Class 6 Science Chapter 8 Notes A Journey through States of Water
Water is one of the most crucial substances on earth. It is fundamental to all forms of life and essential for various natural processes. We need water for drinking and many other activities, like cleaning, washing clothes and utensils, etc. Water is found abundantly on the planet Earth, as about 3/4th of the total earth’s surface is covered with water but only a small amount of this water is suitable for plants, animals, and humans, like water from rivers, lakes, ponds, and groundwater. Most of the water is in the oceans and is not directly usable, as it contains large amounts of salt. Water is unique because it can exist in three distinct states- solid, liquid, and gas, each playing a vital role in our environment.
Different States of Water
As we know water exists in different forms, like ice, water, and water vapour. These forms show different properties and are called states. Different states of water and its properties are
1. Solid State (Ice)
- Water turns into ice when water is kept in a freezer.
- Ice has a definite shape and volume. It retains its shape irrespective of the container it is kept in.
- Ice does not flow or spread.
2. Liquid State (Water)
- When ice is heated, it melts into liquid water.
- Liquid water has a definite volume. It does not have a fixed shape and takes the shape of the container it is kept in.
- It flows or spreads, and moves freely.
3. Gaseous State (Water vapour)
- Heating water turns it into steam or water vapour.
- The water that evaporates during some processes, like the drying of clothes or mopping of floors contributes to the water vapor present in the air around us.
- Water vapour fills the entire space available and has neither a fixed shape nor volume.
- The water vapour spreads out and moved rapidly.
The water vapour is not visible but it is still present in the air. The tiny droplets of water in the water vapor or steam make it visible.
Changing States of Water
Imagine a piece of ice, a glass of water, and steam coming out from a boiling kettle. They might look different, but they are all made of water. This is because water is a substance that can change its state from solid to liquid and liquid to gas on continuous heating. When ice melts, it turns into water. When water heats up, it becomes steam. And when steam cools down, it turns back into water. The processes involved in changing the state of water are
- Melting: The process of conversion of solid into liquid on heating is called melting.
- Freezing: The process of conversion of liquid into solid on cooling is called freezing. Water freezes when cooled below 0°C.
- Evaporation: The process of conversion of liquid into gas on heating is called evaporation. The process of evaporation of water takes place continuously (without any help) even at room temperature. Drying of wet clothes, mopped floors, and sweat from our bodies are examples of evaporation.
- Condensation: The process where water vapour turns back into liquid water when cooled is known as condensation.
Another example of changing state from solid to liquid is the melting of candle wax. The candle wax which is solid melts into liquid wax, when it is heated. Whereas, the liquid wax on cooling, turns into solid candle wax.
Daily Life Example of Water Condensation
When a glass of cold water is taken out from the refrigerator on a warm day, water droplets form on the surface of the glass. This happens because the warm, moist air around the glass, containing water vapour, cools down upon contact with the cold surface and the water vapour condenses into liquid droplets. Also, condensation is often seen on bathroom mirrors after a hot shower, where warm steam from the shower cools and forms droplets on the cooler surface of the mirror.
Atmospheric Water Generator (AWG) machines are used to collect water from humid air to produce drinkable water. This is done by cooling the air, which causes condensation of water vapour.
Evaporation: Self-Disappearing Act of Water
You may notice that water levels in a container or a puddle disappear over time. This is due to evaporation, where the water has turned into water vapour. The process of conversion of water into its vapour state is called evaporation.
Rate of Evaporation
The evaporation of water can be faster or slower, depending on several factors, which are as follows.
1. Temperature:
Higher temperatures increase the rate of evaporation. When water is heated, its molecules gain energy, move faster, and escape more easily into the air as vapour. Similarly, lower temperatures slow down the rate of evaporation. Cooler water has less molecular energy, so fewer molecules have enough energy to escape the liquid state.
