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Gases

Gases hold huge amounts of energyand their molecules are spread out as much as possible. When compared to solids or liquids, those spread out gaseous systems can be compressedwith very little effort. Scientists and engineers use that physical trait all of the time. Combinations of increased pressure and decreased temperature force gases into containers that we use every day.

You might have compressed air in a spray bottle or feel the carbon dioxide rush out of a can of soda. Those are both examples of gas forced into a smaller space at greater pressure. As soon as the gas is introduced to an environment with a lower pressure, it rushes out of the container. The gas molecules move from an area of high pressure to one of low pressure.

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You might hear the term "vapor." Vapor and gas mean the same thing. The word vapor is used to describe gases that are usually liquids at room temperature. Good examples of these types of liquids include water (H₂O) and mercury (Hg). They get the vapor title when they are in a gaseous phase. You will probably hear the term “water vapor” which means water in a gas state. Compounds such as carbon dioxide (CO₂) are usually gases at room temperature. Scientists will rarely talk about carbon

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Gases are everywhere. You may have heard about the atmosphere. The atmosphere is an envelope of gases that surrounds the Earth. In solids, atoms and molecules are compact and close together. Liquidshave atoms that are spread out a little more. The molecules in gases are really spread out, full of energy, and constantly moving around in random ways.

What is another physical characteristic of gases? Gases can fill a container of any size or shape. It doesn't matter how big the container is. The molecules spread out to fill the whole space equally. Think about a balloon. No matter what shape you make the balloon, it will be evenly filled with the gas molecules. Even if you make a balloon animal, the molecules are spread equally throughout the entire shape.

Liquids can only fill the bottom of a container, while gases can fill it entirely. The shape of liquids is very dependent on gravity, while less dense gases are light enough to have a more freedom to move.

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The energy you can measure with a thermometer is really the average energy of all the molecules in the system. There are always a few molecules with a lot of energy and some with barely any energy at all. There is a variety, because the molecules in a liquid can move around. The molecules can bump into each other, and when they hit... Blam! A little bit of energy moves from one molecule to another. Since that energy is transferred, one molecule will have a little bit more and the other will have a little bit less. With trillions of molecules bouncing around, sometimes individual molecules gain enough energy to break free. They build up enough power to become a gas once they reach a specific energy level. In a word, when the molecule leaves, it has evaporated.

The rate of evaporation can also increase with a decrease in the gas pressure around a liquid. Molecules like to move from areas of higher pressure to lower pressure. The molecules are basically sucked into the surrounding area to even out the pressure. Once the vapor pressure of the system reaches a specific level, the rate of evaporation will slow down.

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Sometimes a liquid can be sitting in one place (maybe a puddle) and its molecules will become a gas. That's the process called evaporation. It can happen when liquids are cold or when they are warm. It happens more often with warmer liquids. You probably remember that when matter has a higher temperature, the molecules have a higher energy. When the energy in specific molecules reaches a certain level, they can have a phase change. Evaporation is all about the energy in individual molecules, not about the average energy of a system. The average energy can be low and the evaporation still continues.

You might be wondering how that can happen when the temperature is low. It turns out that all liquids can evaporate at room temperature and normal air pressure. Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor. Not all of the molecules in a liquid have the same energy. When you have a puddle of water (H₂O) on a windy day, the wind can cause an increased rate of evaporation even when it is cold out.

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ntermolecular forces are found in all substances. Some of the forces bring molecules together while others push them apart. Solids are locked together and you have to force them apart. Gases bounce everywhere and spread out. Many liquids want to stick together because of cohesive (sticky) forces that pull the molecules together.

When you place a drop of water on a piece of glass, you will see it stay together as a drop. Cohesive forces keep the drop from spreading out. Cohesive forces also keep water molecules together if there is a drip on your faucet. The water sticks together until it is too heavy. It drips when the weight of the water drop overcomes the cohesive forces holding it all together.

Evaporation occurs when individual liquid molecules gain enough energy to escape the system and become a gas. The extra energy allows individual molecules to overcome the intermolecular forces within the liquid.

