IGCSE Chemistry · ⁨IGCSE الكيمياء⁩

Everything around you is made of tiny particles 粒子 — these can be atoms 原子, molecules 分子, or ions 离子. The kinetic particle theory 粒子动理论 says these particles are always moving. How close the particles are, how they are arranged, and how they move decides whether matter is a solid 固体, a liquid 液体, or a gas 气体.

Properties you can observe

You do not need a microscope to tell the three states apart. They behave in different ways:

• A solid has a fixed shape and a fixed volume 体积. It does not flow and you cannot compress 压缩 it (squeeze it smaller).
• A liquid has a fixed volume but no fixed shape. It flows and takes the shape of its container. It is almost impossible to compress.
• A gas has no fixed shape and no fixed volume. It flows and spreads out to fill the whole container. A gas is easy to compress.

The table below sums up these properties. Density 密度 means how much mass 质量 is packed into a given volume.

The particle picture

The kinetic particle theory explains these properties by looking at three things: the separation 间距 of the particles (how far apart they are), their arrangement 排列 (the pattern), and their motion 运动 (how they move).

Strong forces of attraction 引力 hold the particles together. In a solid these forces are strong enough to hold every particle in place, so a solid keeps its shape. In a liquid the forces are weaker, so particles can move around. In a gas the particles move so fast that the forces hardly act at all, so the gas spreads out.

When you heat or cool a substance, it can change from one state to another. You must know the name of each change.

A pure solid melts at one fixed temperature, the melting point 熔点. A pure liquid boils at one fixed temperature, the boiling point 沸点. The same substance freezes at its melting point and condenses at its boiling point.

A few substances change straight from solid to gas without melting first. This is called sublimation 升华. In the photo below, warmed solid iodine in a beaker turns directly into a purple gas; the gas then cools on the round flask of ice above and turns back into a solid.

Explaining changes of state with the particle theory

Each change of state is really a change in the energy 能量 of the particles.

• When you heat a solid, the particles gain energy and vibrate faster. At the melting point the particles have enough energy to break away from their fixed places and slide around — the solid melts.
• When you heat a liquid, the particles move faster. At the boiling point they have enough energy to fully escape the forces of attraction and become a gas.
• Cooling does the opposite. The particles lose energy, move more slowly, and the forces of attraction pull them back together.

During a change of state the energy goes into breaking the forces of attraction, not into making the particles move faster. This is why the temperature stays the same while a substance is melting or boiling.

Heating and cooling curves

A heating curve 加热曲线 is a graph of temperature against time as you heat a substance steadily. A cooling curve 冷却曲线 is the same graph as the substance cools.

On a heating curve there are two flat (level) parts:

• The first flat part is at the melting point. Here solid and liquid are both present. The heat energy breaks the forces holding the solid together, so the temperature does not rise.
• The second flat part is at the boiling point. Here liquid and gas are both present, and the temperature again stays constant.

A cooling curve is the reverse. It has a flat part at the boiling point (the gas condenses) and a flat part at the melting point (the liquid freezes). As the particles slow down, thermal energy 热能 is released to the surroundings.

A gas pushes on the walls of its container. This push, spread over the area of the wall, is the pressure 压强 of the gas. Pressure comes from the gas particles hitting the walls.

Effect of temperature

If you heat a fixed mass of gas while keeping the pressure the same, its volume increases.

Using the particle theory: heating gives the particles more kinetic energy 动能, so they move faster. They hit the walls harder and more often. To keep the pressure the same, the gas must take up more space, so the volume gets bigger.

If instead the volume is fixed (a sealed, rigid container), heating the gas makes the pressure rise, because the faster particles collide 碰撞 with the walls harder and more often.

Effect of pressure

If you increase the pressure on a fixed mass of gas while keeping the temperature the same, its volume decreases. Squeezing the gas into a smaller space means the particles hit the walls more often, which is what a higher pressure means.

Diffusion 扩散 is the spreading of particles from a region where they are crowded to a region where they are spread out — that is, from high concentration 浓度 to low concentration. It happens because particles are always moving in random directions.

Diffusion explains why you can smell food from across a room: the smell particles move and mix with the air until they reach your nose. Diffusion happens in gases and in liquids, but not in solids, because solid particles cannot move from place to place.

Rate of diffusion and molecular mass

Lighter gas particles move faster than heavier ones at the same temperature. So a gas with a smaller relative molecular mass 相对分子质量 (a smaller mass for each molecule) has a faster rate 速率 of diffusion.

