Read the notes. Press Solid, Liquid and Gas on the picture, and Compress each one. Then press Heat again and again: the solid melts, then boils. Then answer the checks.
States of matter
Everything is made of particles. How the particles are arranged and how they vibrate or move decide the state: solid, liquid or gas.
Remember
Anything that has mass and fills space is matter. All matter is made of very small pieces, much too small to see: particles. The particles are never still: in a solid they vibrate in fixed places, and in a liquid or a gas they move from place to place. Forces of attraction hold them together: strong in a solid, weaker in a liquid and very weak in a gas.
| Solid | Liquid | Gas | |
|---|---|---|---|
| Particles | touch, in a regular pattern | touch, with no pattern | far apart |
| Movement | vibrate in fixed places | slide past each other | move fast in all directions |
| Shape | fixed | takes the shape of its container | fills its container |
| Volume: the space it fills (cm³) | fixed | fixed | fills its container |
| Flows? | no | yes | yes |
| Can you compress it? | no | no | yes |
Heating, cooling and compressing
When you heat a substance, its particles gain energy: in a solid they vibrate faster, and in a liquid or a gas they move faster. In a solid, the gaps between the particles get bigger, so the solid gets bigger: it expands. The particles themselves do not get bigger. When a substance cools, its particles lose energy, vibrate or move more slowly and come closer together, so it gets smaller: it contracts.
You can push a gas into a smaller volume, because its particles are far apart, with empty space between them: you can compress a gas. You cannot compress a solid or a liquid: their particles are already close together, with almost no empty space between them. A hotter gas pushes back harder, because its particles move faster and hit the walls harder: the same push compresses it less. A balloon gets bigger in hot water and smaller in ice water.
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “the little bits” | particles |
| “it shakes (in a solid)” | the particles vibrate |
| “the particles move (in a solid)” | the particles vibrate in fixed places |
| “it gets bigger when it is hot” | it expands |
| “it gets smaller when it is cold” | it contracts |
| “you can squash it” | you can compress it |
| “how much space it takes” | its volume |
Read the notes. Press each change of state on the picture. Then press Heat and cool water above the picture.
Changes of state
Heating or cooling can change the state of a substance. The particles stay the same; only how they are arranged and how they vibrate or move changes.
Remember
- A solid changes to a liquid: melting. A liquid changes to a solid: freezing.
- A liquid changes to a gas: evaporating, or boiling when bubbles of gas form all through the liquid. A gas changes to a liquid: condensing.
- A solid changes directly to a gas, with no liquid in between: sublimation. A gas changes directly to a solid, with no liquid in between: deposition.
Watch the real thing
Melting, evaporating, boiling and sublimation need energy: you heat the substance, and its particles gain energy and vibrate or move faster. In melting, they leave their fixed places and start to slide past each other. In evaporating, boiling and sublimation, they escape from the other particles and move far apart. Freezing, condensing and deposition happen when it cools: the particles lose energy and move more slowly. In condensing, they come close together again; in freezing and deposition, they become fixed in a regular pattern.
A pure substance (only one substance, with nothing else mixed with it) melts and freezes at the same temperature. That temperature is its melting point, and also its freezing point: water melts and freezes at 0 °C. Water boils at 100 °C: its boiling point. So the temperature decides the state of a substance: below its melting point it is a solid, between its melting point and its boiling point it is a liquid, and above its boiling point it is a gas. Each substance has its own melting point and boiling point. A liquid also evaporates slowly from its surface at lower temperatures, as when a puddle dries.
Water as a gas in the air is water vapour. Drops on a cold window in winter are water vapour from the air that has condensed. Dry ice (solid carbon dioxide) shows sublimation: it never makes a puddle.
A change of state makes no new substance, so it is a physical change: the mass stays the same, and you can reverse it: it is reversible. Dissolving sugar in tea and bending a wire are physical changes too: no new substance forms. When a new substance forms, as in burning, cooking or rusting, the change is a chemical change.
