Particle Model of Matter

A gas has no fixed shape and no fixed volume — it spreads out to completely fill whatever container it is put in. A solid, in contrast, has both a fixed shape and a fixed volume, which do not change unless a force is applied to it.

In a solid, the particles are already packed very closely together, touching their neighbours, with almost no empty space between them. Since there is very little room left for the particles to be pushed any closer together, solids strongly resist compression.

Benefit: Because ice floats and forms an insulating layer on the surface of lakes and rivers, the liquid water underneath stays several degrees warmer than the freezing air above, allowing fish and other aquatic organisms to survive through winter. If ice sank instead, bodies of water could freeze from the bottom up, likely killing most aquatic life.

Drawback: Because water expands as it freezes (its particles spread into a less dense arrangement), water trapped inside pipes, engine blocks, or cracks in rock can burst them when it freezes, causing damage to buildings, plumbing and roads (frost damage) — a costly problem in cold climates.

Benefit: Compressing a gas squeezes its particles much closer together, allowing a far larger mass of gas (measured at normal pressure) to be stored in a small, portable cylinder than could otherwise fit. This makes it possible for a diver to carry hours of breathable air, or a camper to carry enough fuel gas in a small canister.

Drawback: Gas cylinders are under very high pressure, so if they are damaged, punctured, or heated too much (increasing the pressure further as the particles move faster and collide more forcefully with the container walls), they can rupture or explode violently. This poses a serious safety risk, which is why compressed gas cylinders must be stored, transported and used carefully (e.g. away from heat, secured upright).

Benefit: Cooling the gas removes enough particle kinetic energy for the forces of attraction between particles to pull them from being widely spread apart (as a gas) into a closely packed liquid arrangement, shrinking its volume by around $600$ times. This makes it possible to transport a genuinely useful amount of energy by ship to places with no pipeline, greatly increasing global access to natural gas as a fuel.

Concern: The cooling process itself uses a large amount of energy, and the specialised insulated tankers and cooling/re-heating plants are expensive to build and run, meaning LNG has a bigger carbon footprint than piped gas before it is even burned. In addition, if a tanker or storage tank were damaged, the very cold liquid would rapidly turn back into a large volume of flammable gas, which is a serious safety hazard that must be carefully managed with strict safety regulations.

This is called sublimation. Example: solid carbon dioxide ("dry ice"), or iodine crystals, both turn directly into a gas at room temperature and pressure without passing through a liquid stage.

The freezing point of pure water is also $0\,^\circ\text{C}$. For a pure substance, the melting point and freezing point are always the same temperature — melting (solid $\to$ liquid) and freezing (liquid $\to$ solid) are simply the reverse of each other, occurring at the one temperature where solid and liquid can exist together in balance.

While the water is changing state (boiling), all of the extra heat energy supplied is being used to give the particles enough energy to completely overcome the forces of attraction holding them together as a liquid, rather than to increase their average kinetic energy any further. Since temperature is a measure of average particle kinetic energy, and that is not increasing during the change of state, the temperature stays constant until all of the liquid has turned into gas.

Benefit: Because evaporation absorbs a large amount of heat energy from its surroundings as the refrigerant particles use that energy to overcome the forces holding them together as a liquid, this cycle can continuously remove heat from a small enclosed space (a fridge) or a room (air conditioning). This keeps food safe from bacterial growth for much longer, and makes hot climates safe and comfortable to live and work in.

Drawback: Running the compressor needed to keep this cycle going uses a significant amount of electricity, contributing to a building's energy consumption and carbon footprint. In addition, some refrigerant gases (such as older CFCs, and some HFCs still used today) are powerful greenhouse gases, or damage the ozone layer, if they leak into the atmosphere — which is why safer refrigerants and proper disposal or recycling of old fridges are now important.

Benefit: Melting down existing metal to reuse it uses far less energy than extracting and refining brand-new metal from ore (for aluminium, recycling can use around $5\%$ of the energy of producing new metal). It also reduces the amount of mining needed, conserving natural resources and reducing the habitat destruction and pollution associated with mining.

Drawback: Melting metal (heating it past its melting point) still requires a large amount of energy and typically involves high-temperature furnaces, which can release significant emissions if the electricity or fuel used comes from burning fossil fuels. Collecting, sorting, and transporting scrap metal to recycling facilities also uses energy and resources.

