A Journey through States of Water | CBSE Class 6 Science Notes

This note covers the three states of water, their properties, melting and freezing, evaporation and condensation, investigations of changing states, conditions affecting evaporation, cooling by evaporation, cloud formation, rain, the water cycle and the careful use of water.

How can the same water exist in three different states?

Forms of the same substance

A state is a form in which a substance exists. Water has three states: solid, liquid and gas. Ice is its solid state, ordinary water is its liquid state, and water vapour is its gaseous state.

The different forms do not behave alike. Ice feels hard and can be held in the hand. Liquid water flows and splashes, whereas ice does not. These differences in behaviour do not mean that ice and liquid water are different substances.

An ice cube left in a cup on a table changes into liquid water. Water kept in the freezer of a refrigerator changes into ice. Observing both changes helps connect the solid and liquid forms of the same substance.

Definition: Water vapour is water in its gaseous state. It is invisible and is present in the air around us, even at room temperature, meaning the temperature of the surroundings.

Comparing shape and spreading

Move an ice cube between containers of different shapes. Its shape remains the same. Pour liquid water between containers and its shape changes. Volume means the space occupied by the water; its volume remains constant during this change of container.

The spreading of liquid water and water vapour should be distinguished. Water poured on a clean surface spreads while retaining its volume. Water vapour spreads into the entire space available to it. A gas, like a liquid, does not possess a fixed shape.

Other familiar solids include stones, wood and glass. Milk and oil are examples of liquids. Oxygen and carbon dioxide are examples of gases. These examples help relate the states of water to the behaviour of other substances around us.

Where does water go when a wet surface dries?

Evaporation in daily life

Evaporation is the conversion of liquid water into water vapour. The liquid may disappear from view, but it has changed state. Evaporation takes place continuously, even at room temperature, so water need not be placed on a hot pan before it can evaporate.

Wet clothes dry, a mopped floor dries, and sweat on the body dries through evaporation. Water left on washed utensils also disappears after some time. These familiar observations connect drying with the production of invisible water vapour in the surrounding air.

Investigating a steel plate

Seeping means passing through a material. To investigate whether water disappears by seeping through a steel plate, examine both sides of the plate rather than looking only at the water on its upper surface.

1. Place a tablespoon of water on a steel plate.
2. Observe whether any water reaches the other side of the plate.
3. Continue observing at regular intervals as the water disappears.
4. Use the observations to consider whether the water seeped through the plate or changed into vapour.

Photograph: Steel plate with a tablespoon of water (NCERT Class 6 Figure 8.1). The photograph shows a steel plate with a small patch of water on it. It illustrates the tablespoon of water used to investigate its disappearance.

Water on the plate changes into water vapour. For a puddle on the ground, consider both evaporation and the possibility of water entering the soil. Do not assume that the explanation for water disappearing must be identical on every surface.

Visible steam and invisible vapour

Water sprinkled on a hot pan while making dosa changes into steam. Steam is water vapour, some of which changes into tiny liquid droplets. Those droplets make the steam visible; water vapour itself is invisible.

Note: A visible cloud above hot water should not be described as visible water vapour. The visible part contains tiny water droplets.

Why do droplets appear outside a cold tumbler?

Observing before explaining

Condensation is the conversion of water vapour into liquid water. When water vapour in the air touches a cold surface, liquid droplets form. A droplet is a tiny drop of water, rather than water in its gaseous state.

To observe the effect, take cold water in a glass tumbler and add a few ice cubes. Leave it undisturbed for five minutes. Examine the outside of the tumbler and record both what is seen and the questions the observation raises.

Initially, small droplets appear on the outer surface. They join to form larger drops. The activity can also be tried with a metal container. Keep the observation, droplets appearing outside, separate from the explanation of where that water came from.

Photograph: A glass tumbler containing cold water and ice cubes (NCERT Class 6 Figure 8.2). The photograph shows a glass tumbler containing cold water and ice cubes, with a hand pointing towards its outer surface.