The water cycle: the Sun heats the sea and water evaporates; high in the sky the water vapour cools and condenses into the tiny drops of clouds; the drops fall to the ground as rain, snow or hail: precipitation.
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “ice turns into water” | it melts |
| “water turns into ice” | it freezes |
| “water turns into steam” | it evaporates (or boils) |
| “steam turns back into water” | it condenses |
| “a solid goes straight to a gas” | sublimation |
| “a gas goes straight to a solid” | deposition |
| “rain, snow or hail” | precipitation |
| “a new thing is made” | a chemical change |
Read the notes. Press Protons, Neutrons and Electrons on the picture. Then press The elements above the picture.
Atoms and elements
An element is made of only one type of atom. What is inside an atom decides which element it is.
Remember: inside an atom
The smallest particle of an element is an atom. In the centre of every atom is a tiny core: the nucleus. Some very small particles have an electric charge: positive (+) or negative (−). A positive charge and a negative charge attract each other.
- In the nucleus are particles with a positive charge (+): protons.
- Also in the nucleus are particles with no charge: neutrons. A particle with no charge is neutral.
- Around the nucleus move very small particles with a negative charge (−): electrons.
An atom has the same number of protons and electrons, so the charges cancel: the atom has no overall charge. The number of protons decides the element: every carbon atom has 6 protons.
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “the middle of the atom” | the nucleus |
| “plus and minus” | a positive and a negative charge |
| “it has no charge” | it is neutral |
Extension · counting charges
Remember: elements
A pure substance made of only one type of atom is an element. It cannot be split into simpler substances. Each element has a short code of one or two letters: its chemical symbol. It starts with a capital letter, sometimes followed by one small letter (Fe, never FE or fe). The table of all the elements is the periodic table. The 13 elements you need:
metalnon-metal
Some elements are shiny, and heat and electricity pass through them easily: they conduct heat and electricity. These elements are metals. Most metals are solid at the temperature of a normal room, room temperature (about 20 °C); mercury is a liquid. The other elements are non-metals: most are dull and do not conduct electricity. At room temperature some are solids (carbon, sulfur), many are gases (hydrogen, helium, nitrogen, oxygen, chlorine) and one, bromine, is a liquid. Mercury and bromine are the only two elements that are liquids at room temperature.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “the letters for an element” | its chemical symbol |
| “lets electricity through” | conducts electricity |
| “normal temperature” | room temperature |
Read the notes. Press each box of particles on the picture. Then press Count the particles, and Iron and sulfur, above the picture.
Molecules and compounds
Atoms can join together. What joins to what decides whether you have an element, a compound or a mixture.
Remember
Atoms can join together; what holds two atoms together is a bond. A particle made of two or more atoms joined together is a molecule.
- Some molecules have only one type of atom: they are still an element. Hydrogen (H₂), oxygen (O₂), nitrogen (N₂) and chlorine (Cl₂) are made of two atoms of the same element joined: they are diatomic.
- When two or more different elements are chemically joined, the substance is a compound: water H₂O, carbon dioxide CO₂, methane CH₄, ammonia NH₃, glucose (a sugar) C₆H₁₂O₆. In a compound, the elements are always in the same ratio: in water, always 2 hydrogen atoms for each oxygen atom. Salt (sodium chloride, NaCl) is a compound too, but it is not made of molecules: see A molecule or a lattice? below.
Symbols and small numbers can show the atoms in a substance: this is its chemical formula. The small number after a symbol counts the atoms of that element; no number means one. CO₂ is 1 carbon atom and 2 oxygen atoms.
Reading a picture of particles
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “atoms stuck together” | atoms joined by bonds: a molecule |
| “two of the same atom” | a diatomic molecule |
| “the letters and numbers” | the chemical formula |
Remember: a molecule or a lattice?
A molecule has an exact number of atoms. Every water molecule has 3 atoms: 2 hydrogen and 1 oxygen (H₂O). A bigger drop of water has more molecules, but each molecule still has exactly 3 atoms.