Benefit: This process provides a reliable source of clean, salt-free drinking water in regions with very little natural fresh water, since evaporation naturally separates water particles from the dissolved salt (only the water turns into vapour, not the salt), and condensing that vapour gives pure water. This allows large, dry coastal populations to have a stable water supply.

Concern: Heating a huge volume of seawater until it evaporates requires an enormous amount of energy, making thermal desalination expensive and, if powered by fossil fuels, a significant source of carbon emissions. The leftover concentrated salty water (brine) also has to be disposed of, and pumping it back into the sea in large quantities can harm marine ecosystems near the outflow point by making the local water far saltier and warmer than normal.

Diffusion is the net (overall) movement of particles from a region of higher concentration to a region of lower concentration, caused by the particles' own random motion, continuing until the particles are evenly (uniformly) spread out.

Diffusion happens easily in gases and liquids, because their particles are free to move around and change position. In a solid, particles are held in fixed positions by strong forces of attraction and can only vibrate on the spot — they cannot move from place to place to spread out, so diffusion essentially does not happen in solids.

Gas particles (perfume vapour) are constantly moving randomly and quickly in all directions. Through this random motion, particles naturally spread out from where they are concentrated (near the bottle) into the surrounding air where there are fewer of them, eventually reaching all parts of the room. This is diffusion, and it happens on its own, without needing to be stirred.

In a gas, particles are much further apart, move much faster, and collide with each other far less often, so they can travel large distances quickly and freely. In a liquid, particles are packed close together and constantly collide with their close neighbours, which slows down how quickly they can spread through the liquid — so diffusion in a gas is much faster than in a liquid.

Brownian motion is the continuous, random, jittery movement of small particles (such as pollen grains or smoke particles) that can be observed under a microscope, caused by these particles being constantly and unevenly bombarded by fast-moving, invisibly small particles (air or water molecules) around them. It provides strong evidence that gas and liquid particles are in constant, random motion, even though the individual gas or liquid particles themselves are far too small to see directly.

Benefit: By reducing the concentration of oxygen inside the sealed packet, the concentration gradient driving oxygen diffusion into the food (and supporting bacterial growth and browning reactions) is much smaller, so food stays fresher for significantly longer. This reduces food waste for both shops and households, and allows fresh food to be transported further or stored longer before it needs to be eaten.

Drawback: This packaging typically uses more plastic material (to make an airtight seal that stops outside air diffusing back in) than simpler packaging, adding to plastic waste. The mixed layers of different plastics used for a good seal can also be difficult or impossible to recycle, so the environmental benefit of less food waste has to be weighed against the extra packaging waste produced.

Benefit: Because diffusion spreads pollutant gases from a smokestack outward and upward through a very large volume of air, releasing them from a tall chimney means the gas is diluted (spread out and mixed with far more air) by the time it reaches ground level near people, reducing the concentration people actually breathe in near the source. This predictable spreading behaviour also lets planners keep new housing or schools a safe distance from major pollution sources.

Drawback: Diffusion means pollutant gases inevitably spread beyond the immediate area of the factory or road, so people and ecosystems far from the original source can still be affected over time as the gases gradually diffuse and get carried by wind over long distances. Pollution cannot simply be "contained" at its source, which is why controlling emissions at the source, rather than just relying on dilution and diffusion, is still necessary.

Benefit: Because these gases diffuse rapidly from the lungs into the blood and reach the brain, they allow doctors to safely and reliably keep a patient unconscious and free of pain during long or complex operations that would otherwise be impossible to carry out. This diffusion-based delivery method is fast-acting and its effects can also be reversed relatively quickly once the gas supply stops and it diffuses back out of the body, making it a controllable and essential tool in modern medicine, saving countless lives.

Concern: Because these gases inevitably diffuse away from the patient (some remains in exhaled breath and some escapes from equipment), and hospitals worldwide use large volumes of them every day, gases like nitrous oxide that leak or vent into the atmosphere contribute a meaningful amount to global greenhouse gas emissions and climate change, given how much more strongly they trap heat than carbon dioxide. Hospitals are increasingly investing in gas-capture and recycling systems to reduce how much anaesthetic gas is released into the atmosphere, balancing patient care against environmental impact.