--- title: "Separating Mixtures" book: "School Chemistry — Grades 1 to 12" subject: chemistry language: en chapter: 3 exercises: 11 source: https://one-course.com/books/chemistry/1/en/chapter/3-separating-mixtures license: CC-BY-NC-SA-4.0 credit: "One Chemistry Book, One Course (one-course.com)" --- # Chapter 3 — Separating Mixtures After a storm, the puddles in the road are brown: rain water mixed with soil, sand and bits of leaves. Yet the water that comes out of a kitchen tap is clear, and it too was rain once, before it ran into a river. How can muddy water be made clean again? Things that have been mixed can be taken apart. This chapter shows how, one method at a time. **You already know.** To [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) is to put different things together. A solid such as sugar or salt [dissolves](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve) in water: it disappears from sight, and the clear liquid is a solution. A dissolved solid is still there: when the water dries away, the solid is left behind. Sand and soil do not [dissolve](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve) ([Chapter 2](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#ch-g2-mixing-and-dissolving)). ## 3.1 Sorting by hand and sieving When the pieces of a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) are big enough to see and to pick up, the simplest way to separate them is by hand: raisins can be picked out of muesli one by one. When there are too many pieces, a sieve does the sorting for us. **Definition 3.1 (Sieving).** *Sieving* is separating the pieces of a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) by their size, with a sieve: a net or a plate full of holes. The small pieces fall through the holes; the big pieces stay in the sieve. **Example 3.2 (Gravel and sand).** A builder shakes a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) of gravel and sand over a sieve. The fine sand falls through into a heap below; the gravel stays on the mesh. ![Sieving a mix of gravel and sand: the pieces are sorted by their size.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/fig-3dc81a8a632c.svg) *[Sieving](#def-g3-separating-mixtures-sieve) a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) of gravel and sand: the pieces are sorted by their size.* ![A kitchen sieve: the fine flour goes through, the lumps stay behind.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/img-d8f135ed1ebc.jpg) *A kitchen sieve: the fine flour goes through, the lumps stay behind.* ## 3.2 Letting settle and decanting **Definition 3.3 (Settling and decanting).** A [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) of water and of a solid that does not [dissolve](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve) can be left to rest: the solid sinks slowly to the bottom. This is called *settling*. Once it has settled, the clear water above can be poured gently into another container, leaving the solid behind: this is *decanting*. ![Settling, then decanting a mix of soil and water.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/fig-92ff825e1d08.svg) *[Settling](#def-g3-separating-mixtures-decant), then [decanting](#def-g3-separating-mixtures-decant) a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) of soil and water.* **Remark 3.4 (Settling takes time).** Big, heavy grains settle in a few minutes; very fine clay can take a day or more, and the water stays a little cloudy meanwhile. [Decanting](#def-g3-separating-mixtures-decant) never removes every grain: a little cloudiness is poured off with the water. ## 3.3 Filtering **Definition 3.5 (Filtering and filtrate).** *Filtering* is pouring a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) through a filter: a sheet of paper, a cloth or a layer of sand with holes too small to let the solid pieces through. The solid stays on the filter; the clear liquid that passes through is called the *filtrate*. ![Filtering muddy water through a paper filter.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/fig-ec0c2958aaf5.svg) *[Filtering](#def-g3-separating-mixtures-filter) muddy water through a paper filter.* **In the lab — Filtering muddy water.** The teacher folds a round sheet of filter paper into a cone and sets it in a glass funnel above a beaker. Muddy water is poured into the cone, a little at a time, never above the edge of the paper. Clear water drips into the beaker; the soil makes a brown layer on the paper. When the [filtrate](#def-g3-separating-mixtures-filter) is cloudy, the paper is torn or the water went over its edge. **Proposition 3.6 (A filter does not stop what is dissolved).** A filter holds back the solid pieces that are not dissolved. It does not hold back what is dissolved: salty water poured through a filter comes out just as salty, and sweet tea just as sweet. ![Making coffee is filtering: the grounds stay in the paper, the coffee is the filtrate.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/img-a1a257093cb9.jpg) *Making coffee is [filtering](#def-g3-separating-mixtures-filter): the grounds stay in the paper, the coffee is the [filtrate](#def-g3-separating-mixtures-filter).* ## 3.4 Evaporating the water away **Definition 3.7 (Evaporating).** To get back a solid that is dissolved in water, the water is left to dry away, in the sun or on a gentle heat: this is *evaporating*. The water goes into the air; the solid stays in the dish. **Example 3.8 (Salt from salty water).** A little salty water is poured into a shallow dish and left on a sunny window sill. Days later there is no water left, only a white crust of salt: the salt that was dissolved. ![Evaporating, in the laboratory: the dish is warmed gently on a hotplate by the teacher. The salt stays in the dish.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/fig-6b13cdfa5cdb.svg) *[Evaporating](#def-g3-separating-mixtures-evaporate), in the laboratory: the dish is warmed gently on a hotplate by the teacher. The salt stays in the dish.* ## 3.5 Which method for which mixture? **Method 3.9 (Choosing a separation).