Particles, Dissolving, and Separating Mixtures

A spoon is an object. Metal is a material the spoon can be made from. A property tells you something about that material. Tests help you find properties that looks alone can hide.

Scratches and breaks

Hardness means how difficult a material is to scratch or dent. In a scratch test, the harder material scratches the softer one. Your fingernail can scratch candle wax or soap. It cannot scratch a glass marble, so glass is harder than your nail.

Hard does not mean unbreakable. Glass is hard but brittle: a fall can make it shatter. Soft rubber can stretch without breaking. Scratching, bending, and breaking are different tests.

What happens to light?

Hold each sample between a flashlight and a wall. Compare the light reaching the wall. Also look through each sample to check whether the view is sharp or blurry. This helps you tell transparent and translucent materials apart.

Color and shine are properties too. A reflective surface bounces light back well, like a mirror. But several white powders can look alike. Sugar, salt, baking soda, and flour need more than a color check to tell them apart.

Try a magnet

A magnet pulls on iron, nickel, cobalt, and most steel. A steel paper clip is a useful test object. Aluminum foil and copper wire are metals, but a magnet does not attract them. Many stainless steel spoons are not attracted either. Test the actual object.

At a recycling center, a magnet can lift steel cans away from aluminum cans. Their different responses make sorting possible.

Make the comparison fair

Change the material and keep the test the same. Use the same magnet at the same distance. Keep the flashlight position fixed. In a scratch test, use the same tool and pressure.

Record results in a table with a row for each sample. For example, write, "Glass marble: hard, transparent, not magnetic." A table lets you compare several properties without relying on memory.

Different materials let electricity and heat pass through at different speeds. These properties help explain the parts of a cable, a pan, or a cooler.

Test electricity with a circuit

An electrical conductor lets electricity flow easily. An electrical insulator does not. Copper, aluminum, iron, and steel are good conductors. Plastic, rubber, glass, and dry wood are insulators.

Use batteries for circuits, never outlets. Keep water away from plugged-in devices.

1. Make a circuit with a battery, wires, and a bulb.
2. Leave a gap between two wire ends.
3. Touch both ends to your sample, filling the gap.
4. Record whether the bulb lights.

If it lights, the material conducts. If it stays dark, too little electricity passes to light that bulb. Check the connections before judging the sample. A dark bulb can mean an insulator or a weak conductor.

Aluminum foil and a steel paper clip light a bulb in a working test circuit. A plastic ruler, rubber band, and wooden craft stick keep it dark. Graphite, the material inside a pencil, is a nonmetal that conducts. It may make the bulb glow dimly.

A cable has copper inside to carry electricity. Its plastic cover is an insulator that keeps your hands away from the conductor.

Test heat traveling

A thermal conductor lets heat travel easily. A thermal insulator slows heat moving through it. Metals conduct heat well. Wood, plastic, wool, cork, foam, and trapped air slow it down.

Here is a spoon test to watch with an adult. Put a small bead on each spoon handle using equal dabs of butter. Use metal, plastic, and wooden spoons with handles of similar length. An adult pours hot water into a cup and stands the spoons in it to the same depth. The butter on the metal handle melts first. Its bead falls first because heat travels up metal fastest.

Keep warmth in or out

A pan's metal body carries heat to food. Its wooden or plastic handle slows heat reaching your hand. Oven mitts and cup sleeves also slow heat transfer.

Insulators do not make heat. A coat slows heat leaving your warm body. Wool around an ice cube slows heat entering from warmer air. So an insulator can help keep something warm or cold.

Metal and wood left in the same room reach the same temperature. Metal feels colder against your warm hand because it takes heat away faster. A thermometer helps you check temperature without confusing it with how something feels.

An umbrella and a towel both meet water. But they need very different properties. Start with the job, then choose a material that can do it.

Name the useful properties

A waterproof material keeps water from passing through. An absorbent material soaks water up. Plastic sheet is waterproof; a sponge is absorbent.

