What is 'matter'?
'Matter' is everything that occupies space (has volume) and has mass. It makes up everything in the universe in some shape or form; it’s everything in the universe. It makes up our planet and the whole universe.
On Earth, all matter exists in one of three different states: solid, liquid or gas.
Do you know humans are made of all three main states of matter?
The state in which a substance exists at room temperature is called its standard state. For example, at room temperature water exists as a liquid. Some substances exist as gases at room temperature (oxygen and carbon dioxide), while others, like water, exist as liquids. Most metals exist as solids at room temperature. Mercury has the interesting properties of being both a metal and a liquid in its standard state.
Each of these states is made up of tiny particles. The state of matter depends on the number of particles they are made up of.
Solid
Something is usually described as a solid if it can hold its own shape and is hard to compress. For example, water in solid form is ice. In a solid, the molecules are closely packed together and they have a high density.
Liquid
A liquid like water can flow or run but it can’t be stretched or squeezed. In liquids, the molecules are particularly close together but not as close together as solid matter; molecules have the ability to move around and slide past each other. A liquid matter has no shape of its own, it will take on the shape of the container it is being held. Examples of liquid are water, milk, juice, petrol, lemonade, etc.
Gas
Gas can flow, expand, and can be squeezed. Water in gas form is steam. If it is in an unsealed container it escapes. In gases, the molecules are much more spread out than in solids or liquids, and they collide randomly with one another. A gas will fill any container, but if the container is not sealed, the gas will escape. Gas can be compressed much more easily than liquid or solid.
Changing states of matter
Matter can exist in a solid, liquid, or gaseous state, and the state a substance is in can be largely determined by its temperature. Each substance has a unique threshold temperature after which it changes its state. After that threshold temperature is crossed, the substance will change its phase, thus changing the state of the matter. Under conditions of constant pressure, the temperature is the primary determinant of a substance’s phase.
Depending on its temperature, matter can change state. Melting, freezing, boiling, evaporation, condensation, sublimation, and deposition are ways in which a material changes state.
At low temperatures, molecular motion decreases and substances have less internal energy. Molecules will settle into low-energy states relative to one another and move very little, which is characteristic of solid matter. As temperature increases, additional heat energy is applied to the constituent parts of a solid, which causes additional molecular motion. Molecules begin to push against one another and the overall volume of a substance increases. At this point, the matter has entered the liquid state. A gaseous state exists when molecules have absorbed so much heat energy due to increased temperatures that they are free to move around one another at high speeds.
If the pressure is constant, the state of a substance will depend entirely on the temperature it is exposed to. For this reason, ice melts if taken out of a freezer and water boils out of a pot if left at too high a temperature for too long. Temperature is merely a measurement of the amount of heat energy present in the surroundings. When a substance is placed in the surroundings of a different temperature, heat is exchanged between the substance and the surroundings, causing both to achieve an equilibrium temperature. So when an ice cube is exposed to heat, its water molecules absorb heat energy from the surrounding atmosphere and begin to move more energetically, causing the water ice to melt into liquid water.
Melting is the process of changing a solid into a liquid. When a solid is heated, the particles are given more energy and start to vibrate faster. At a certain temperature, the particles vibrate so much that their ordered structure breaks down. At this point, the solid melts into a liquid. The temperature at which this change from solid to liquid happens is called the melting point. Each solid has a set melting point at normal air pressure. At lower air pressure, such as up a mountain, the melting point lowers.
Evaporation is the process of changing a liquid into a gas. If you leave some water in a wide-mouth container, you will notice that some of the water will disappear after some time. Liquid water changes to a gas (water vapors) – this is evaporation. It occurs when a liquid turns into a gas far below its boiling point. There are always some particles in a liquid that have enough energy to break free from the rest to become a gas.
Condensation is the process of changing a gas into a liquid. For example, water vapors in the air cool down and change into tiny drops of liquid water (dew) on leaves and windows in the morning after a cold night. Cooler objects often absorb energy from hotter objects.
Freezing is the process of changing a liquid into a solid. It is the opposite of melting. For example, lava is liquid rock, which erupts through a volcano at temperatures as high as 1,500OC (2,732OF) through a volcano. However, the red-hot lava cools as it meets the Earth’s surface, and turns back into solid rock again.
Boiling – When a liquid is heated, the particles are given more energy. They start to move faster and farther apart. At a certain temperature, the particles break free of one another and the liquid turns to gas. This is the boiling point. The boiling point of a substance is always the same; it does not vary. For example, water boils when it reaches its boiling point of 100ºC (212ºF). This is the temperature at which water turns to steam. Steam is an invisible gas. When it reaches the lid it cools back to a liquid.
Sublimation is the conversion of a solid into a gas without it becoming liquid. The easiest example of sublimation might be dry ice. Dry ice is solid carbon dioxide (CO2). Amazingly, when you leave dry ice out in a room, it just turns into a gas without it becoming liquid. Have you ever heard of liquid carbon dioxide? It can be made, but not in normal situations. Coal is another example of a compound that will not melt at normal atmospheric pressures. It will sublimate at very high temperatures.
Deposition is the conversion of a gas to a solid. It occurs when a gas becomes a solid without going through the liquid state of matter. Closer to the poles one can see frost on winter mornings. Those little frost crystals on plants build up when water vapor from the air becomes solid on the leaves of plants.
Chemical Changes versus Physical Changes
It is important to understand the difference between chemical and physical changes. Physical changes are usually about the physical state of matter, and chemical changes happen when molecular bonds are broken or created during a chemical reaction. Chemical changes take place at a molecular level.
No Change to Molecules
Stretching a rubber band, filling the air in a balloon, or crushing a can, are all examples of physical changes. These are changes only in the shape of the items. There are no changes in the state of matter because the energy at the molecular level did not change. In a physical change, no change in molecules occurs, molecules are still the same with no new chemical bonds created or broken.
Similarly, melting ice cubes, boiling water or freezing liquid water are all physical changes by adding energy. Changes in phase or state of matter i.e. solid to liquid, liquid to gas, liquid to solid are all physical changes. Physical actions such as changing temperature or pressure can cause physical changes. For example, in melting ice or freezing liquid water no chemical change takes place, the water molecules are still water molecules.
Changing the molecules
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. When hydrogen peroxide changes to water, one may see bubbles of oxygen (O2) gas. Those bubbles are evidence of 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.
When iron is exposed to oxygen gas in the air, iron rusts. This process can be seen over a long period of time. The molecules change their structure as the iron is oxidized, eventually becoming iron oxide. Rusty pipes in abandoned buildings are real-world examples of the oxidation process.
Change can be reversible or irreversible
A reversible change is a change that can be changed back again. For example, if an ice cube is melted it becomes water but we can freeze it again to become an ice cube so it can return to its original state. Melting and heating are examples of reversible changes.
An irreversible change is a change that cannot be changed back again. For example, if a cake mixture is baked it becomes a cake and we cannot turn it back into a mixture. The change is irreversible because a chemical reaction has taken place. Burning or mixing a liquid with the bicarbonate of soda is an example of irreversible changes.
A quick snapshot of certain terms and related phase changes:
Fusion/melting – Solid to a liquid
Freezing – Liquid to a solid
Vaporization/boiling – Liquid to a gas
Condensation – Gas to a liquid
Sublimation – Solid to a gas
Deposition – Gas to a solid
