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[Slide 1] 
[Prof. Shijin Kozhumal] Hello everyone. Welcome to the lecture on states of matter. So we are going to look into fires from different types of fuels: solid fuels, liquid fuels, gaseous fuels. So before starting on that, let's have a quick look at different states of matter. 
[Slide 2]
So the first question: what is matter? So matter is anything that has mass and takes up space. So it should have a mass and it should have a volume associated with it or it takes up space. 
So different types of states of matter. First, solids. They have definite shape and volume. The molecules are closely packed so there is much stronger molecular forces. They are rigid materials most of the time. 
And liquids on the other hand have definite volume as do solids but no definite shape. The liquids assume the shape of their container. So the molecular forces are slightly weaker than they are in solids. So they deform much more easily. 
And gas. They have no definite shape or definite volume. So it lacks both definite shape and volume. It can have a different volume but not different shape. Gases don’t have definite shape or different volume. So definitely the molecules are weakly packed or the molecular forces are very small. They are separated from one another. So gases can be compressed. 
And the fourth one which we may not use that commonly is plasma. Plasma is similar to gas but it's highly ionized. So it doesn't have any shape or volume. They're highly ionized and how positive and negative charged ions. It happens either at extremely low pressures or at very high temperatures. So you know fire scenarios we almost never encounter extreme low pressures, so the plasma state is possible only at very high temperatures. Technically speaking, even in the normal fires there will be a few molecules that are ionized but we don't have to consider that as plasma state because that's insignificantly small. But if combustion happened at extremely high temperatures, intonations and shock waves, it may be there may be significant amount of plasma state. 
So graphically. So solids, the molecules are closely packed. So stronger molecular forces and they have definite shape and definite volume. In liquids they have definite volume but it takes the shape of the container. They have definite volume but not definite shape. In gases, there is no definite shape or definite volume. And plasma, very similar to gases but molecules are highly ionized. And as we move in this direction, the energy level is increasing. I hope this is something you already knew. 
[Slide 3]
And now there are names for all the physical processes that is possible between these phases. That's solid, liquid, gas, and plasma stage. As they are increasing in level of energy or enthalpy. So if a solid changes into liquid, we call it melting. And if the liquid changes back to solid, we call it freezing. These two words we use in our daily life. And if a liquid converts into a gas we call it vaporization. And if that gas converts back to the liquid it's called condensation. And solids directly converting into gas, so is a direct conversion without going through the liquid phase, we call that sublimation. So if it first melts and then vaporizes, we don't call it sublimation. Sublimation is a direct conversion of a solid to a gas without becoming a liquid. Sublimation. And the reverse of that process, that is a direct conversion of a gas to the solid, is called deposition. And a gas changing to plasma is called ionization. And a plasma state or ions lose their ionic state and coming back to the normal state is called recombination. A lot of new terms but if you remember this figure it's easy to understand. Melting, solid to liquid. Freezing, liquid to solid. And vaporization, liquid to gas. Condensation, gas to liquid. Solid changing into gas without going through that intermediate liquid state is called sublimation. And its reversal, gas changing into solid, is called deposition. And gas getting ionized into a plasma state is called ionization. And from a plasma state to a gaseous state change is called recombination. A lot of new words, but I hope it's not that hard to understand. 
[Slide 4]
Let's look at some of the definitions. All of these changes in states of matter are determined by two key properties, two key parameters: temperature and pressure. If we know the temperature and pressure of a material, we can decide what state it is in. So most substances can undergo through different states of matter from solid to liquid to gas and sometimes even plasma depending on what the temperature and pressure are. 
First of the important points for our analysis and for fire combustion is freezing or melting point. It is by definition the lowest temperature at which a substance can exist in liquid state. Both are the same point. The name changes based on which direction we are moving. If we are moving from a solid to liquid, we call it melting. And if we are moving from a liquid to a solid, we call it freezing. But either way it is the lowest temperature at which a substance can exist in liquid state. If we keep on reducing the temperature after this exact point, called freezing point, the liquid will freeze into a solid. Or if we are increasing the temperature at this exact same point, a solid will melt into a liquid. 
We know from water that these two points are commonly known to us. 
[Slide 5]
The next one is boiling point. Boiling point, the definition of boiling point, is the highest temperature at which the substance can exist in liquid state. Freezing or melting point is the lowest temperature at which a substance can exist in the liquid state. This is the highest point, highest temperature, at which the substance can exist in the liquid state. So if we heat a liquid beyond this temperature or at this temperature, it will transform into a gas. The temperature at which a substance changes from liquid to a gas, that's your water to steam, is called boiling point. And a few key things about boiling of most liquids is that the densities of most solids and liquids are very high compared to that of gases. Gases are very low density. So for example in the case of water. If liquid water, where it changes to a gaseous state, its volume increases almost 1600 times. Say for example if you take one liter of water and if you boil it at the same atmospheric pressure, you will get around 1600 liters of steam at the same pressure. So this causes a big threat. If we try to boil it inside a strongly closed container it will explode because of the increased pressure. There are certain benefits and certain challenges to it. Whenever there is water boiling, it expands. 
So sublimation, as we just mentioned, is a transition of a substance directly from solid into the gas phase without passing through the intermediate liquid phase. If it passes through the intermediate liquid state, like ice melting into water and water converted into steam, this is first melting and then vaporization. But if some material, if ice, is directly converted into steam without going to water that process is called sublimation. 
I hope this kind of reviews some of the fundamental ideas about different states of matter and changes in states of matter. In the next video we'll look at the properties. 
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