Testing possible explanations

The suggestion that water has seeped out is a possible explanation to investigate. Simply looking at the water level may not settle it: a small change might not be significant enough to be seen. A tall, narrow bottle makes slight changes in level more noticeable.

Another useful comparison is a tumbler containing water at room temperature. Investigating whether water seeps from that tumbler helps examine the suggestion that the glass itself allows water to pass through. An explanation should be supported by observations, rather than accepted because it sounds possible.

Dew drops, the drops seen on plants, provide another setting for thinking about condensation. Water drops also collect on the inner side of a steel plate covering a utensil in which water is boiled. In each case, distinguish liquid droplets from invisible vapour.

The key change in condensation is from gas to liquid. It is different from ice becoming liquid water, because ice begins in the solid state. Naming the initial and final states helps identify the process correctly.

What does the weighing experiment show about condensation?

Recording evidence

A digital weighing balance is an instrument used here to measure mass, the quantity of matter in the weighed materials. It allows changes in the reading to be recorded while droplets form on a cold tumbler.

1. Half-fill a glass tumbler with water and add a few ice cubes.
2. Cover the tumbler with a small steel plate and place it on a digital weighing balance.
3. Record the initial reading, then record a reading after every five minutes.
4. Continue the observations for 30 minutes and compare the readings with the prediction made before observing.

The recording times are 0, 5, 10, 15, 20, 25 and 30 minutes. These times organise the observations; they are not mass measurements. The readings must come from the balance during the activity, so they should not be filled with guessed values.

You may observe droplets on the tumbler. Water vapour from the surrounding air touches its cold surface and changes into liquid water. The balance reading increases as water collects on the outside.

Recognising the limit of a conclusion

Note: The increase in the balance reading alone does not conclusively establish that no water is seeping through the tumbler, or that all the water outside it is due to condensation.

Repeat the activity with the water level marked using a permanent marker or visible tape. The water level does not go down, while extra water collects on the outside. This modified activity shows that water is not seeping out and that the additional water comes from condensation.

The distinction matters: the first activity records an increase, while the modified activity also follows the water level inside. The additional observation addresses the suggested explanation of seepage. Scientific reasoning involves asking whether the evidence actually answers the question being investigated.

A prediction is an expectation made before observing the result. Compare that expectation with the recorded observations, and revise the explanation when needed. Do not turn a possible explanation into a definite conclusion before examining the relevant evidence.

How do heating and cooling change the state of water?

Melting and freezing

Melting is the change from a solid to a liquid. Freezing is the change from a liquid to a solid. Supplying heat changes ice into liquid water, while placing water in a cold environment such as a freezer changes it into ice.

Take ice out of the freezer and it melts into water. On further heating, liquid water changes into water vapour. These observations connect changes of state with heating and cooling, while evaporation also occurs continuously at room temperature.

The diagram places three state boxes in a row with arrows in both directions between neighbouring boxes. Solid and Melts are supplied; the remaining spaces are for Liquid, Gas, Freezes, Evaporates and Condenses.

See Fig. 8.5 in your NCERT textbook

Following each change in the correct direction

Read a change-of-state diagram by following an arrow from its starting state to its final state. From solid to liquid, the process is melting; the opposite arrow represents freezing. From liquid to gas, the process is evaporation; the opposite arrow represents condensation.

Changes of state also occur in other substances. A candle is made of wax, which can be changed into a liquid. Cooling liquid wax changes it back into a solid. Coconut oil can be seen in its solid state during winter.

Atmospheric Water Generator machines collect water from humid air to produce drinkable water by cooling the air and condensing its water vapour. Humid air means air containing water vapour. The process resembles drops forming outside a tumbler containing ice-cold water.

How can an investigation show the effect of exposed area?

Changing one condition

Exposed area is the area of water open to the air. Water spread over a plate has a larger exposed area than the same amount in a small bottle cap. Increasing this area makes evaporation faster.

