What this chapter is about
Water is everywhere around us — in rivers, clouds, ice cubes and even in the air we breathe. This chapter helps you understand how water changes from one form to another. You will learn about the three states of water: solid (ice), liquid (water) and gas (water vapour).
You already see these changes in daily life. Ice melts in your glass of lemonade. Wet clothes dry on the clothesline. Dew forms on grass in the morning. This chapter explains why these changes happen and how heat plays a role in each change.
After studying this chapter, you should be able to name the three states of water, describe how water moves from one state to another, and explain everyday events like boiling, freezing and evaporation using simple science.
Key ideas
- States of matter means the different forms in which a substance can exist. Water has three states: solid (ice), liquid (water) and gas (water vapour).
- Melting is when a solid turns into a liquid. Ice melts into water when it gets warm.
- Freezing is when a liquid turns into a solid. Water freezes into ice when it gets very cold.
- Evaporation is when a liquid slowly turns into a gas without boiling. Wet clothes dry because water evaporates into the air.
- Boiling is when a liquid turns into a gas quickly at a certain temperature. Water boils at 100 degrees Celsius.
- Condensation is when a gas turns into a liquid. Water vapour in air becomes tiny water droplets on a cold glass.
- The water cycle is the continuous movement of water between the earth and the sky through evaporation, condensation and rainfall.
- Heat is needed to change water from solid to liquid or from liquid to gas. Removing heat changes water from gas to liquid or from liquid to solid.
Formulas and facts to remember
- Water freezes at 0 degrees Celsius and boils at 100 degrees Celsius.
- Ice is lighter than liquid water, which is why ice cubes float in a glass of water.
- Water vapour is invisible. The white mist you see from a boiling kettle is actually tiny water droplets, not pure vapour.
- Evaporation happens at all temperatures, not just at 100 degrees Celsius.
- During condensation, water vapour loses heat and becomes liquid.
- The water cycle has no beginning or end — it keeps going forever.
- Adding heat makes particles in water move faster. Removing heat slows them down.
Worked examples
Example 1: Why does ice melt in your hand?
Suppose you hold an ice cube in your palm. Your body temperature is about 37 degrees Celsius. The ice is at 0 degrees Celsius or lower. Heat flows from your warm hand to the cold ice. This extra heat makes the ice melt and turn into water. That is why your hand feels cold — it is losing heat to the ice.
Example 2: Why do wet clothes dry faster on a sunny day?
Riya hangs her washed uniform outside. On a sunny day, the air is warm. Heat from the sun gives energy to the water in the cloth. The water particles gain enough energy to escape into the air as water vapour. This is evaporation. On a cloudy or rainy day, there is less heat, so evaporation is slower and clothes take longer to dry.
Example 3: Why do water droplets form on a cold bottle?
Arjun takes a bottle of cold water from the fridge. Within minutes, tiny droplets appear on the outside of the bottle. The air around the bottle contains water vapour. When this vapour touches the cold surface, it loses heat and turns into liquid water. This change from gas to liquid is condensation.
Common mistakes
- Thinking that steam and water vapour are the same thing → Steam you can see is tiny water droplets; true water vapour is invisible.
- Believing evaporation only happens when water boils → Evaporation can happen at any temperature, even on a cool day.
- Confusing melting with dissolving → Melting is a state change; dissolving is when a substance mixes into a liquid like sugar in tea.
- Thinking ice sinks in water → Ice floats because it is less dense than liquid water.
- Assuming the water cycle starts with rain → The water cycle is continuous with no fixed starting point.
Quick revision
- Water exists in three states: solid (ice), liquid (water) and gas (water vapour).
- Melting, freezing, evaporation, boiling and condensation are changes of state.
- Heat is added to change solid to liquid or liquid to gas.
- Heat is removed to change gas to liquid or liquid to solid.
- The water cycle moves water between earth and sky endlessly.