Vapours

A saturated vapour is a gas that's in equilibrium with its liquid. This means the rate at which liquid molecules escape equals the rate at which gas molecules rejoin the liquid. Think of boiling water in a pot—the steam above it is saturated because water keeps evaporating and condensing simultaneously.

An unsaturated vapour, however, hasn't reached this balance yet. More molecules are still escaping from the liquid than returning to it. On a humid Lagos afternoon, the water vapour in the air is usually unsaturated because water continues evaporating from surfaces.

The key difference? Saturated vapours can condense easily if cooled slightly, while unsaturated vapours need more cooling before condensation begins. Temperature and pressure matter greatly—increase either and unsaturated vapour becomes saturated.

An unsaturated vapour is a gas formed when a liquid evaporates, but it hasn't reached its maximum capacity yet. Think of it like a room that can hold many people—when only a few are inside, the room isn't full. Similarly, unsaturated vapour can still accept more vapour molecules without condensing back to liquid.

In Nigeria's harmattan season, the air becomes very dry because the moisture in the atmosphere is unsaturated. The water vapour present hasn't reached the saturation point, so more evaporation continues to happen from water bodies and soil. This is why clothes dry so quickly during harmattan.

The key difference from saturated vapour is that unsaturated vapour won't condense even if cooled slightly. It only condenses when it becomes saturated first, meaning it has reached maximum vapour density at that temperature.

When a liquid evaporates in a closed container, vapour molecules escape upward while some condense back into liquid. Eventually, evaporation and condensation happen at equal rates, creating equilibrium. At this point, the vapour is saturated and exerts a pressure called saturated vapour pressure (SVP).

SVP depends only on temperature, not on the volume of the container or the amount of liquid present. When you heat water in a closed pot, more molecules escape as vapour, increasing the pressure until a new equilibrium forms at the higher temperature. Think of boiling water in a pressure cooker in a Nigerian kitchen—as temperature rises, vapour pressure increases dramatically, which is why the cooker hisses and builds pressure.

Understanding SVP helps explain why clothes dry faster on hot days and why water boils at lower temperatures at higher altitudes where atmospheric pressure is lower.

The boiling point is the temperature at which a liquid changes into a gas or vapour. At this temperature, the heat energy is strong enough to break apart the forces holding liquid molecules together, allowing them to escape as vapour throughout the liquid. This happens when the vapour pressure of the liquid equals atmospheric pressure pushing down on it.

Think of boiling water in your mother's kitchen at home. At 100°C under normal atmospheric pressure, water molecules gain enough energy to transform into steam that rises from the pot. Different liquids have different boiling points because their molecular forces vary. For example, alcohol boils at around 78°C, which is lower than water because its molecules are held together less strongly.

The boiling point depends on atmospheric pressure too. At higher altitudes in Jos or Abuja where air pressure is lower, water boils at temperatures below 100°C. This matters for cooking times in different regions.

Saturated vapour pressure (SVP) is the maximum pressure a vapour exerts when in equilibrium with its liquid at a fixed temperature. To determine SVP using a barometer tube, you invert a mercury-filled tube over a mercury surface in a dish. The mercury drops, leaving a space above it. If you introduce vapour into this space, the vapour exerts pressure, pushing the mercury column down further. The difference between the original mercury height and the new height equals the SVP.

Think of it like this: on a hot Nigerian afternoon, water evaporates from a pan. If you trapped that water vapour in a confined space above mercury, it would push down on the mercury just as atmospheric pressure does, but by a smaller amount because vapour molecules move more freely than atmospheric gas molecules.

The dew point is the temperature at which air becomes saturated with water vapour and condensation begins to occur. When air cools to this temperature, invisible water vapour turns into visible liquid water droplets. Think of it this way: warm air can hold more moisture than cold air, so as temperature drops, the air eventually reaches a point where it cannot hold all its water anymore.

A perfect Nigerian example is early morning in Lagos or Jos. During harmattan season, the air cools overnight and reaches its dew point, causing the grass and car windscreens to become wet with dew. This isn't rain—it's condensation from the surrounding air. The cooler the morning, the lower the dew point, and the wetter the surfaces become. Understanding this concept helps explain why your bathroom mirror fogs up after a hot shower.

Humidity is simply the amount of water vapour present in the air around you. When you feel sticky during the rainy season in Lagos, that's high humidity at work. Relative humidity, however, is the ratio of the actual amount of water vapour in the air to the maximum amount the air can hold at that particular temperature, expressed as a percentage.

Think of it like this: warm air can hold more moisture than cold air. So on a hot afternoon, the air might hold only 50% of its maximum water capacity, while on a cool morning, the same amount of vapour could represent 90% of the air's capacity. That's why you feel more uncomfortable in the morning even though both times have similar moisture content.

Relative humidity determines whether sweat evaporates from your skin. Low relative humidity means faster evaporation and comfort; high relative humidity means slow evaporation and that sticky feeling.

The wet and dry bulb thermometer method measures humidity in the atmosphere. You have two identical thermometers placed side by side. One bulb stays dry while the other has wet cloth wrapped around it. The wet bulb reads lower because water evaporates from the cloth, cooling it down. The difference between the two readings tells you how much moisture is in the air.

In Lagos during harmattan season, the dry bulb might read 28°C while the wet bulb reads 22°C. This large difference means the air is very dry because much evaporation occurred. During rainy season, both thermometers read closer together, showing high humidity since less evaporation happens when air already contains moisture.

Vapours are gases formed when liquids evaporate below their boiling point. Think of it like this: when you leave a bucket of water in the sun, water molecules escape into the air as an invisible gas—that's vapour. The key difference from boiling is that vapour formation happens at the surface of the liquid at any temperature, while boiling happens throughout the liquid at a specific temperature.

In Nigeria, you see this daily. When petrol sits in a container, you smell fumes—those are petrol vapours escaping. Similarly, wet clothes dry because water molecules become vapours and float away into the air.

For numerical problems, remember that vapour pressure increases with temperature. Use the ideal gas law and understand saturated versus unsaturated vapours. Saturated vapour means the air is holding maximum water or liquid molecules and cannot hold more.

A vapour is a gas that forms when a liquid evaporates below its boiling point. Think of it this way: when you leave a cup of water on your table, water molecules escape from the surface and become invisible water vapour in the air. This happens even without boiling! A gas, however, is a substance that's already in gaseous form at room temperature, like oxygen or nitrogen in the air around you.

The key difference is temperature and origin. Vapour comes from a liquid or solid that hasn't reached its boiling point yet. When you smell petrol at a filling station in Lagos, that's petrol vapour escaping from the liquid below its boiling point. A true gas doesn't need to come from evaporation.

Understanding this distinction helps you solve thermodynamics problems correctly. You must recognise whether a substance is behaving as a vapour (liquid-to-gas transition) or already exists as a gas.