An investigation should make clear what changes, what stays the same and what is measured. In this comparison, the exposed area changes. The amount of water remains the same, and the two containers are kept near each other.

1. Take water in a small bottle cap.
2. Place the same amount of water on a plate.
3. Keep the plate and bottle cap near each other.
4. Record how long the water takes to evaporate completely in each container.

The comparison is between less exposed area in the bottle cap and more exposed area on the plate. The measured result is the time needed for complete evaporation. Record the actual times observed rather than assuming a fixed duration for either container.

Explaining the result

Water spread on the plate evaporates faster because more of its area is exposed to air. The result links the changed condition, exposed area, to the observed difference in evaporation. It does not require invented amounts of water or invented timing results.

The same way of organising an investigation can be used for other conditions. State the condition being changed, identify the conditions being kept the same, and record the time for evaporation. Comparing observations is more useful when these choices are clear.

Keep a record of the method as well as the outcome. For example, writing that equal amounts were used explains an important part of the comparison. Writing only that one container dried first leaves out how the investigation was arranged.

These activities combine observation, questioning, prediction and reasoning. They ask for evidence of how evaporation changes with conditions, rather than a memorised drying time. A result should describe what was observed under the conditions used in that activity.

How do sunshine, moving air and humidity affect evaporation?

Sunlight compared with shade

Take identical bottle caps and put an equal amount of water in each. Place one in sunlight and the other in shade. Observe them after every 15 minutes, and record how long the water takes to evaporate completely in each.

Water evaporates faster from the cap in sunlight than from the cap in shade. It is also a common observation that clothes dry faster on a hot sunny day. The activity can be repeated on a windy or rainy day.

The drawing shows a bottle cap in the shade beside a building and another in the sunlit open area. An enlarged view shows water inside a cap.

See Fig. 8.6 in your NCERT textbook

Air movement and humidity

With increased air movement, water evaporates faster. It is a common observation that clothes dry faster on a windy day. A fan moves air and helps sweat evaporate, which is connected with the cooling felt when sitting under a fan.

Humidity is the amount of water vapour in the air. On rainy days, the air contains more water vapour and is more humid. If the amount of water in the air is already high, water evaporates slowly.

Keep the difference between a condition and a result clear. Humidity describes the water vapour already present in the air. The time taken for water to evaporate is an outcome that can be observed while comparing conditions.

A fan can help wet clothes dry even though moving air also gives a cooling sensation. Increased air movement speeds evaporation, while evaporation itself causes cooling. These two effects can occur together and should not be treated as contradictory.

How does evaporation produce cooling in everyday life?

Water in an earthen pot

Evaporative cooling means the cooling effect produced when water evaporates. A matka is an earthen pot used for storing water. Water seeps through its surface and evaporates, producing a cooling effect on the water in the pot.

Sprinkling water on a floor or roof during summer is another example of this cooling effect. Sweat evaporating from the body also cools us. Moving air from a fan helps sweat evaporate and therefore helps us feel cooler.

Making a pot-in-pot cooler

A pot-in-pot cooler is a simple model made with two earthen pots and moist sand. It demonstrates cooling without electricity. The pots must differ in size so that the smaller one can be placed inside the larger one.

1. Put a layer of sand at the bottom of the larger earthen pot.
2. Place the smaller pot in the centre of the larger pot.
3. Fill the space between the two pots with more sand.
4. Pour water into the sand area.
5. Cover the smaller pot with a lid or a wet jute sack.

The drawing shows a small pot inside a large pot, with a layer of sand between them. The labels identify Small pot, Large pot, Layer of sand, Wet jute sack and Lid.

See Fig. 8.7 in your NCERT textbook

Allow 4 to 5 hours for the model to cool down. This time range can be influenced by many conditions. Add water regularly to keep the sand moist, and observe how the arrangement creates a cooling effect inside the pots.

Place vegetables and fruits inside and examine them daily for a week to investigate freshness. The observation period is not a promise that every fruit or vegetable stays fresh for a week. The number of days must be found by observation under the conditions used.

