Chapter Notes

A Journey through States of Water

Water is a unique substance that exists in three different forms or states in our daily lives: solid, liquid, and gas. These states can be observed through simple phenomena around us.

When an ice cube is left at room temperature, it gradually changes into water. This demonstrates that ice and water are two forms of the same substance. Although they are the same substance, their behaviors differ significantly; for example, water flows and splashes, while ice does not.

Investigating Water's Disappearing Act

Have you ever noticed water puddles disappearing after it rains, or water drying up on washed utensils? This disappearance of water is a common phenomenon.

One might initially think the water seeps into the ground or through surfaces. However, an activity involving water on a steel plate shows that water does not seep through non-porous surfaces like steel. This means the water must go somewhere else.

The water actually changes into a gaseous state called water vapour. This process, where water converts into its vapour state, is called evaporation. Evaporation occurs continuously, even at room temperature.

• Drying of wet clothes on a clothesline.
• A mopped floor drying up.
• Sweat disappearing from our bodies, making us feel cooler.
• Hand sanitiser disappearing as you rub it on your hands.

When water is sprinkled on a hot pan (like when making dosa), it quickly turns into steam. Steam is essentially water vapour, though some parts of it might convert into tiny water droplets visible to the eye.

Therefore, the disappearance of water from puddles is often due to a combination of water seeping into the ground and evaporation into the air.

Another Mystery

When cold water with ice cubes is placed in a glass tumbler, tiny water droplets appear on the outer surface of the glass after a few minutes. These tiny droplets combine to form bigger drops. This phenomenon raises the question: where do these water droplets come from?

The water droplets do not seep out from inside the glass. Instead, they form because the water vapour present in the air comes into contact with the cold surface of the glass. When water vapour touches a cold surface, it loses heat and changes back into its liquid state, forming water droplets. This process of conversion of water vapour into its liquid state is called condensation.

• Dew drops forming on plants in the morning. This happens because the air cools down overnight, and the water vapour in the air condenses on the cold surfaces of leaves.
• Water droplets accumulating on the inner side of a steel plate covering a utensil of boiling water. The steam (water vapour) from the boiling water rises, touches the cooler plate, and condenses.

To further confirm that the water droplets on the glass tumbler are due to condensation and not seepage, an experiment can be done. If a glass tumbler with ice water is weighed on a digital balance, its mass will gradually increase as water droplets form on its outer surface. This increase in mass comes from the water vapour in the surrounding air condensing onto the cold glass. Additionally, if the water level inside the glass is marked, it will be observed that the internal water level does not decrease, confirming that water is not seeping out.

What are the Different States of Water?

Water is commonly found in three distinct states:

1. Solid state (Ice)
2. Liquid state (Water)
3. Gaseous state (Water Vapour)

Each state has different properties:

• Ice (Solid state)

  • Shape: Retains its own fixed shape, regardless of the container it is placed in.
  • Ability to flow: Does not flow.
  • Ability to spread: Does not spread.

• Water (Liquid state)

  • Shape: Does not have a fixed shape; it takes the shape of the container it is kept in.
  • Ability to flow: Flows easily.
  • Ability to spread: Spreads out while keeping its volume constant.

• Water Vapour (Gaseous state)

  • Shape: Does not have a fixed shape; it spreads out to fill the entire available space.
  • Ability to flow: Flows freely as it spreads.
  • Visibility: It is invisible to us, even though it exists at room temperature and is present in the air around us. Water vapour in the air primarily comes from evaporation processes like drying clothes or mopping floors.

• Solids: Stones, wood, glass.
• Liquids: Milk, oil.
• Gases: Oxygen, carbon dioxide. The smell of food cooking in the kitchen reaches us because the gaseous particles carrying the smell spread through the air.

How can We Change the States of Water?

The state of water can be changed by either heating or cooling.