A flexible material bends without breaking. A rigid material keeps its shape. Fabric can bend around your body. A wooden shelf needs to stay rigid under its load.

Use four steps when you choose:

1. Say what the object needs to do.
2. Name the properties it needs.
3. Compare possible materials using tests.
4. Choose a material and give your evidence.

Work through a window choice

A window must let daylight in, give a clear view, and keep rain out. That calls for a transparent, waterproof material. It also needs to resist scratches. Glass fits these jobs. Clear plastic can fit too. Wood blocks the view, while ordinary cloth lets rain through.

An answer such as "Glass is nice" gives no useful reason. "Glass is transparent, so you can see outside" connects a property to the job.

Use measurements to decide

Leo tested three materials for wiping spills. He used equal-sized squares, 50 mL of water, and 30 seconds each time.

For the paper towel, subtract the water left from the starting water: 50 minus 38 is 12 mL. It absorbed the most in this test, so it is the best spill wiper of these samples. The plastic would suit a rain cover instead.

Give each part its own job

One object can use several materials. A lamp cable needs copper inside for conduction and flexible plastic outside for insulation. A pan needs a metal body for heat transfer and an insulating handle. A material's usefulness depends on the job you want that part to do.

An inflated ball feels different from a flat one. Nothing visible was poured inside, yet air has changed it. Air is one clue that matter can be present without being seen.

Matter has mass and fills space

Matter is anything that has mass and takes up space. Wood, water, and air are matter. Light and sound are not matter.

Blowing up a balloon shows that air takes up space. Pumping a basketball makes it a little heavier because air has mass. An open cup that looks empty already contains air.

You can see air keeping water out of a cup. Push a cup straight down into water, upside down, with a paper towel tucked inside. Trapped air occupies the cup, so water cannot fill that same space. The towel stays dry while the cup stays upside down without tilting and the air remains trapped.

Small pieces explain big clues

All matter is made of particles far too small to see. A magnifying glass or classroom microscope cannot show these particles. Even one grain of salt contains a huge number of them.

Think about these observations:

• Sugar becomes invisible when stirred into water, but it is still present.
• A popcorn smell reaches you across a room.
• Food coloring slowly spreads through still water.
• A puddle dries as water moves into the air.

Particles spreading and moving help explain each clue. Invisible does not mean destroyed. The particles can be too spread out or too small for your eyes to detect.

Draw a useful model

A particle model uses simple marks to explain matter. Draw a small circle for each particle. Use matching circles for one substance. Use a different color or size for a second substance, and add a key that explains your symbols.

The circles are a model, not a photograph. Real particles are not little colored toy balls. The drawing leaves out many particles so you can see the arrangement clearly.

There is empty space between particles. Do not fill the gaps with tiny patches of air: air also consists of particles. When matter gets warmer, its particles move faster. They do not grow bigger. Keep each circle the same size when you draw the same substance at different temperatures.

A wooden block keeps its shape. Water changes shape when poured. Air spreads through a whole container. Their particles explain these different behaviors.

Compare the arrangements

Draw solids as closely packed circles in rows. Draw liquid circles close together in an uneven arrangement. Spread gas circles far apart across the whole container. Solid particles still move: they jiggle without leaving their fixed places.

Pour 100 mL of water from a narrow glass into a wide bowl. Its shape changes, but its volume stays 100 mL. Its particles slide past each other while remaining close together.

Why air can be squashed

Air trapped inside a covered syringe can be pushed into less space. Its particles move closer together. Water in a covered syringe hardly squashes because its particles are already close. Solids are also hard to squash. Squashing a gas changes the gaps, not the size of its particles.

Sand and sugar can pour, but each grain is a solid. The grains slide over each other and form heaps. This movement of whole grains is different from liquid particles sliding past one another.

Follow a change of state

Heating gives particles more energy. During melting, solid particles leave their fixed places and start sliding. Ice melts at 0 °C (32 °F). During freezing, liquid particles settle into fixed places again.