A surahi is another clay vessel used to keep water cold in summer. It provides another familiar example of using a clay container for cool drinking water, alongside the matka and the pot-in-pot cooling model.

How do clouds form and give rain?

From water vapour to falling drops

The atmosphere is the thin layer of air surrounding the Earth. Condensation helps bring evaporated water back to the Earth's surface. As air moves higher above the surface, it becomes cooler and cooler.

1. At certain heights, the air becomes cool enough for its water vapour to turn into droplets.
2. These droplets are generally formed around dust particles.
3. The small droplets float in the air and form clouds.
4. Many droplets join to make larger drops of water.
5. Some drops become so heavy that they fall; these falling drops are rain.

Under special conditions, water might also fall as hail or snow, which are solid forms of water.

Investigating the role of dust particles

A bottle activity demonstrates how very small dust particles help clouds form. Use an empty, discarded plastic bottle with a capacity of one litre, a measure of the volume it can hold. Pour about one cup of water into it and close the lid tightly.

Quickly squeeze and release the bottle continuously for about 2 to 3 minutes. Observe the space above the water. Repeat after adding a small burnt piece of newspaper to the water, and compare the observations.

In the activity with the burnt newspaper, some haziness appears above the water. The burnt paper supplies very small, invisible dust particles. Water vapour condenses around them, forming the cloud-like haziness observed inside the bottle.

Note: Handle burning paper carefully during the bottle activity. Record the observations rather than inventing a measured size or number of droplets.

The activity connects a small-scale observation with a process occurring in the atmosphere. Both involve condensation around particles. Remember the word generally when describing droplets forming around dust particles in clouds, and the phrase some drops when explaining which drops become heavy enough to fall.

What is the water cycle, and why must water be used wisely?

Following water through the environment

The water cycle is the circulation of water between the Earth's surface and the atmosphere. Water from the oceans and the Earth's surface evaporates into the air. It returns as rain, hail or snow and ultimately flows back to the oceans.

This circulation includes both changes of state and movement between places. Evaporation changes liquid water into gas. Condensation changes water vapour into liquid droplets. Rain returns water to the surface, connecting the atmosphere with water on the Earth.

The drawing shows clouds, falling drops, snow-covered mountains and water on and below the land beside the ocean. Arrows indicate movement. The word bank supplies Cloud, Lake, Ocean, River, Groundwater, Evaporation, Condensation, Rain and Snow.

See Fig. 8.9 in your NCERT textbook

Groundwater means water beneath the ground. When labelling a water-cycle drawing, distinguish places where water is stored, such as a lake or ocean, from processes such as evaporation and condensation. Rain and snow show ways in which water returns from the atmosphere.

Recognising the limits of usable water

Only a small portion of the water available on Earth is fit for use by plants, animals and humans. Most of the water is in the oceans and cannot be used directly. The existence of the water cycle does not remove this limitation.

Water is used for drinking and many other activities. As population rises, the number of people using water increases. Growing demand causes water shortages in many parts of the world, making careful use of water important.

Avoid wasting water and keep water bodies free from pollution. These responsibilities follow from the limited portion available for use and the increasing demand.

The complete journey connects several familiar observations: wet surfaces dry, droplets collect on cold surfaces, clouds form and rain falls. Understanding the changes of state makes it possible to link these observations within one continuous circulation of water.

Glossary

• State — A form in which a substance exists, such as solid, liquid or gas.
• Ice — The solid state of water, which retains its shape and does not flow.
• Water vapour — Water in its invisible gaseous state, present in the air around us.
• Volume — The space occupied by water, which remains constant when liquid water changes container.
• Evaporation — The conversion of liquid water into water vapour, occurring even at room temperature.
• Condensation — The conversion of water vapour into liquid water, as on a cold surface.
• Melting — The change from a solid state into a liquid state, as ice becomes water.
• Freezing — The change from a liquid state into a solid state, as water becomes ice.
• Humidity — The amount of water vapour in air, which affects how quickly water evaporates.
• Exposed area — The area of water open to air, increased when water spreads across a plate.
• Evaporative cooling — The cooling effect produced by evaporation, seen in water stored in an earthen pot.
• Atmosphere — The thin layer of air surrounding the Earth, involved in the water cycle.
• Water cycle — The circulation of water between the Earth's surface and atmosphere through evaporation and return.