Salt (sodium chloride, NaCl) is a compound, but it is not made of molecules. It is made of particles with a charge, ions: sodium ions (+) and chloride ions (−). They are held in a regular pattern that repeats in every direction: a lattice. The whole crystal is one lattice, so no small piece of it has an exact number of ions: a bigger crystal has more ions. So NaCl does not count the ions. It gives their ratio: 1 sodium ion for each chloride ion.
Metals are the same. A piece of copper is a lattice of copper atoms: it has no molecules, and no exact number of atoms. Its formula is its symbol: Cu.
Remember: a compound has new properties
What a substance is like and how it behaves are its properties. A compound has different properties from the elements that make it. Iron is grey and magnetic; sulfur is yellow. Heated together, they react and make iron sulfide: a dark grey solid that is not magnetic. Sodium (a soft metal) and chlorine (a poisonous gas) make salt, which you can eat.
In a mixture, each substance keeps its own properties, you can mix the substances in any ratio, and you can separate them by a physical method, such as a magnet. You can separate the elements of a compound only by a chemical reaction.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “what it is like” | its properties |
| “a new thing is made” | a new substance forms: a compound |
Read the notes. Shine the torch through the three test tubes and leave them to stand. Then press Graph: salt on roads above the picture.
Mixtures
Most things around you are mixtures. Their substances are not joined, so you can separate them.
Remember
A substance is a type of matter that has specific properties and a uniform composition: it is the same all through, such as water, salt or oxygen.
When two or more substances are mixed but not chemically joined, you have a mixture. Each substance keeps its own properties, so you can separate them by physical methods. You can change how much of each substance there is (its composition can vary): one spoon of sugar in a cup of water, or five spoons, both make a mixture.
Only one substance, with no other substance mixed with it, is a pure substance: one element or one compound. Pure water, such as distilled water, melts at exactly 0 °C and boils at exactly 100 °C. Adding another substance changes these temperatures: salt water freezes below 0 °C and boils above 100 °C.
Mixtures around you: air (nitrogen, oxygen, argon, carbon dioxide and other gases), seawater (water with salts dissolved in it) and fruit juice. A metal mixed with other elements is an alloy: brass (copper and zinc), bronze (copper and tin), steel (iron and carbon).
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “only one thing in it” | a pure substance |
| “things mixed together” | a mixture |
| “metals mixed together” | an alloy |
Three kinds of mixture
In water, the particles of some solids, such as sugar and salt, spread between the water particles until you cannot see the solid: the solid dissolves. A solid that does not dissolve, such as chalk or sand, is insoluble in water.
| Solution | Suspension | Colloid | |
|---|---|---|---|
| What is spread through it | particles of a dissolved solid, far too small to see | small pieces of an insoluble solid | tiny droplets or pieces, too small to see |
| Can you see through it? | yes: it is transparent (it may have a colour) | no: it is cloudy | no: it is cloudy |
| Leave it to stand (keep it still) | nothing settles | the pieces settle to the bottom | nothing settles |
| Examples | salt water, sugar water | chalk powder or flour stirred into water | milk, fog |
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “see-through” | transparent |
| “it sinks to the bottom” | it settles |
| “cloudy, with bits that sink” | a suspension |
Extension · a torch in a colloid
A torch beam shows inside a colloid, because its tiny droplets scatter the light: the Tyndall effect.
Reading a graph
In winter, salt is spread on icy roads. The graph shows the freezing point of water with different masses of salt dissolved in it.
To read a graph: find your value on one axis, go straight to the line, then straight across (or down) to the other axis, and read its scale. Drag the red ruler on the graph. Freezing points below 0 °C are negative: write the minus sign (−).
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “the line goes down” | as the mass of salt increases, the freezing point decreases |
Read the notes. Make the drink on the picture stronger and weaker. Then use the graph and the chart above the picture.
Solutions
A solution forms when a solute dissolves in a solvent. You can say how strong it is, and how much of the solute will dissolve.
Remember
When the particles of a substance spread between the particles of a liquid, until you cannot see the solid, the substance dissolves. The substance that dissolves is the solute. The liquid it dissolves in is the solvent. Together they make a solution. Sugar does not melt in tea: it dissolves (melting is a change of state). No new substance forms, and the mass stays the same: 10 g of salt in 100 g of water makes 110 g of salt water.
