CLASS 6 SCIENCE · CHAPTER 9
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.
Many useful materials occur mixed with unwanted substances. Pulses may contain small stones, harvested crops may include stalks and husks, and water may contain insoluble particles. Separation helps remove unwanted components or obtain a desired substance. The method depends on the properties of the materials, such as size, shape, weight, solubility or whether one component is attracted by a magnet. Before choosing a method, identify the components and decide what needs to be collected.
Handpicking is useful when unwanted pieces are large enough to be seen and removed by hand, as when small stones are picked out of pulses. Threshing separates grains from stalks by beating or using an appropriate process. Winnowing uses moving air to separate lighter husk from heavier grains. These methods appear in food preparation and farming. They work because the components differ in size, attachment or weight, and the method is chosen to take advantage of that difference.
Sieving separates solid particles of different sizes using a sieve. Smaller particles pass through its openings while larger ones remain. Sedimentation occurs when heavier insoluble particles settle at the bottom of a liquid if the mixture is left undisturbed. Decantation can then be used to carefully pour the upper liquid into another container without disturbing the settled material. These methods are useful in different situations, and the mixture’s properties determine which will work best.
Filtration separates an insoluble solid from a liquid by passing the mixture through a filter. The liquid passes through while the solid is retained, depending on the filter and particle size. Evaporation can be used to obtain a dissolved solid from a solution: as the liquid changes to vapour, the dissolved material remains behind. For example, salt can be obtained from salt solution by evaporating water. The two methods serve different purposes: filtration is for insoluble solids, while evaporation can recover a dissolved solid.
Some mixtures need more than one separation step. A method should be selected according to the properties of each component and the desired result. Magnetic separation can remove iron-containing pieces from a mixture; churning can separate butter from curd; and oil and water may form separate layers that can be separated carefully. Filtration is similar in purpose to the filtering action of nasal hair or masks in trapping particles, though the materials and circumstances differ. Safe, careful handling and a clear record of the procedure help make separation effective.
Key idea: Many useful materials occur mixed with unwanted substances.
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.