Common errors and misconceptions

• Misconception: Ice and liquid water must be different substances because they behave differently. Correct: They are different states of the same substance, as shown by freezing water and melting ice.
• Misconception: Evaporation happens only when water is heated on a stove. Correct: Evaporation takes place continuously, even at room temperature, including while clothes and mopped floors dry.
• Misconception: Water vapour is the visible cloud in steam. Correct: Water vapour is invisible; tiny water droplets make the steam visible.
• Misconception: An increased balance reading conclusively rules out seepage from a cold tumbler. Correct: The first weighing activity alone does not establish this conclusively; repeat it with the water level marked.
• Misconception: A fan cannot help wet clothes dry because it makes us feel cooler. Correct: Increased air movement makes evaporation faster, and evaporation also causes cooling.
• Misconception: The pot-in-pot cooler guarantees a week of freshness. Correct: Observe the contents daily for a week to investigate freshness; the number of days depends on conditions.
• Misconception: The water cycle makes all water directly usable. Correct: Only a small portion is fit for use by plants, animals and humans; most water is in oceans and cannot be used directly.

Exam-style questions with model answers

Q1. Define evaporation and condensation, giving the starting and final states for each. [2 marks]

1. Evaporation is the conversion of liquid water into water vapour, so the change is from liquid to gas.
2. Condensation is the conversion of water vapour into liquid water, so the change is from gas to liquid.

Q2. An ice cube retains its shape when moved between containers. Liquid water changes shape when poured into another container. What do these observations show about their shapes, and what happens to the liquid water's volume? [3 marks]

1. The ice cube is water in the solid state. It retains its own shape when placed in a differently shaped container.
2. Liquid water has no fixed shape. It takes the shape of the container into which it is poured, rather than retaining its previous shape.
3. The volume of the liquid water remains constant during the change of container. A change of shape should not be confused with a change of volume.

Q3. A glass tumbler is half-filled with water and ice, covered with a small steel plate and placed on a digital weighing balance. Droplets form outside and the reading increases. Explain the process, the limit of this evidence, the modification needed and the observation supporting the conclusion. [4 marks]

1. Water vapour from the air meets the cold outer surface and condenses into liquid droplets. Water collecting outside increases the balance reading.
2. The increase alone does not conclusively show that no water is seeping through the wall or that all external water comes from condensation.
3. Repeat the activity with the water level marked on the tumbler using a permanent marker or visible tape.
4. The marked water level does not fall while extra water collects outside. This modified activity shows that water is not seeping out and that the extra water comes from condensation.

Q4. Equal amounts of water are placed in a bottle cap and on a plate kept near each other. The plate exposes more water to the air. Identify the changed condition, one condition kept the same and the measurement to record; then explain the expected result. [4 marks]

1. The changed condition is the exposed area of water. The plate gives a larger area open to the air than the bottle cap.
2. The amount of water is kept the same in both containers. This is part of arranging the comparison of exposed areas.
3. Record the time taken for the water to evaporate completely in each container. These times must be measured during the activity.
4. Water on the plate evaporates faster because its exposed area is larger. The bottle cap has less area open to the air.

Q5. Explain how a pot-in-pot cooler is assembled and used. Include the sand arrangement, adding water, covering the smaller pot, the cooling time with its qualification, and investigating freshness. [5 marks]

1. Place a layer of sand at the bottom of a larger earthen pot. Put a smaller earthen pot in its centre and fill the gap between them with sand.
2. Pour water into the sand area. Add water regularly to keep the sand moist as the arrangement produces a cooling effect.
3. Cover the top of the smaller pot with a lid or a wet jute sack to complete the simple cooler.
4. Allow 4 to 5 hours for the cooler to cool down. This time range can be influenced by many conditions.
5. Place vegetables and fruits inside and observe them daily for a week. Investigate how long they remain fresh rather than assuming a guaranteed storage period.