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Another trait of liquids is that they are difficult to compress. When you compress something, you take a certain amount of material and force it into a smaller space or volume. You force the atoms closer together. Most solids are very difficult to compress while gases are easier. You can find compressed gases in SCUBA air tanks. Liquids are in the middle, but tend to be difficult to compress because the molecules are already close together. You probably can’t compress a liquid with your hands. It takes a lot of force.

Many shock absorbers found in cars and trucks have compressed liquids, such as oils, in sealed tubes. Without shocks, there would be a very rough ride for the driver and a lot of stress on the structure of the car. The shocks counter the extremes of the up and down motion by acting as a dampening device.

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Liquids are the second state of matter we will talk about. Solids are objects you can hold and maintain their shape. Gases are floating around you or trapped in bubbles. Liquids are found between the solid and gas states. Examples of liquids at room temperature include water (H₂O), blood, and even honey. If you have different types of molecules dissolved in a liquid, it is called a solution. Honey is a solution of sugar, water, and other molecules.

Liquids fill the shape of any container they are in. If you pour water in a cup, it will fill up the bottom of the cup first and then fill the rest. If you freeze that cup of water, the ice will be in the shape of the cup.

The top of a liquid will usually have a flat surface. That flat surface is the result of gravity pulling on the liquid molecules. Let’s go back to the cup for a moment. If you put an ice cube (solid) into the cup, it will sit there and not change shape. As the cube warms and melts, the liquid water will fill the bottom of the cup and have a flat surface on top.

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Solids can be made of many things. They can have pure elements or a variety of compounds inside. When you have a solid with more than one type of compound, it is called a mixture. Most rocks are mixtures of many different compounds. Concrete is a good example of a man-made solid mixture.

Granite is a mixture you might find when you hike around a national park. Granite is made of little pieces of quartz, mica, and other particles. Because all of the little pieces are spread through the rock in an uneven way, scientists call it a heterogeneous mixture. Heterogeneous mixtures have different concentrations of compounds in different areas of the mixture. For example, there might be a lot of quartz and very little feldspar in one part of the granite, but only a few inches away those amounts might flip.

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Chemical changes happen on a much smaller scale. While some experiments show obvious chemical changes, such as a color change, most chemical changes are not visible. The chemical change as hydrogen peroxide (H₂O₂) becomes water cannot be seen since both liquids are clear. However, behind the scenes, billions of chemical bonds are being created and destroyed. In this example, you may see bubbles of oxygen (O₂) gas. Those bubbles are evidence of the chemical changes.

Melting a sugar cube is a physical change because the substance is still sugar. Burning a sugar cube is a chemical change. Fire activates a chemical reaction between sugar and oxygen. The oxygen in the air reacts with the sugar and the chemical bonds are broken.

Iron (Fe) rusts when it is exposed to oxygen gas in the air. You can watch the process happen over a long period of time. The molecules change their structure as the iron is oxidized, eventually becoming iron oxide (Fe₂O₃). Rusty pipes in abandoned buildings are real world examples of the oxidation process.

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When you step on a can and crush it, you have forced a physical change. However, you only changed the shape of the can. It wasn't a change in the state of matter because the energy in the can did not change. Also, since this was a physical change, the molecules in the can are still the same molecules. No chemical bonds were created or broken.

When you melt an ice cube (H₂O), you have a physical change because you add energy. You added enough energy to create a phase change from solid to liquid. Physical actions, such as changing temperature or pressure, can cause physical changes. No chemical changes took place when you melted the ice. The water molecules are still water molecules.

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Chemical Changes Versus Physical Changes

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Chemical Changes Versus Physical Changes

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It is important to understand the difference between chemical and physical changes. Some changes are obvious, but there are some basic ideas you should know. Physical changes are usually about physical states of matter. Chemical changes happen on a molecularlevel when you have two or more molecules that interact. Chemical changes happen when atomic bonds are broken or created during chemical reactions.