During evaporation, water particles escape from a liquid's surface. This happens in an ordinary puddle. During boiling, gas bubbles form throughout the water. Water boils at 100 °C (212 °F) at sea level. Those bubbles contain water vapor, not air.

Cooling slows particles down. Water vapor condenses into liquid drops, as on a cold window. Water vapor itself is invisible.

Ice, liquid water, and water vapor contain the same water particles. Only their arrangement and motion change. In a sealed bag, 50 g of ice becomes 50 g of liquid water. No water particles were lost during melting.

Stir sugar into water and the grains shrink out of sight. You have made a mixture with its own name: a solution. The sugar is still there, spread through the water.

Name the three parts

Dissolving happens when solute particles spread evenly through a solvent. Moving water particles meet the sugar's surface. Sugar particles break away and spread among water particles. You cannot see the individual particles.

Soluble or insoluble?

A substance is soluble in a liquid if it dissolves there. Sugar, salt, instant coffee, and drink powder dissolve in water. Sand, gravel, flour, and chalk are insoluble in water.

Sand grains remain visible and sink. Flour makes water cloudy and settles slowly. A solid that still leaves visible bits has not dissolved completely. It may be insoluble, still dissolving, or more than the water can dissolve.

A solution is see-through and looks the same throughout. Its dissolved solute does not settle out just because you stop stirring. A colored solution can still be transparent. You can see through water colored with a little food coloring; transparent does not mean colorless.

Dissolving and melting are different

Melting changes a solid into its own liquid when heated. Ice becomes liquid water without another substance being added. Dissolving involves a solute and a solvent. It can happen in cold water.

Say, "The sugar dissolved in my tea." Hot tea does not turn sugar into liquid sugar. It helps the sugar mix among the water particles. You can recover a dissolved solid such as salt by slowly evaporating its water.

The solvent matters

Sugar and salt dissolve in water, but they do not dissolve in cooking oil. You must name the solvent when describing solubility. Water dissolves many substances, but it does not dissolve everything.

Solutes can also be gases or liquids. Carbon dioxide dissolved in soda makes it fizzy. Oxygen dissolved in pond water helps fish breathe. Vinegar mixes evenly into water. Cooking oil does something different: it forms a separate layer. Shaking breaks that layer into drops, but the oil separates again when left standing.

One spoonful of salt can dissolve completely. Keep adding salt to the same water, and some will stay at the bottom. The amount of water and its temperature help set this limit.

When no more dissolves

A saturated solution cannot dissolve any more of that solute at that temperature. Extra solid remains even after you stir well and wait.

Imagine adding salt in 5 g portions to 100 mL of water at about 20 °C. The first 35 g dissolve. Your next portion brings the total to 40 g. About 36 g can dissolve, leaving about 4 g on the bottom. The solution above that solid is saturated.

Solubility tells how much solute can dissolve in a certain amount of solvent at a certain temperature. At about 20 °C, 100 mL of water dissolves about 36 g of salt or about 200 g of sugar. Sugar is much more soluble in water.

Change the water

Twice as much water can dissolve twice as much salt at the same temperature. So 200 mL can dissolve about 72 g. Compare different solutes using equal amounts of water at the same temperature.

Warmer water dissolves more of most solids. The increase depends on the substance. Near boiling, the amount of sugar can reach nearly 490 g for the water sample above. Salt rises only to about 39 g. Heating does not increase every solid's solubility by the same amount.

Gases behave differently. Less gas stays dissolved in warmer water. An open warm soda loses its carbon dioxide faster and becomes flat sooner.

Amount and speed are different

Stirring or crushing helps a solid dissolve faster. Neither changes the final limit for the same amount of water at the same temperature. Stirring a saturated salt solution cannot make unlimited salt dissolve.

Concentrated or dilute?

A concentrated solution has a lot of solute compared with its solvent. A dilute solution has little solute compared with its solvent.

