A Journey through States of Water — NCERT Solutions, AI Tutor & Practice

Understand: Water is a single substance that can take different forms depending on temperature.

The three states:

1. Solid – water becomes solid when it is very cold. Example: ice cubes, snow, hailstones.
2. Liquid – the ordinary flowing form of water. Example: water in a river, rain, water in a glass.
3. Gas – water in the form of water vapour (invisible gas). Example: the vapour that leaves a boiling kettle (the visible 'steam' is actually tiny water droplets).

Conclude: The same water can change from one state to another when it is heated or cooled.

Understand: Water can slowly turn into vapour without boiling.

Definition: Evaporation is the process in which water changes from the liquid state to the gaseous state (water vapour) at the surface, at temperatures below its boiling point.

Examples:

1. Wet clothes drying on a clothesline in sunlight.
2. Water sprinkled on a floor disappearing after some time.
3. Sweat drying from our skin.

Conclude: Evaporation happens slowly and only from the surface of the water.

Understand: Evaporation can be fast or slow depending on the surrounding conditions.

Factors that increase the rate of evaporation:

1. Higher temperature – more heat gives water particles more energy to escape.
2. Larger surface area – more surface exposed means faster evaporation (a spread-out puddle dries quicker than a deep one).
3. Faster wind/air movement – blowing air carries away vapour, allowing more to escape.
4. Lower humidity – dry air holds more vapour, so evaporation is faster.

Conclude: On a hot, dry, windy day, water evaporates fastest.

Understand: Condensation is the reverse of evaporation.

Definition: Condensation is the process in which water vapour (gas) changes into liquid water on cooling.

Explaining the cold glass:

1. The air around us always contains invisible water vapour.
2. The cold surface of the glass cools the air touching it.
3. This cooled water vapour loses energy and turns into tiny liquid droplets.
4. These droplets collect on the outer surface of the glass.

Conclude: The water on the glass comes from water vapour in the air, not from inside the glass.

Understand: Water changes state at particular fixed temperatures.

Melting point: the temperature at which a solid (ice) changes into a liquid (water). For water it is about $0^\circ\text{C}$.

Boiling point: the temperature at which a liquid (water) rapidly changes into a gas (vapour) throughout the liquid. For water it is about $100^\circ\text{C}$ (at normal sea-level pressure).

Conclude: Ice melts at $0^\circ\text{C}$ and water boils at $100^\circ\text{C}$.

Understand: Water keeps circulating between the Earth's surface and the atmosphere.

Steps of the water cycle:

1. Evaporation: Heat from the Sun evaporates water from oceans, rivers and lakes into water vapour.
2. Transpiration: Plants also release water vapour into the air.
3. Condensation: The vapour rises, cools at higher altitudes, and condenses into tiny droplets forming clouds.
4. Precipitation: When droplets grow heavy, they fall as rain, snow, or hail.
5. Collection/Runoff: The fallen water flows through rivers back into the oceans, and the cycle repeats.

Conclude: The water cycle involves evaporation, condensation, and precipitation, continuously recycling Earth's water.

Understand: Water behaves unusually when it freezes.

Analyze:

• When water freezes, the water particles arrange in a way that takes up more space, so the volume increases (water expands on freezing).
• Because volume increases but mass stays the same, ice becomes less dense than water, which is why ice floats on water.

Everyday consequence: A completely filled glass bottle of water may crack or burst in the freezer, because the expanding ice pushes against the walls. Similarly, water pipes can burst in very cold weather.

Conclude: Water is special — it expands when it freezes, unlike most substances.

Understand: Both dew and fog result from condensation, but they form differently.

Formation of dew:

1. At night, surfaces such as grass, leaves and metal cool down.
2. Water vapour in the air touching these cold surfaces condenses.
3. It forms tiny water droplets on the grass — this is dew.

Fog:

• Fog forms when water vapour in the air condenses into tiny droplets that float in the air itself (not on a surface), reducing visibility.

Difference: | Dew | Fog | |-----|-----| | Droplets form on cool surfaces | Droplets float in the air | | Seen on grass/objects | Seen as a cloud near the ground |

Conclude: Dew settles on surfaces, while fog is suspended in the air.

Understand: The question tests the effect of surface area on evaporation.

Analyze:

• When the handkerchief is spread out, a large surface of the wet cloth is exposed to the air.
• More exposed surface means more water can evaporate at the same time.
• When folded, only a small area is exposed and much water is trapped inside layers, so evaporation is slower.

Conclude: A spread-out handkerchief dries faster because a larger surface area speeds up evaporation.

Understand: This uses the cooling effect of evaporation.

Analyze:

1. When water is sprinkled on a hot floor or roof, it begins to evaporate.
2. To change from liquid to vapour, water needs heat.
3. This heat is taken from the floor/roof and the surrounding air.
4. As heat is removed, the surroundings become cooler.

Conclude: Sprinkling water cools the area because evaporation absorbs heat from the surface — this is why we also feel cool when sweat evaporates from our skin.

Understand: Each process is a change from one state of water to another.

Matching:

| Process | State Change | |---------|--------------| | (a) Freezing | Liquid $\rightarrow$ Solid | | (b) Boiling | Liquid $\rightarrow$ Gas | | (c) Condensation | Gas $\rightarrow$ Liquid | | (d) Melting | Solid $\rightarrow$ Liquid |

Conclude: Heating causes melting and boiling; cooling causes condensation and freezing.

Understand: The water cycle recycles water, but usable freshwater is limited.

Analyze:

1. Most of Earth's water is salty ocean water, which we cannot drink or use directly.
2. Only a small fraction is fresh water, and much of that is locked in ice.
3. The water cycle may not deliver rain evenly — some regions face droughts.
4. Human activities pollute rivers and lakes, reducing clean water.
5. Growing population increases demand for the limited freshwater.

Conclude: Even though water is recycled, clean and usable freshwater is scarce, so we must conserve it by avoiding wastage and preventing pollution.