Q6. Describe how water vapour in rising air can lead to rain. Include cooling, droplet formation, clouds, larger drops and falling rain; retain the qualification about hail or snow. [5 marks]

1. As air moves higher above the Earth's surface, it becomes cooler and cooler. At certain heights it becomes cool enough for its water vapour to change state.
2. The water vapour condenses into liquid droplets. These droplets are generally formed around dust particles.
3. The small droplets float in the air and form clouds. Cloud droplets are liquid water, distinct from the invisible water vapour from which they formed.
4. Many small droplets join together to form larger drops of water. The drops therefore do not all remain separate tiny droplets.
5. Some drops become heavy enough to fall as rain. Under special conditions, water might also fall as hail or snow.

Q7. Explain why clothes commonly dry faster on windy days but slowly on rainy days. Define the term used for the amount of water vapour in air. [3 marks]

1. Increased movement of air makes water evaporate faster. This explains the common observation that wet clothes dry faster on a windy day.
2. Humidity is the amount of water vapour in the air. Rainy days are more humid because the air contains more water vapour.
3. When the amount of water in the air is already high, evaporation is slow. Clothes therefore commonly dry slowly under rainy-day conditions.

Q8. Describe the water cycle and explain two reasons why careful use of water remains necessary despite this circulation. [3 marks]

1. Water evaporates from the oceans and Earth's surface into the atmosphere, returns as rain, hail or snow, and ultimately flows back to the oceans.
2. Only a small portion of Earth's water is fit for use by plants, animals and humans. Most is in oceans and cannot be used directly.
3. Population growth increases the number of people using water. Increasing demand causes shortages in many parts of the world, so water should be used wisely.

Key takeaways

• Ice, liquid water and water vapour are three states of the same substance, with different properties of shape and spreading.
• Evaporation changes liquid water into invisible water vapour and takes place continuously, even at room temperature.
• Condensation changes water vapour into liquid droplets, as seen on the outside of a cold tumbler.
• The first weighing experiment alone is inconclusive about seepage; marking the water level adds the observation needed to investigate it.
• Melting changes a solid into a liquid, while freezing changes a liquid into a solid.
• Greater exposed area and increased air movement make evaporation faster; high humidity makes evaporation slower.
• Evaporation causes cooling, illustrated by sweat drying, water in earthen pots and the pot-in-pot cooler.
• Cloud droplets generally form around dust particles; many join together, and some become heavy enough to fall as rain.
• The water cycle circulates water between the Earth's surface and atmosphere, but only a small portion is fit for use by plants, animals and humans.

Test yourself

What happens to an ice cube's shape when it is transferred between differently shaped containers?

It retains its own shape because it is water in the solid state.

Why does water disappearing from a steel plate not mean that it has ceased to exist?

The liquid water changes into water vapour through evaporation and becomes invisible in its gaseous state.

What makes steam visible if water vapour is invisible?

Tiny water droplets formed from some of the vapour make steam visible.

What extra observation is obtained by marking the water level in the condensation activity?

The water level does not go down while extra water collects on the outside of the tumbler.

Why is an equal amount of water used in the bottle cap and plate comparison?

The amount of water is kept the same while the exposed area is changed.

Why does a fan help us feel cooler when we are sweating?

Moving air helps sweat evaporate faster, and evaporation produces a cooling effect.

What qualification should accompany the 4 to 5 hour cooling time for the pot-in-pot model?

The time range can be influenced by many conditions; it is not an unconditional guarantee.

Why does the existence of the water cycle not justify wasting water?

Only a small portion of Earth's water is fit for use, most water is in oceans and cannot be used directly, and increasing demand causes shortages.