States of water and their properties
An ice cube taken out of the freezer turns to water on the table, and a wet floor slowly turns dry: water appears as ice, as water and as invisible vapour.
Water exists in three states: solid (ice) with its own shape and volume, liquid (water) with its own volume but the shape of its vessel, and gas (water vapour) that spreads everywhere. Water vapour is invisible; the white cloud over a hot cup is tiny droplets of water.
In the figure: Particles are packed in ice, loose in water and far apart in vapour.
Rule 1: Shape and volume — Compare how each state behaves in a vessel.
- Easy: Ice keeps its shape in any glass.
- Medium: Water takes the shape of the glass but its volume stays the same.
- Exam-type: Vapour fills the whole room and has no fixed shape or volume.
Rule 2: Seeing the invisible vapour — Steam you can see is not vapour.
- Easy: Real water vapour is a colourless gas in the air.
- Medium: The white cloud over a kettle is water droplets formed when vapour cools.
- Exam-type: Ice melts at 0 °C and water boils at 100 °C at sea level.
Name the state first, then describe its shape and volume.
Calling the visible white “steam” water vapour.
"Ice fixed, water fits, vapour spreads."
Practice questions
- 1Ice and water are —
- 2Water flows but ice —
- 3Ice retains its shape —
- 4Gases —
- 5Which of these is a solid?
- 6The solid state of water is —
- 7The gaseous state of water is —
- 8Water takes the shape of —
Show answers (try them yourself first)
- 1two states of the same substance
- 2does not flow
- 3irrespective of the container it is placed in
- 4do not have a fixed shape
- 5Stone
- 6ice
- 7water vapour
- 8the container in which it is kept
Evaporation
Wet clothes on a line dry even without boiling; the water goes into the air as invisible vapour.
Evaporation is the slow change of a liquid into vapour from its surface. It happens at every temperature, not only at the boiling point, and it needs no bubbles. The water that leaves is not lost; it goes into the air.
In the figure: The water level falls over days as vapour leaves the surface.
Rule 1: Where and when it happens — Look at the surface of the liquid.
- Easy: A puddle on the road slowly dries up.
- Medium: Evaporation happens only at the surface, so the inside of the liquid stays still.
- Exam-type: It happens at room temperature too.
Rule 2: Evaporation or boiling — Both make vapour but in different ways.
- Easy: Evaporation is slow, only from the surface, and at any temperature.
- Medium: Boiling is fast, bubbles form throughout the liquid, and it needs 100 °C for water.
- Exam-type: Washing dries on a sunny day because evaporation is quicker when warm.
To check evaporation, mark the water level at the start and compare it later.
Thinking the lost water has disappeared; it is only in the air as vapour.
"Slow, from the top, at any heat."
Practice questions
- 1The conversion of water into its vapour is called —
- 2Evaporation takes place continuously, even at —
- 3Water sprinkled on a hot dosa pan disappears because it —
- 4If water disappears from a steel plate without seeping through it, it has been converted into —
- 5Water left on washed utensils dries up because it —
- 6Which of these is an example of evaporation?
- 7Which of these also shows evaporation?
- 8Steam is actually —
Show answers (try them yourself first)
- 1evaporation
- 2room temperature
- 3gets converted into steam (water vapour)
- 4water vapour
- 5evaporates
- 6Drying of wet clothes
- 7A mopped floor drying
- 8water vapour, some part of which converts into droplets
Condensation
Water droplets appear on the outside of a glass of cold water. They did not leak from inside; they came from the air.
Condensation is the change of water vapour into liquid water when it cools. Air always contains some vapour. When the air touches a cold surface, the vapour loses heat and turns into tiny droplets. Dew on grass and fog are examples.
In the figure: Vapour from the air cools on the cold glass and forms droplets outside.
Rule 1: Cooling makes drops — The colder the surface, the more the drops.
- Easy: Dew forms on grass on a cool night.
- Medium: A mirror fogs up when warm breath touches it.
- Exam-type: Fog is a cloud of droplets near the ground.
Rule 2: Evaporation and condensation are opposites — Liquid to vapour and back again.
- Easy: Evaporation: liquid to vapour, heat is taken in.
- Medium: Condensation: vapour to liquid, heat is given out.
- Exam-type: The same water goes round again and again.
Ask: where did the vapour come from, and what cooled it?
Believing the drops on a cold glass came through the glass from the inside.
"Cold surface catches the vapour."
Practice questions
- 1Which machine collects water from humid air to produce drinking water?