A classic experiment shows this. Cotton wool soaked in ammonia 氨气 ($\text{NH}_3$) is put at one end of a long glass tube. Cotton wool soaked in hydrogen chloride 氯化氢 ($\text{HCl}$) is put at the other end. Both gases diffuse along the tube and meet to form a white ring of ammonium chloride 氯化铵 ($\text{NH}_4\text{Cl}$).

Ammonia has $M_r = 17$ and hydrogen chloride has $M_r = 36.5$. Ammonia is lighter, so it diffuses faster and travels further along the tube. The white ring forms nearer the hydrogen chloride end.

Worked example. Bromine ($M_r = 160$) and ammonia ($M_r = 17$) are released at the same moment from opposite ends of a long tube. Which travels further before they meet? Compare the relative molecular masses. Ammonia's $M_r$ is far smaller, so its molecules move faster at the same temperature and it diffuses more quickly. Ammonia therefore travels the greater distance, and the two gases meet nearer the bromine end. The rule is always "lighter means faster" - argue from $M_r$, never from how big the formula looks on paper.

• During melting and boiling the temperature stays constant (the flat parts of a heating curve), because the energy breaks the forces between particles instead of making them move faster.
• Evaporation happens only at the surface and at any temperature; boiling happens throughout the liquid at one fixed temperature.
• Explain gas behaviour with the particle theory: heating a gas at fixed volume raises the pressure because the particles hit the walls harder and more often — the particles themselves do not get bigger.
• Diffusion is faster for lighter particles (smaller $M_r$) and cannot happen in solids. In the ammonia/hydrogen chloride tube the lighter ammonia travels further, so the white ring forms nearer the HCl end.

A formula 化学式 shows which atoms 原子 are in a substance, and how many of each. There are two kinds of formula you must know.

• The molecular formula 分子式 is the actual number of each atom in one molecule 分子. For glucose it is $\text{C}_6\text{H}_{12}\text{O}_6$.
• The empirical formula 实验式 is the simplest whole-number ratio 比例 of the atoms or ions 离子 in a compound 化合物. For glucose it is $\text{CH}_2\text{O}$.

Working out a formula

If you are given a model or diagram, just count the atoms of each element and write them as a formula.

For an ionic compound 离子化合物 you can work out the formula from the charges on the ions. The total positive charge must balance the total negative charge, because the compound has no overall charge. Some common ions:

For example, $\text{Na}^{+}$ and $\text{O}^{2-}$: you need two $\text{Na}^{+}$ to balance one $\text{O}^{2-}$, so the formula is $\text{Na}_2\text{O}$.

An equation shows how reactants 反应物 (the starting substances) change into products 生成物 (the substances made).

• A word equation 文字方程式 uses names: magnesium + oxygen → magnesium oxide.
• A symbol equation 化学方程式 uses formulae and must be balanced 配平 — the same number of each atom on both sides.

You add state symbols 状态符号 in brackets to show the physical state: $(s)$ solid, $(l)$ liquid, $(g)$ gas, and $(aq)$ for aqueous 水溶液 (dissolved in water).

Ionic equations

An ionic equation 离子方程式 shows only the ions that actually change. Ions that are the same on both sides are spectator ions 旁观离子 and are left out. For example, when an acid reacts with an alkali:

The relative atomic mass 相对原子质量 ($A_r$) of an element 元素 is the average mass 质量 of its atoms compared to $\tfrac{1}{12}$ of the mass of one $^{12}\text{C}$ atom.

The relative molecular mass 相对分子质量 ($M_r$) is the sum of the relative atomic masses of all the atoms in the molecule. For ionic compounds we use the relative formula mass 相对式量, found the same way from the formula.

Reacting masses by simple proportion

You can sometimes find a reacting mass without the mole. If 24 g of magnesium makes 40 g of magnesium oxide, then 12 g of magnesium (half as much) makes 20 g of magnesium oxide.

The mole 摩尔 (symbol mol) is the unit for the amount of substance 物质的量. One mole of any substance contains $6.02 \times 10^{23}$ particles 粒子 (atoms, ions or molecules). This number is the Avogadro constant 阿伏伽德罗常数.

The molar mass 摩尔质量 is the mass of one mole, in grams per mole (g/mol). Its number is the same as the $A_r$ or $M_r$. The key relationship is:

Worked example. How many moles are in 36 g of water? Molar mass of water $= 18$ g/mol.

To find the number of particles, multiply the moles by the Avogadro constant.

Volumes of gases

At room temperature and pressure (r.t.p.), one mole of any gas takes up the same volume 体积. This molar gas volume 摩尔气体体积 is $24 \text{ dm}^3$.

