This chapter explores the nature of water and how it exists in different forms, and how it moves around the Earth. It starts with a simple observation about ice and water – are they the same substance, or different?

Observing that ice melts and turns into water shows that ice and water are indeed the same substance, but they are in different states or forms.

Although they are the same substance, their properties can be different (e.g., ice is hard and can be held, water flows and splashes).

A wise saying from Thirukkural reminds us of the importance of water: "If it does not rain well, even the mighty ocean will be drained."

Investigating Water’s Disappearing Act

Observing Water Disappearance

We often notice water disappearing from places. Examples include:

• Water puddles on the ground after rain disappearing over time.
• Water left on washed utensils drying up.
• Sweat on our body disappearing.

Where does this water go? Does it soak into the surface, or does something else happen?

An activity observing water on a steel plate shows that water does not seep through the solid surface of the plate. The water eventually disappears from the surface.

The Process Of Evaporation

When water seems to disappear from a surface, it is changing its state. The process where water changes from its liquid state into its gaseous state (invisible water vapour) is called evaporation.

Evaporation happens continuously from the surface of water, even at room temperature.

Examples of evaporation in daily life:

• Drying of wet clothes.
• Drying of a mopped floor.
• Sweat evaporating from our skin, causing cooling.
• Water sprinkled on a hot pan turning into steam (water vapour).
• Hand sanitiser disappearing from hands.

Water vapour is invisible. What we sometimes see as 'steam' contains tiny liquid water droplets suspended in the air, which makes it visible.

Disappearance of water from puddles is due to both seeping into the ground and evaporation into the air as water vapour.

Another Mystery

Water Droplets On Cold Surfaces

Have you ever seen tiny water droplets appear on the outer surface of a glass or container holding cold water or ice? This is another interesting phenomenon involving the change of state of water.

Initially, it might seem like water is leaking or seeping through the container, but activities demonstrate this is not the case (e.g., the water level inside does not drop). Also, these droplets appear even on non-porous surfaces like steel.

The Process Of Condensation

The water droplets on the outside of a cold surface come from the air around the container. The air always contains some amount of invisible water vapour.

When this water vapour in the air comes into contact with a cold surface, it cools down. As it cools, it changes back from its gaseous state (water vapour) to its liquid state (water droplets). This process is called condensation.

Examples of condensation:

• Dew drops forming on plant leaves, especially in the cool morning.
• Water drops forming on the inner side of a lid covering a pot of boiling water (steam cools and condenses on the lid).
• Water droplets forming on the outside of a cold drink bottle.

Evidence For Condensation

An activity measuring the mass of a glass of cold water with ice cubes over time on a digital weighing balance can provide evidence. As condensation occurs, water collects on the outer surface, and the total mass of the glass and its contents (including the condensed water) will slightly increase.

Combining this with the observation that the water level inside the glass does not decrease (or only decreases due to the ice melting) confirms that the external water is not coming from inside the glass but from the surrounding air.

The amount of water vapour present in the air is called humidity.

What Are The Different States Of Water?

Water is a substance that can exist naturally in three different states:

Solid State (Ice)

In its solid state, water is known as ice.

Properties of ice:

• Has a fixed shape (it retains its shape regardless of the container).
• Does not flow or spread easily.

Liquid State (Water)

In its liquid state, water is simply called water.

Properties of water:

• Has no fixed shape; it takes the shape of the container it is in.
• Can flow and spread.
• Has a fixed volume (a specific amount of water will occupy the same volume regardless of the container shape).

Gaseous State (Water Vapour)

In its gaseous state, water exists as water vapour or steam (when visible due to tiny suspended water droplets).

Properties of water vapour:

• Has no fixed shape or volume.
• Spreads out to fill the entire available space.
• Is usually invisible at room temperature.

Properties Of Different States

Property	Ice (Solid state)	Water (Liquid state)	Water vapour (Gaseous state)
Shape	Fixed	Takes shape of container	No fixed shape (fills container)
Ability to flow	No	Yes	Yes (spreads easily)
Ability to spread	No	Yes (on a surface)	Yes (fills space)
Volume	Fixed	Fixed	No fixed volume (fills container)
Visibility	Visible	Visible	Invisible (usually)

Many other substances besides water can also exist in solid, liquid, and gaseous states (e.g., wax, oil, ghee, oxygen, carbon dioxide).

The smell of food cooking reaching us is an example of gases (aroma molecules) spreading through the air, which is also matter in gaseous state.

How Can We Change The States Of Water?

Water can change from one state to another, and these changes are usually caused by adding or removing heat.

Changing States With Heating And Cooling

• To change ice (solid) into water (liquid), you need to supply heat (e.g., by leaving it at room temperature or heating it).
• To change water (liquid) into water vapour (gas), you need to supply more heat (e.g., by heating it to boiling or through evaporation at lower temperatures).
• To change water vapour (gas) into water (liquid), you need to remove heat (cool it down) through condensation.
• To change water (liquid) into ice (solid), you need to remove heat (cool it down) by placing it in a cold environment like a freezer. This process is called freezing.

Melting And Freezing

• The process of conversion of a solid into a liquid state is called melting. (Ice to Water).
• The process of conversion of a liquid into a solid state is called freezing. (Water to Ice).

