important in relation to water?

Understanding density and expansion of water is important because it affects many natural processes, such as ocean currents and weather patterns. It also plays a crucial role in determining the behavior of substances that are dissolved in water.

How does the density of water change with temperature?

The density of water decreases as the temperature increases. This is because the molecules in water gain energy and move further apart, making the overall volume of water larger and decreasing its density.

What is the significance of the solid-liquid line on a P-T graph of water?

The solid-liquid line on a P-T graph of water represents the melting point of water. At this point, water transitions from a solid to a liquid state, and the density remains constant throughout this phase change.

How does the P-T graph of water differ from other substances?

The P-T graph of water is unique because it shows a decrease in density when the temperature increases, unlike most other substances which show an increase in density with increasing temperature. This is due to the hydrogen bonding between water molecules, which causes water to expand as it gains energy.

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