2. Surface Area:
Evaporation is a surface phenomenon. Increasing the surface area of water exposed to air speeds up evaporation. For example, spreading water out in a steel plate allows more water to be exposed to air as compared to the water filled in the cap of a bottle. Decreasing the surface area, such as using a small container, limits the area available for evaporation and thus slows down the process.
3. Air Movement (Wind):
Increased air movement or wind accelerates evaporation. Moving air carries away the water vapour that has formed above the liquid surface, reducing humidity and allowing more water to evaporate.
4. Humidity:
Humidity is the amount of water vapour present in the air. Lower humidity levels in the air enhance evaporation. When the air is dry, it has a greater capacity to absorb additional water vapour, leading to faster evaporation. High humidity levels mean the air already has more amount of water or has more humidity, so the rate of evaporation decreases because the air can hold less additional water vapour.
Examples showing how these factors influence the rate of evaporation are
- Drying Clothes: Clothes dry faster on a hot, windy day as compared to a cold and not-so-windy day because higher temperatures and air movement increase the rate of evaporation of water from the clothes. On rainy days, the amount of water vapour in the air is more, therefore, rainy days are more humid. So, it is difficult to dry clothes on rainy days as the rate of evaporation is less due to more humidity.
- Puddles: A large puddle present in the area getting direct sunlight will evaporate faster than a small puddle present in the shade, due to increased surface area and higher temperature.
- Drying Dishes: Dishes placed on a dish rack in an area with warm, dry air will dry faster, as the increased air circulation and heat help the water on the dishes to evaporate more quickly.
Cooling Effect of Evaporation
When water evaporates, it needs heat to change into a gas. The heat comes from the things around it (surroundings). As the heat is used up for evaporation, the water and the surface where it was, become cooler and give a cooling effect. Examples showing the cooling effect of evaporation are
(i) When you sweat, your body releases water onto your skin. As the sweat (water) evaporates into the air, it takes heat away from your skin, making you feel cooler.
(ii) On a hot day, the water in a matka (earthen pot) stays cool, so when you drink it, it feels refreshing. Matkas are made up of clay, which has tiny holes. When you fill it with water and leave outside, the water seeps through the surface of the matka and starts to evaporate. As the water evaporates, it makes the water inside the matka cooler than the temperature of the air around it.
(iii) Similar to matkas, surahis are also made up of clay. They are used to store water and have a similar evaporation and cooling process. The cooling effect helps keep the water inside the surahi cool, which is especially nice to drink in summer days.
Plants take in water from the soil through their roots. The water travels up through the plant’s stem (or trunk) to the leaves. In the leaves, some of this water evaporates into the air through the stomata. The process by which plants lose water through tiny openings (stomata) in their leaves is called transpiration. Just like how sweating cools you down, transpiration provides a cooling effect to the plant.
Formation of Clouds to Give Rain
As a result of evaporation, water vapour from the earth rises higher above the earth’s surface as water vapour is lighter than air. It cools down and changes into tiny water droplets, which are generally formed around the dust particles. These small droplets float in the air and form clouds. Many droplets join together to form bigger drops of water. Some drops became so heavy that they started falling in the form of rain.
The Water Cycle
The water cycle is the continuous movement of water on, above, and below the surface of the earth. It describes how water changes its form and travels through different stages in nature. Water evaporates from the surface of the earth, rises into the atmosphere, cools down condenses to form clouds, and falls again to the surface as precipitation. This circulation of water is known as the water cycle.
It is crucial for distributing fresh water across the planet. It supports all forms of life, helps regulate climate, and replenishes groundwater.
→ Melting: The process where solid ice turns into liquid water upon heating.
→ Freezing: The process where liquid water turns into solid ice upon when cooled below 0°C.
→ Evaporation: The process where liquid water turns into a water vapour or steam when heated.
→ Condensation: The process where water vapour turns back into liquid water when cooled.
→ Humidity: The amount of water vapour or moisture present in the air.
→ Water Cycle: The circulation of water between the Earth’s surface and atmosphere.
→ Precipitation: The term used to describe any form of water, like, rain, snow, or hail, that falls on the Earth from the atmosphere.