Two scoops of drink powder in 500 mL of water make a more concentrated drink than two scoops in 1,000 mL. Adding water spreads the same powder through more liquid. It makes the solution more dilute without removing any powder. Concentrated describes the comparison of amounts; saturated says a dissolving limit has been reached.

A sugar cube can take several minutes to dissolve. You can shorten the wait by warming the water, stirring it, or breaking the cube into smaller pieces. Each change helps particles leave the solid's surface.

Explain the three changes

Warmer water: water particles move faster. They meet the solid more often and harder, helping solute particles break away more quickly.

Stirring: moving liquid carries dissolved particles away from the solid. Fresh water reaches the surface and can take in more solute particles.

Smaller pieces: breaking a lump exposes more surface to water. More solute particles can leave at the same time. Use equal masses when comparing a cube with loose grains.

Crushing does not make each tiny particle smaller. It makes smaller pieces, each still containing many particles.

Plan the temperature test

Ask, "Does water temperature change how quickly a sugar cube dissolves?"

1. Use the same kind of cup and the same amount of water.
2. Use matching sugar cubes of the same mass.
3. Change only the water temperature.
4. Keep stirring the same, or do not stir any cup.
5. Time from adding the cube until no solid remains visible.

Use warm tap water; an adult handles hot water. A test with hot, stirred water and cold, still water changes two things. You cannot tell which change caused the different time.

Read a set of results

Ava used 100 mL of water, matching cups, and one matching cube each time. She did not stir.

The warmer water gave shorter times in her test. Repeat each test to see whether the times are similar. If one result is far from the rest, check the method and repeat that test.

A fair stirring test would keep temperature and piece size fixed. A fair piece-size test would keep temperature and stirring fixed. Always time the same endpoint: all the solid has dissolved.

Stirring and smaller pieces speed dissolving but do not raise solubility. Once the solution is saturated, extra solid stays. Warming can affect both speed and the amount of most solids that dissolves.

A scale can help you find sugar that your eyes cannot see. Dissolving spreads its particles through water. It does not remove them, so their mass still counts.

Add the two masses

Mass measures the amount of matter in something. A scale or balance measures it in grams (g) or kilograms (kg). When mass is conserved, the total stays the same.

The solution's mass equals the solvent's mass plus the solute's mass. For example, 300 g of water and 25 g of sugar make 325 g of solution. If you know the solution is 325 g and contains 25 g of sugar, subtract to find 300 g of water. These masses leave out the container.

The same idea works during melting. A sealed bag containing 50 g of ice still contains 50 g of water after the ice melts.

Compare the same things twice

To investigate dissolving, keep all the equipment in both readings:

1. Put the water cup, spoon, and paper holding dry salt on the scale together.
2. Record the total mass.
3. Tip the salt into the water and stir.
4. Return the spoon and empty paper to the scale beside the cup.
5. Compare the new total with the first one.

The paper and spoon belong in both readings. Salt stuck to them still counts if they remain on the scale. Adding or removing equipment would change the reading for a different reason.

Explain an unexpected reading

Look for matter that entered or left. A splash or spilled grains reduce the measured total if they leave the scale. A cup left open for days loses water particles by evaporation. Gas escaping from a fizzy mixture carries mass away too.

Also check the scale's zero and which objects were included. A lower reading does not mean dissolved salt became massless.

Use mass, not the water level

The liquid level need not rise by the space the dry grains used to occupy. In a simple particle picture, solute particles spread among water particles. The measured volume is not a reliable way to add their masses.

If you dissolve 20 g of salt and later evaporate all the water, about 20 g of dry salt remains. Both readings count the same salt particles, first spread out in solution and later gathered into crystals.

Trail mix is easy to recognize as a mixture. Salt water looks like one material, but it is a mixture too. Looking for separate pieces is not the only way to decide.

One substance or several?

A mixture contains two or more substances together. In simple mixing, each substance keeps its properties. Iron in sand still responds to a magnet. Dissolved salt is still salt.