Worked example. What volume does $0.25$ mol of carbon dioxide occupy at r.t.p.?

Concentration of solutions

The concentration 浓度 of a solution 溶液 can be given in $\text{g/dm}^3$ or in $\text{mol/dm}^3$.

Remember to convert volume: $1 \text{ dm}^3 = 1000 \text{ cm}^3$, so divide a volume in $\text{cm}^3$ by 1000.

Most calculations follow the same steps: change the known amount into moles, use the balanced equation to find the moles of what you want, then change back into mass, volume or concentration.

Worked example. What mass of magnesium oxide forms when 12 g of magnesium burns completely? $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. ($A_r$: Mg $= 24$, O $= 16$.)

Moles of Mg $= 12 / 24 = 0.5$ mol. The ratio Mg : MgO is $1 : 1$, so $0.5$ mol of MgO forms. Its molar mass is $24 + 16 = 40$ g/mol, so the mass is $0.5 \times 40 = 20$ g — the same answer as the simple-proportion method above.

Limiting reactant

When two reactants are mixed, often one runs out first. This is the limiting reactant 限量反应物. It decides how much product can form; any other reactant is in excess (left over).

Worked example. 8 g of hydrogen reacts with 8 g of oxygen: $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$. Which is the limiting reactant, and what mass of water forms? ($A_r$: H $= 1$, O $= 16$.)

Moles of $\text{H}_2 = 8 / 2 = 4$ mol; moles of $\text{O}_2 = 8 / 32 = 0.25$ mol. The equation needs $2\text{H}_2$ for every one $\text{O}_2$, so $0.25$ mol of $\text{O}_2$ reacts with only $0.5$ mol of $\text{H}_2$. There is far more hydrogen than that, so oxygen is the limiting reactant and the hydrogen is in excess. The oxygen makes $2 \times 0.25 = 0.5$ mol of water, so the mass is $0.5 \times 18 = 9$ g. Always work in moles, not masses — the reactant with the smaller mass is not always the one that runs out.

Titration calculation

In a titration 滴定 you measure the volume of one solution that reacts with another. From the volume and concentration you find the moles of one solute 溶质, then use the equation ratio to find the moles, concentration or volume of the other.

Worked example. $25.0 \text{ cm}^3$ of sodium hydroxide solution is exactly neutralised by $20.0 \text{ cm}^3$ of $0.10 \text{ mol/dm}^3$ hydrochloric acid. $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$. Find the concentration of the sodium hydroxide.

Moles of HCl $= 0.10 \times \dfrac{20.0}{1000} = 0.0020$ mol. The ratio is $1 : 1$, so there are $0.0020$ mol of NaOH in $25.0 \text{ cm}^3$:

Empirical and molecular formulae from data

To find an empirical formula from masses or percentages:

1. Divide each element's mass (or %) by its $A_r$.
2. Divide all the answers by the smallest one to get the simplest ratio.

To find the molecular formula, compare the empirical formula mass with the real $M_r$ and multiply up.

Worked example. A compound contains $40\%$ calcium, $12\%$ carbon and $48\%$ oxygen by mass. Find its empirical formula. ($A_r$: Ca $= 40$, C $= 12$, O $= 16$.)

Divide each percentage by its $A_r$: Ca $= 40/40 = 1$, C $= 12/12 = 1$, O $= 48/16 = 3$. The ratio is $1 : 1 : 3$, so the empirical formula is $\text{CaCO}_3$.

Percentages

• Percentage yield 产率 compares how much product you actually got with the most you could get: $\dfrac{\text{actual}}{\text{theoretical}} \times 100$.
• Percentage composition by mass 质量分数 of an element $= \dfrac{\text{mass of the element}}{M_r} \times 100$.
• Percentage purity 纯度 $= \dfrac{\text{mass of pure substance}}{\text{mass of impure sample}} \times 100$.

Worked example. Find the percentage by mass of nitrogen in ammonium nitrate, $\text{NH}_4\text{NO}_3$. ($A_r$: N $= 14$, H $= 1$, O $= 16$.)

$M_r = (2 \times 14) + (4 \times 1) + (3 \times 16) = 80$. The mass of nitrogen is $2 \times 14 = 28$, so

Worked example. A reaction could make at most 5.0 g of product, but only 4.0 g is actually collected. Find the percentage yield.

Some product is always lost — left behind in the apparatus, or used up in side reactions — so the yield is almost never $100\%$.

Worked example. A 50 g sample of impure calcium carbonate contains 45 g of pure calcium carbonate. Find its percentage purity.