** To take a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) apart, ask: 1. Are the pieces big and of different sizes? Sort by hand, or sieve. 2. Is there a solid that does not [dissolve](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve) , in a liquid? Let it settle and decant, or filter for clearer water. 3. Is there a solid dissolved in the liquid? Evaporate the liquid: the solid stays. Several steps are often needed, one after the other. **Example 3.10 (Sand, salt and water).** A jar holds water with sand and salt in it. The salt has dissolved; the sand has not. 1. Filter: the sand stays on the paper. 2. The [filtrate](#def-g3-separating-mixtures-filter) is clear, but it is salty water. Evaporate it: the salt stays in the dish. Two methods, one after the other, give back the sand and the salt. **Example 3.11 (Clean water from a river).** A water-treatment plant makes river water fit to drink in several steps: 1. the river water passes through screens, big grids that stop branches, leaves and plastic: a kind of [sieving](#def-g3-separating-mixtures-sieve) ; 2. it rests in large tanks where mud settles to the bottom; 3. it is filtered through thick beds of sand that hold back the finest grains; 4. a very small amount of a product that kills germs is added, so that the water stays safe in the pipes. ![The steps of a water-treatment plant. Under each step: what it removes.](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/fig-a14754376a13.svg) *The steps of a water-treatment plant. Under each step: what it removes.* ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/img-3dd4abaab05d.jpg) ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g3-separating-mixtures/img-2fbbf857ec60.jpg) *[Settling](#def-g3-separating-mixtures-decant) tanks of a water-treatment plant.* *Panning for gold: the river water carries the light sand away; the heavy grains stay in the pan.* **Remark 3.12 (Clear is not always clean).** Water that has been filtered can look perfectly clear and still contain dissolved substances, and germs far too small to see. That is why nobody drinks water from a puddle or a stream, even after [filtering](#def-g3-separating-mixtures-filter) it. ## 3.6 Exercises **Exercise 3.1 ★.** Which method would you use to separate peas from rice grains? **Solution of Exercise 3.1.** [Sieving](#def-g3-separating-mixtures-sieve): the rice grains are smaller than the peas and fall through a sieve with the right holes. (Sorting by hand also works, slowly.) **Exercise 3.2 ★.** A glass of muddy water is left on a table for an hour. What happens to the soil? What is this called? **Solution of Exercise 3.2.** The soil sinks slowly and forms a layer at the bottom; the water above becomes clearer. This is [settling](#def-g3-separating-mixtures-decant). **Exercise 3.3 ★.** What is a [filtrate](#def-g3-separating-mixtures-filter)? Give the [filtrate](#def-g3-separating-mixtures-filter) when coffee is made. **Solution of Exercise 3.3.** The [filtrate](#def-g3-separating-mixtures-filter) is the liquid that passes through the filter. When coffee is made, the [filtrate](#def-g3-separating-mixtures-filter) is the coffee in the jug; the grounds stay in the paper. **Exercise 3.4 ★.** Salty water is poured through a filter paper. Is the [filtrate](#def-g3-separating-mixtures-filter) salty? Explain. **Solution of Exercise 3.4.** Yes. The salt is dissolved, and a filter does not hold back what is dissolved: it passes through with the water. **Exercise 3.5 ★.** How can the sugar be got back from a glass of sugar water? **Solution of Exercise 3.5.** By [evaporating](#def-g3-separating-mixtures-evaporate) the water: in the sun, or gently heated, the water goes into the air and the sugar stays in the dish. **Exercise 3.6 ★.** Look at the figure of the sieve. Which pieces stay in the sieve? Why do the others fall through? **Solution of Exercise 3.6.** The gravel stays in the sieve. The grains of sand are smaller than the holes, so they fall through. **Exercise 3.7 ★.** Match each [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) with a method — [sieving](#def-g3-separating-mixtures-sieve), [filtering](#def-g3-separating-mixtures-filter) or [evaporating](#def-g3-separating-mixtures-evaporate): (a) sand and pebbles; (b) chalk powder in water; (c) salt in water. **Solution of Exercise 3.7.** (a) [Sieving](#def-g3-separating-mixtures-sieve). (b) [Filtering](#def-g3-separating-mixtures-filter) (or [settling](#def-g3-separating-mixtures-decant) and [decanting](#def-g3-separating-mixtures-decant)). (c) [Evaporating](#def-g3-separating-mixtures-evaporate). **Exercise 3.8 ★.** Look at the figure of the water-treatment plant. How many steps does the water go through? Which step removes the mud? **Solution of Exercise 3.8.** Four steps: screens, [settling](#def-g3-separating-mixtures-decant) tanks, sand filters, germs killed. The [settling](#def-g3-separating-mixtures-decant) tanks remove the mud. **Exercise 3.9 ★.** Put these steps in order to get back the sand and the salt from a jar of sand, salt and water: evaporate the [filtrate](#def-g3-separating-mixtures-filter); filter; collect the sand from the paper. **Solution of Exercise 3.9.** Filter; collect the sand from the paper; evaporate the [filtrate](#def-g3-separating-mixtures-filter) to get the salt. **Exercise 3.10 ★★.** Can a filter make sea water fit to drink? Explain why not. **Solution of Exercise 3.10.** No. The salt of sea water is dissolved, and a filter does not hold back what is dissolved: the [filtrate](#def-g3-separating-mixtures-filter) is still salty water. **Exercise 3.11 ★★.** Tea leaves are left in a teapot full of tea. Name two methods, seen in this chapter, that separate the tea from the leaves. **Solution of Exercise 3.11.** Pouring the tea off gently once the leaves have settled ([decanting](#def-g3-separating-mixtures-decant)), or pouring it through a strainer or a filter ([sieving](#def-g3-separating-mixtures-sieve) or [filtering](#def-g3-separating-mixtures-filter)).