A pure substance contains just one substance, such as pure water, sugar, or iron. The everyday word "pure" can mean something different. "100% pure orange juice" still contains water, sugars, and other substances. Tap water also contains dissolved substances, so it is a mixture.

Air is another mixture. It includes gases such as nitrogen, oxygen, carbon dioxide, and water vapor. Its parts are invisible to your eyes.

Find a useful difference

A separating method works by using a property that differs between the parts.

• Different piece sizes: choose a sieve.
• Magnetic and nonmagnetic materials: choose a magnet.
• An insoluble solid in liquid: settle and pour off, or filter.
• A dissolved solid: evaporate the liquid to recover the solid.

Salt water stays mixed because dissolved salt does not settle like sand. Its see-through appearance does not make it pure water.

Mixing can be undone

Mixing and dissolving are reversible changes: you can get the substances back. Removing water from salt water leaves salt behind.

Some substances react when brought together. Baking soda and vinegar fizz because a new gas forms. A sieve or filter cannot simply return the original baking soda and vinegar. The key difference is whether simple mixing kept the substances, or a change made new ones.

Two dry solids can be mixed without losing their properties. A sieve uses the sizes of their pieces. A magnet uses whether they are attracted to it.

Choose holes that fit between the sizes

A sieve is a mesh with holes. Small pieces fall through; larger ones stay above. Place a bowl underneath to collect what falls.

To separate sand and pebbles, choose holes bigger than the sand grains but smaller than the pebbles. Pour the dry mixture into the sieve and shake gently. Sand reaches the bowl, while pebbles remain in the mesh.

Sand grains range from about 0.06 mm to 2 mm across. Smaller bits include silt and clay; larger pieces are gravel. A sieve with 2 mm holes can separate typical sand from larger gravel.

The actual pieces matter more than the material names. Fine sand and table salt usually have similar-sized grains, so they can pass through together. Large rock salt pieces mixed with fine sand could be sieved if the holes fit between their sizes.

A kitchen strainer holds pasta while water drains. A garden sieve keeps stones out of soil. Sifting flour can remove large lumps.

Pull out the magnetic part

A magnet can remove iron filings from dry sand. Move it over the mixture so the iron sticks and the sand stays behind.

Put the magnet inside a plastic bag first. The filings collect on the bag's outside. Hold it over a clean dish and remove the magnet from the bag. The filings drop off into the dish.

This works for magnetic iron and most steel. It does not remove aluminum or copper. A shiny metal appearance does not tell you whether a magnet will work.

Keep the method useful

Work on a dry mixture when using these methods. Wet sand can clump, block sieve holes, or cling to iron filings. Removing the dry solids first makes later steps easier.

Dissolved salt passes through a sieve with water. Its particles are far smaller than the holes. Making the holes suitable for sand and pebbles will not make the sieve catch dissolved salt.

Muddy water holds solid bits mixed through a liquid. You can remove many of those bits by waiting, pouring carefully, and filtering. Each step depends on the solid staying undissolved.

Wait, then pour

During settling, insoluble solid bits sink to the bottom. The layer they form is sediment. The liquid above becomes clearer.

Decanting means gently pouring off the upper liquid. Tip the container slowly and stop before the sediment follows. Some fine particles may still float in the poured-off liquid. Settling and pouring do not catch every bit.

Set up a filter

1. Fold filter paper into a cone that fits a funnel.
2. Place the funnel over a collecting jar.
3. Pour the mixture slowly into the paper.
4. Keep the liquid below the paper's top edge.

Water goes through tiny openings in the paper. Sand grains stay behind because they are too big to pass through. The solid left on the paper is the residue. The liquid collected below is the filtrate.

Do not poke wet paper. It tears easily, and a tear would let solid grains through. Pouring above the paper's edge also lets the mixture go around the filter.