If a substance can dissolve in a solvent, it is soluble in it; if it cannot, it is insoluble. This depends on the solvent: permanent-marker ink is insoluble in water, but it dissolves in ethanol.
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “the stuff that dissolves” | the solute |
| “the liquid it goes into” | the solvent |
| “sugar melts in tea” | sugar dissolves in tea |
| “it cannot dissolve” | it is insoluble |
Extension · why water dissolves sugar and salt
A water molecule has a slightly negative end (its oxygen atom, δ−) and a slightly positive end (its hydrogen atoms, δ+): water is polar. Opposite charges attract: they pull towards each other.
A sugar molecule also has slightly positive parts and slightly negative parts. Water molecules pull each sugar molecule away from the solid and surround it: each one turns the end with the opposite charge towards it. The sugar molecule stays whole. In salt, water molecules surround each ion: the oxygen end faces a sodium ion (+), and a hydrogen end faces a chloride ion (−). On the picture, press 🥄 Add a spoon, then Stir, and watch the lens. Do it again, then try Salt.
Oil does not dissolve in water: its molecules are not polar, so water does not attract them.
Concentrated or dilute
How strong a solution is depends on the mass of solute in each cm³ of solution: its concentration. A solution with many solute particles in each part is concentrated; a solution with few is dilute. Add more solute to the same volume: more concentrated. Add more solvent (water): more dilute.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “a strong drink” | a concentrated solution |
| “a weak drink” | a dilute solution |
| “the amount of sugar” | the mass of sugar, in g |
Solubility
The greatest mass of a solute that can dissolve in 100 g of water, at a particular temperature, is its solubility. When no more solute can dissolve, the solution is saturated. Most solids are more soluble in hot water. The graph shows the solubility of potassium nitrate at each temperature. Drag the red ruler on the graph.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “how much will dissolve” | its solubility |
| “no more will dissolve” | the solution is saturated |
Investigations
In an experiment, the thing you change is the independent variable. The thing you measure is the dependent variable. The things you keep the same, so that the test is fair, are the control variables. On a graph, the independent variable is on the horizontal axis (the x-axis), and the dependent variable is on the vertical axis (the y-axis).
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “the thing I changed” | the independent variable |
| “the thing I measured” | the dependent variable |
| “the things I kept the same” | the control variables |
Extension · dissolving faster
A solid dissolves faster in warmer water, when you stir it, and when its pieces are smaller.
Choosing with a chart
Sometimes a job needs two properties at once. Read the chart for both, and choose the one that is best at both. The chart shows four liquids: water, ethanol, ethyl ethanoate and propanone. A liquid with a lower boiling point evaporates more quickly.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “it dries quickly” | it evaporates quickly: it has a low boiling point |
Read the notes. Run the filtration and the sieve on the picture. Then press Layers of liquids above the picture.
Sieve, filter, decant, magnet
You can separate a mixture because its substances have different properties.
Remember
Each method uses a property that is different in the substances: size, solubility, magnetism, boiling point or density. The mass in each cubic centimetre of a substance is its density, in g/cm³ (grams in each cm³). A liquid with more mass in each cm³ is denser. A less dense liquid floats on a denser liquid that it does not mix with.