• From Solid to Liquid (Melting): When ice is heated, it absorbs energy and changes into liquid water. This process is called melting.
• From Liquid to Gas (Evaporation/Boiling): When water is heated further, it absorbs more energy and converts into water vapour. This process is called evaporation.
• From Liquid to Solid (Freezing): When water is cooled (e.g., by placing it in a freezer), it loses heat and changes into solid ice. This process is called freezing.
• From Gas to Liquid (Condensation): When water vapour is cooled, it loses heat and changes back into liquid water. This process is called condensation.

• A candle, made of wax, melts into a liquid when heated. This liquid wax can then be cooled to solidify again.
• Coconut oil, which is liquid in warm weather, often solidifies into a solid state during the winter season due to cooling.
• Atmospheric Water Generator (AWG) machines collect drinkable water from humid air by cooling the air, causing water vapour to condense.

How can Water be Evaporated Faster or Slower?

The rate at which water evaporates is affected by several conditions:

• Exposed Area: A larger exposed surface area allows water to evaporate faster. For instance, water spread on a plate evaporates quicker than the same amount of water in a bottle cap because the plate offers a larger surface area to the air.
• Temperature: Water evaporates faster at higher temperatures. Clothes dry faster on a hot sunny day because the increased heat provides more energy for water molecules to escape into the air.
• Air Movement (Wind): Increased air movement, like on a windy day, speeds up evaporation. The moving air carries away the water vapour, allowing more liquid water to evaporate. This is why clothes dry faster on a windy day.
• Humidity: The amount of water vapour already present in the air (humidity) affects evaporation. If the air is already saturated with water vapour (high humidity, like on a rainy day), water evaporates more slowly because there is less space for additional water vapour.

To dry clothes faster on a rainy day, one could use a fan to increase air movement or place them in a warmer area.

Cooling Effect

Evaporation causes a cooling effect. When water evaporates, it absorbs heat from its surroundings to change into its gaseous state. This removal of heat from the surface or substance causes the temperature of that surface or substance to drop, leading to a cooling sensation.

• Earthen pots (matka or surahi): Earthen pots have tiny pores on their surface. Water slowly seeps through these pores to the outside surface of the pot. As this water evaporates from the outer surface, it takes latent heat from the remaining water inside the pot, making the water inside cooler.
• Sweating: When we sweat, the sweat evaporates from our skin, taking heat from our body and making us feel cooler, especially when there's a fan or wind.
• Sprinkling water on floors/roofs: In summer, sprinkling water on the floor or roof causes the water to evaporate, absorbing heat from the surface and cooling it down.
• Hand sanitiser: When rubbed on hands, the alcohol in the sanitiser evaporates quickly, taking heat from the skin and producing a cooling sensation.

A simple pot-in-pot cooler can be made using two earthen pots and sand. By keeping the sand between the pots moist with water, the evaporation of water from the sand and the outer pot creates a cooling effect inside the inner pot, which can keep vegetables and fruits fresh for longer. The effectiveness of this cooler depends on conditions like ambient temperature, humidity, and air movement.

How Do Clouds Give Us Rain?

Condensation plays a crucial role in bringing evaporated water back to the Earth's surface as rain. This process, along with evaporation, forms the water cycle.

1. Evaporation: Water from oceans, lakes, rivers, and the Earth's surface evaporates into the atmosphere, becoming water vapour.
2. Rising Water Vapour: Water vapour is lighter than air, so it rises higher into the atmosphere.
3. Cooling and Condensation: As the air containing water vapour rises, it encounters cooler temperatures at higher altitudes. The air cools down, and the water vapour loses heat.
4. Cloud Formation: At a certain height, the air becomes so cool that the water vapour condenses around tiny particles like dust (which act as condensation nuclei) to form microscopic water droplets. These small droplets float in the air and collectively form clouds.
5. Precipitation: Many small water droplets within clouds join together to form bigger, heavier drops. When these drops become too heavy to float in the air, they fall to the Earth's surface as rain. Under special atmospheric conditions, water can also fall as hail (ice pellets) or snow (ice crystals).

This continuous circulation of water from the Earth's surface to the atmosphere and back again, involving evaporation, condensation, and precipitation, is known as the water cycle.