CLASS 6 SCIENCE · CHAPTER 8
These notes follow the chapter’s sequence and explain its central ideas, examples and activities. Read each section, connect it with the textbook observations, and use the quick check to review the main concepts.
Puddles shrink after rain, and wet clothes dry even when water is not poured away. These observations show that liquid water can change into water vapour and mix with the surrounding air. This process is evaporation. It can occur at the surface of a liquid and does not require the water to boil. Water vapour is not usually visible, so the water may appear to have disappeared even though it has changed state.
Water vapour can change back into liquid water through condensation. Droplets forming on a cool surface provide an example: water vapour in the surrounding air cools and condenses. Evaporation and condensation are opposite changes of state. The amount and rate of evaporation depend on conditions such as exposed surface area, temperature and movement of air. These factors help explain why a spread-out wet cloth or moving air can make drying faster.
Water occurs as solid ice, liquid water and gaseous water vapour. Ice has a definite shape, liquid water flows and takes the shape of its container, and water vapour spreads through the space available. These are different states of the same substance. In the natural world, water may be found as ice or snow in cold regions, as liquid in rivers and lakes, and as vapour in the air. Changes between these states are part of the water cycle.
When ice gains enough heat, it melts into liquid water. When liquid water loses heat sufficiently, it freezes into ice. Heating liquid water can produce water vapour; boiling is a rapid change that occurs under suitable conditions. Cooling water vapour can cause condensation. These changes help explain everyday observations such as ice melting, water freezing and droplets appearing on a cool surface. During an investigation, observe and record what changes and what conditions are applied.
Evaporation can produce a cooling effect because the process takes energy from the surroundings. This helps explain why evaporation from a wet surface can make it feel cooler. Water evaporating from water bodies and land contributes water vapour to the atmosphere. As moist air cools, vapour condenses into tiny droplets that form clouds; droplets can combine and fall as rain. The chapter links everyday changes in water with larger processes in nature and reminds us that water is a resource we must use responsibly.
Key idea: Puddles shrink after rain, and wet clothes dry even when water is not poured away.
Observe the examples in the chapter, identify the main idea in each section, and connect the concept with the textbook activities. Use accurate terms, explain your reasoning, and review the key ideas before answering questions.