Examples Of State Change

Besides water, other substances also change states with heating and cooling:

• Wax melts when heated (solid to liquid) and solidifies (freezes) when cooled (liquid to solid).
• Coconut oil, which is liquid in warm weather, solidifies in cold winter (freezing).

The changes of state of water can be represented as a cycle:

Extra Information: Atmospheric Water Generator (AWG) machines produce drinkable water by cooling humid air to condense water vapour, mimicking the formation of dew or the droplets on a cold glass.

How Can Water Be Evaporated Faster Or Slower?

The rate at which water evaporates depends on several conditions.

Conditions Affecting Evaporation Rate

Factors that influence how fast water evaporates include:

• The exposed surface area of the water.
• The temperature.
• The movement of air (wind).
• The amount of water vapour already in the air (humidity).

Effect Of Exposed Area

If you have the same amount of water in a narrow bottle cap (small exposed area) and on a plate (large exposed area), the water on the plate will evaporate much faster.

This is because more of the water surface is in contact with the air, allowing more water molecules to turn into vapour.

Exposed area of water	Time taken for complete evaporation
Less (bottle cap)	Longer time
More (plate)	Shorter time

Effect Of Temperature And Wind

• Temperature: Evaporation happens faster on a hot day (like in sunlight) compared to a cold or shaded day. More heat provides the energy for water molecules to escape into the air. Clothes dry faster on a hot, sunny day.
• Air Movement (Wind): Evaporation is faster on a windy day. Moving air carries away the water vapour that has evaporated from the surface, allowing more water molecules to escape. This is why sitting under a fan makes you feel cooler (sweat evaporates faster).
• Humidity: Evaporation is slower when the air is already humid (contains a lot of water vapour), like on a rainy day. If the air is already nearly full of water vapour, less water can evaporate into it. This is why clothes dry slowly on a rainy day.

Knowing these factors helps us dry things faster (e.g., spreading clothes out, drying them in sunlight or under a fan) or slow down evaporation when needed.

Cooling Effect

Evaporation Causes Cooling

When water evaporates, the process requires energy (heat). This energy is taken from the surface or the substance from which the water is evaporating. As heat is removed from the surface, it becomes cooler.

This is why evaporation causes a cooling effect.

Examples Of Cooling Effect

• Earthen pots (Matka/Surahi): Earthen pots have tiny pores in their walls. Water seeps through these pores to the outer surface and evaporates. This evaporation takes heat from the water inside the pot, keeping the water cool. Stainless steel pots do not have pores, so water does not evaporate from the surface, and there is no such cooling effect.
• Sprinkling water: Sprinkling water on a floor or roof in summer cools it down as the water evaporates.
• Sweating: When sweat evaporates from our skin, it takes heat from our body, making us feel cooler. A fan helps sweat evaporate faster, enhancing the cooling effect.
• Hand sanitiser: Hand sanitiser, which contains alcohol, evaporates quickly from the skin, causing a noticeable cooling sensation.

The Pot-In-Pot Cooler Model

A simple, electricity-free cooler can be made using two earthen pots and sand, based on the principle of evaporative cooling. The space between a smaller inner pot and a larger outer pot is filled with sand, which is kept moist with water. Water evaporates from the outer surface of the larger pot and from the moist sand, drawing heat from the inner pot and keeping its contents (like vegetables or fruits) cool.

Regularly adding water to keep the sand moist is necessary for this cooler to work effectively.

How Do Clouds Give Us Rain?

The processes of evaporation and condensation are fundamental parts of the natural movement of water on Earth.

From Water Vapour To Clouds

Water evaporates from oceans, lakes, rivers, soil, plants, etc., and rises into the atmosphere as invisible water vapour.

As this air containing water vapour rises higher, it encounters colder temperatures. At a certain height, the air becomes cool enough for the water vapour to condense.

During condensation in the atmosphere, the water vapour turns into tiny liquid water droplets (or ice crystals at very high altitudes). These tiny droplets often form around tiny particles like dust or smoke present in the air.

These tiny droplets are very light and remain suspended in the air, forming clouds.

Extra Information: Water vapour is lighter than dry air, which helps the moist air rise in the atmosphere.

Rain, Hail, And Snow Formation

Within the clouds, these tiny water droplets (or ice crystals) collide and merge together, forming larger drops.

When these drops become too heavy for the air currents to support, they fall to the Earth's surface as rain.

Under specific atmospheric conditions (like strong updrafts and very cold temperatures), the falling water can freeze, forming hail, or can form ice crystals that fall as snow.

The Water Cycle

The continuous circulation of water between the Earth's surface and the atmosphere is known as the water cycle.

Key stages of the water cycle include:

• Evaporation: Water from bodies of water and the Earth's surface turns into vapour and rises into the atmosphere.
• Condensation: Water vapour in the atmosphere cools and forms clouds of tiny water droplets.
• Precipitation: Water falls back to Earth as rain, snow, or hail.

The water cycle ensures that water is continuously recycled on our planet.

Importance Of Water And Conservation

While Earth has a large amount of water, most of it is in oceans (salty water) and not directly usable for drinking, agriculture, or most human activities. Only a small fraction is available as fresh water.

With the increasing population, the demand for fresh water is rising, leading to water shortages in many areas.

Therefore, it is crucial to use water wisely, avoid wastage, and protect our water sources (like rivers, lakes, and groundwater) from pollution. Conserving water is essential for the well-being of plants, animals, and humans.

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