- 2The conversion of water vapour into its liquid state is called —
- 3Droplets on the outer surface of a glass of cold water come from —
- 4When water is boiled in a half-filled utensil covered with a steel plate, drops collect on the plate's inner side because of —
- 5To show that water is not seeping out of the tumbler, you can —
- 6An Atmospheric Water Generator collects water by —
- 7Condensation occurs when water vapour comes in contact with —
- 8Dew drops on plants form due to —
Show answers (try them yourself first)
- 1Atmospheric Water Generator
- 2condensation
- 3water vapour in the air condensing
- 4condensation of vapour
- 5mark the water level and see that it does not go down
- 6cooling humid air so vapour condenses
- 7a cold surface
- 8condensation
Changing states, rate of evaporation and cooling
We put water in the freezer to make ice and heat it to make steam: heating or cooling changes the state. A wet cloth on the forehead cools us as it dries.
Heating changes ice to water (melting) and water to vapour (evaporation or boiling); cooling reverses them (condensation, freezing). Evaporation is faster when it is hotter, the surface is wider, the air is moving or the air is dry. Evaporating water takes heat from its surroundings and so causes cooling.
In the figure: Heat taken in turns solid to liquid to gas; heat given out reverses it.
Rule 1: What speeds up evaporation — Four things change how fast water dries.
- Easy: Hot weather dries clothes faster than cold weather.
- Medium: Clothes spread wide dry faster than a folded heap.
- Exam-type: A breeze helps, but damp humid air slows drying.
Rule 2: Evaporation cools — Evaporating water takes heat from what it is on.
- Easy: Sweat evaporating from the skin cools the body.
- Medium: Water in an earthen pot stays cool as water seeps out and evaporates.
- Exam-type: A wet cloth on a hot forehead lowers the heat.
To test a factor, change only that one thing and keep the rest the same.
Saying that evaporation heats the surroundings; it cools them.
"Warm, wide, windy, dry: dries fastest."
Quick table
| Factor | Effect on evaporation |
|---|---|
| Higher temperature | Faster |
| Wider surface | Faster |
| Moving air (wind) | Faster |
| Humid air | Slower |
Practice questions
- 1The conversion of a solid into a liquid is called —
- 2The conversion of a liquid into a solid is called —
- 3Candle wax can be turned into a liquid by —
- 4Water evaporates faster in —
- 5Evaporation causes —
- 6To change ice into water, we must —
- 7To change water into ice, water should be placed in —
- 8Coconut oil may become solid during the —
Show answers (try them yourself first)
- 1melting
- 2freezing
- 3heating it, and back into a solid by cooling it
- 4sunlight than in shade
- 5a cooling effect
- 6supply heat
- 7a cold environment such as a freezer
- 8winter season
Clouds, rain and the water cycle
The same water that left the sea comes back as rain, to fill rivers, wells and fields.
The Sun heats sea, lakes and rivers, and water evaporates. The vapour rises, cools and condenses into tiny droplets that form clouds. When droplets grow heavy they fall as rain, snow or hail. Water flows into rivers, soaks into the soil as groundwater and returns to the sea: this loop is the water cycle.
In the figure: Sun, evaporation, cloud, rain, then back to the sea: the water cycle.
Clouds, rain and the water cycle
In the app you can move this figure with your finger.
Open in the appRule 1: Steps of the cycle — Follow the water in order.
- Easy: Heat from the Sun makes water evaporate from seas and lakes.
- Medium: Vapour cools high up and condenses into clouds.
- Exam-type: Rain, snow or hail brings the water back to the ground.
Rule 2: Why the cycle matters — It gives us fresh water again and again.
- Easy: Sea water is salty, but rain is fresh because only vapour rises.
- Medium: Groundwater fills wells and borewells.
- Exam-type: Saving water and trees helps the cycle continue.
Draw the cycle as a loop and name each arrow: evaporation, condensation, rain, flow.
Saying that clouds are made of water vapour; they are made of droplets.
"Up as vapour, cloud, down as rain."
Practice questions
- 1Clouds form when water vapour in cooled air —
- 2As air moves higher above the Earth's surface, it becomes —
- 3Rain forms when —
- 4In Activity 8.10, haziness (clouds) appears above the water when —
- 5Only a small portion of the Earth's water is fit for use by —
- 6Water droplets in the air are generally formed around —
- 7Clouds are made of —
- 8Under special conditions, falling water may also be —
Show answers (try them yourself first)
- 1turns into tiny droplets
- 2cooler
- 3many droplets join into bigger drops that become heavy and fall
- 4a small burnt piece of newspaper is added
- 5plants, animals and humans
- 6dust particles
- 7tiny water droplets floating in the air
- 8hail or snow