| Method | What it separates | Property |
|---|---|---|
| Sieve | pieces of different sizes (rice and salt), or big pieces from a liquid (pasta and water). It cannot separate a dissolved solid: its particles pass through the holes. | size |
| Filtration | an insoluble solid from a liquid (chalk and water). The solid left on the filter paper in the funnel is the residue; the liquid that passes through into a conical flask is the filtrate. A dissolved solid passes through with the liquid. | size and solubility |
| Decanting | a liquid from a solid that has settled at the bottom: pour the liquid carefully into another container; the solid remains | density |
| Separating funnel | two liquids that do not mix, immiscible liquids, such as oil and water: open the tap, and the lower, denser liquid flows through the tap first | density |
| Magnet | iron or steel from a mixture (iron filings and sulfur powder) | magnetism |
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “the stuff left on the paper” | the residue |
| “the liquid that goes through” | the filtrate |
| “pour off the top” | decant |
| “liquids that do not mix” | immiscible liquids |
Layers of liquids
Liquids that do not mix form layers: the less dense liquid floats on the denser one. Liquids that can mix, such as honey and water, also form layers for a while if you pour them in slowly, one at a time, and do not stir. An object floats on a liquid that is denser than it, and sinks through a liquid that is less dense than it. So it stops on top of the first liquid that is denser than it.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “the heavier liquid” | the denser liquid (the one with more mass in each cm³) |
| “it stays on top” | it floats |
Read the notes. Run the distillation, the evaporation and the chromatography on the picture. Then answer the checks and write two answers.
Evaporation, distillation, chromatography
Three ways to separate a solution: keep the solid, keep the liquid too, or separate the dyes.
Remember: evaporation
Heat the solution in an evaporating basin until the solvent changes to a gas and leaves: this is evaporation. The solute is left as solid pieces with flat faces: crystals. You keep the solid, but the solvent is lost. To grow bigger crystals, heat only until the solution is saturated, then leave it to cool slowly: crystallisation.
Remember: simple distillation
Heat the solution in a flask over a Bunsen burner. The solvent boils and changes to vapour. The vapour moves into a glass tube cooled by cold water, the condenser, where it condenses: it changes to a liquid again. This pure liquid drips into a beaker: it is the distillate. The solute stays in the flask. A thermometer at the top shows the boiling point. The whole method is simple distillation.
Distillation keeps both: the solvent is collected, and the solute remains in the flask. But it needs more apparatus and more energy than evaporation. Collecting a substance again from a mixture is recovering it. Evaporation and distillation can recover all of a dissolved solid; filtration cannot, because dissolved particles pass through the filter paper.
Say it like a scientist
From now on, write the scientific word. It is the word that scores marks; everyday words do not.
| Instead of … | write … |
|---|---|
| “the water you collect” | the distillate |
| “the steam turns back into water” | the vapour condenses |
| “the cooling tube” | the condenser |
| “the water dries up and salt is left” | the water evaporates and salt crystals are left |
Remember: chromatography
The coloured substances in an ink or a food colouring are its dyes. A way to separate them on paper is chromatography. Draw a line in pencil near the bottom of the paper: the start line (pencil does not dissolve). Put a small spot of each sample on the line. Stand the paper in a solvent, with the solvent below the line. The solvent rises through the paper and carries the dyes with it; each dye travels a different distance, so the dyes separate. The top of the wet part of the paper is the solvent front. The pattern of spots you get is the chromatogram.
- Two samples have the same dyes when all their spots match: the same colours at the same heights.
- A sample contains a dye when it has a spot at the same height, with the same colour, as that dye.
- A spot that stays on the start line is a dye that is insoluble in that solvent.
Say it like a scientist
Write the scientific word:
| Instead of … | write … |
|---|---|
| “the colours in the ink” | the dyes |
| “the pencil line” | the start line |
| “how far the solvent got” | the solvent front |
| “the pattern of spots” | the chromatogram |
Extension · the pencil line
Writing an explanation
To explain how a set-up distils a liquid, write each step, using these words:
- The liquid evaporates (or boils) because it is heated.
- The vapour cools and condenses on a cold surface.
- The pure liquid is collected in a separate container.
- The dissolved solid (or dirt) remains in the container, because it does not evaporate.
To explain a chromatogram, write your conclusion first, then the evidence: which spots match, at which heights.
Find the faults in a student’s graphs and table: tap each part that is wrong. Then press Show it on the picture, and compare.
Graphs and tables
In class, you learnt how to draw a perfect table and a perfect graph. Do you remember every rule?
Before you start
Try the checks without looking at the rules. Each fault you find shows its rule. When you finish, press Show it on the picture: it shows the perfect graph and the perfect table, with every rule numbered.