Follow sand and water

When you filter sand mixed with water, wet sand remains on the paper. Water collects underneath. Spread the sand out to dry if you want dry sand back. Coffee filters and tea bags also hold solid pieces while liquid passes through.

Follow salt through the same paper

Dissolved salt behaves differently from a sand grain. Its tiny particles pass through the filter along with water particles. The filtrate from salt water is still salt water.

If you filter a mixture of sand and salt water, sand is the residue and salt water is the filtrate. To recover the salt, the filtrate needs another method: evaporation.

Clear-looking water can still contain dissolved substances and germs. Never drink the water from a science filtering activity. The filter has removed visible solids, not made drinking water.

A dish of salt water can leave white crystals after drying. The salt did not arrive from the air. Its particles were present in the solution all along.

Follow the water out

During evaporation, water particles leave the liquid surface and move into the air. Salt particles stay in the dish. As less water remains, the solution becomes more concentrated and eventually saturated. Salt particles then gather into solid crystals.

Pour salt water into a shallow dish. Leave it in a warm place where it will not be spilled. After the water evaporates, salt crystals remain. This recovers the solid, but it does not collect the water. That water is now in the air as water vapor.

Help evaporation happen faster

Warmth, moving air, and a wide exposed surface speed evaporation. A shallow dish spreads the water out so more surface is open to the air. A deep, narrow jar exposes less.

Evaporation does not require boiling. A puddle dries at ordinary temperatures. Slow evaporation tends to grow larger crystals than fast evaporation.

Recover the same substance

If a solution was made with 20 g of salt, evaporating all the water leaves about 20 g of dry salt. The salt can dissolve again in fresh water. Getting it back shows that dissolving is reversible.

People use the same idea to collect sea salt. Seawater spreads into shallow coastal ponds. Sun and wind remove water, leaving salt behind. A white crust on a dried seaside rock has formed in the same way.

Take care with sugar and heating

Recover sugar from sugar water by slow evaporation in a warm place. Strong heating can brown sugar and make caramel, which contains new substances. Gentle evaporation keeps the separation from turning into a different change.

If heating is used, an adult does it. Never heat a closed container. Heating stops before the dish dries completely because salt can spit as the last water leaves. Let the dish cool before touching it.

A mixture with three materials may need several methods. At each step, ask what you have removed and what remains. The next method must work on what is left.

Plan before you begin

List the materials. Next to each, note a useful property: piece size, response to a magnet, or solubility in water. Choose a method for each difference. Put the dry methods first, before adding water.

Separate iron, sand, and salt

1. Use a magnet on the dry mixture. Iron filings stick; sand and salt remain.
2. Add enough water and stir. Salt dissolves. Sand stays as grains.
3. Filter. Wet sand stays as residue. Salt water passes through as filtrate.
4. Evaporate the filtrate. Water leaves and salt crystals remain. Dry the sand separately.

A magnet used after adding water may carry wet sand with the iron. Evaporating before filtering puts salt crystals back among sand grains. Filtering a dry sand-and-salt mixture keeps both solids together; the dissolving step is what first makes them behave differently.

Adapt the plan

For pebbles, sand, and sugar, first sieve out the pebbles. Add water to dissolve the sugar, filter out the sand, and evaporate the sugar water slowly. Strong heating would change the sugar.

For sandy, muddy seawater, let the solids settle first. Pour off the upper liquid and filter it to catch remaining fine solids. The filtrate still contains dissolved salt. Evaporation recovers that salt.

Check the recovered solids

Compare masses only after the recovered solids are dry. A 50 g dry mixture might give 10 g of iron, 30 g of sand, and 10 g of salt. Together these total 50 g. A smaller total points to spilled grains or material left on equipment. Wet sand can give a misleadingly high reading because its water adds mass.

Recognize when these methods will not work

Fine table salt and granulated sugar can have similar grain sizes. Neither is magnetic, both dissolve in water, and both pass through filter paper when dissolved. Evaporating their mixed solution returns the solids together.