• Always turn masses, gas volumes and concentrations into moles first, use the balanced equation's ratio, then convert back. Never compare masses directly across an equation.
• Key formulas: $n = \dfrac{\text{mass}}{M_r}$, gas volume $= n \times 24\ \text{dm}^3$ at r.t.p., and concentration $= \dfrac{n}{\text{volume in dm}^3}$. Change cm³ to dm³ by dividing by 1000.
• To balance a symbol equation you may only change the big numbers in front of a formula, never the small subscripts inside it.
• The limiting reactant is the one that runs out (fewest moles once you allow for the ratio); it sets how much product forms. The other reactant is in excess.
• Percentage yield = actual ÷ theoretical × 100; percentage purity = mass of pure substance ÷ mass of sample × 100. Both are always 100% or less.

Electrolysis 电解 is the breaking down (decomposition 分解) of an ionic compound 离子化合物 — when it is molten 熔融 or in aqueous 水溶液 solution — by passing an electric current 电流 through it.

It only works when the substance is molten or dissolved, because then the ions 离子 are free to move and carry the charge. A solid ionic compound cannot be electrolysed because its ions are locked in place.

The electrolytic cell

The set-up is called an electrolytic cell 电解池. Two electrodes 电极 (solid conductors) dip into the electrolyte 电解质 — the molten or aqueous substance being broken down.

• The anode 阳极 is the positive ($+$) electrode.
• The cathode 阴极 is the negative ($-$) electrode.

The electrodes are often inert 惰性 (they do not react), such as platinum 铂 or carbon/graphite 石墨.

What is formed at each electrode

There is a simple rule:

• Metals 金属 or hydrogen 氢气 are formed at the cathode.
• Non-metals 非金属 (other than hydrogen) are formed at the anode.

For aqueous solutions, the product can depend on whether the solution is dilute 稀 or concentrated 浓. Here are the three Core examples:

Worked example. Predict the products of electrolysing molten zinc chloride, $\text{ZnCl}_2$. Because it is molten, only zinc ions and chloride ions are present - there is no water to complicate things. Zinc is a metal, so it forms at the cathode: $\text{Zn}^{2+}$ ions gain electrons and silvery zinc appears. Chlorine is a non-metal, so it forms at the anode: pale green chlorine gas bubbles off. Always check the state first. A molten compound simply gives you its own two elements, while an aqueous one brings water into the competition - which is why concentrated sodium chloride solution gives hydrogen at the cathode rather than sodium.

How the charge moves

During electrolysis:

• Electrons 电子 move through the wires (the external circuit 外电路) from the power supply.
• At the cathode, positive ions gain electrons. Gaining electrons is reduction 还原.
• At the anode, negative ions lose electrons. Losing electrons is oxidation 氧化.
• Inside the electrolyte, the ions move: positive ions go to the cathode and negative ions go to the anode.

You can write a half-equation 半反应式 for each electrode. For molten lead(II) bromide:

Electrolysis of copper(II) sulfate

The product at the anode depends on the electrode:

• With inert carbon electrodes: copper 铜 forms at the cathode and oxygen forms at the anode. The blue colour of the solution slowly fades as copper is removed.
• With copper electrodes: copper forms at the cathode, while the anode itself dissolves into the solution. The blue colour stays the same. This is used to purify copper.

Electroplating

Electroplating 电镀 means covering a metal object with a thin layer of another metal. This improves its appearance and its resistance to corrosion 腐蚀 (rusting and wearing away).

To electroplate an object:

• the object to be coated is made the cathode;
• the plating metal is made the anode;
• the electrolyte is a solution containing ions of the plating metal.

A fuel cell 燃料电池 uses hydrogen and oxygen to make electricity directly. The only chemical product is water.

It is useful to compare a fuel cell with a normal petrol (gasoline) engine in a vehicle.

Advantages of the fuel cell: the only product is water, so it does not pollute the air, and it changes chemical energy into electricity efficiently. Disadvantages: hydrogen is hard to store and transport safely, and producing the hydrogen can still use energy from fossil fuels.

• At the cathode (negative) you get a metal or hydrogen; at the anode (positive) you get a non-metal. Positive ions move to the cathode, negative ions to the anode.
• A molten compound just gives its two elements. An aqueous solution can give something different (concentrated sodium chloride gives hydrogen and chlorine, not sodium).
• Reduction is gain of electrons (at the cathode); oxidation is loss of electrons (at the anode). Remember OIL RIG.
• In a half-equation the atoms and the charges must balance, with the electrons on the correct side: $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$ (cathode).
• To electroplate an object, make it the cathode, make the plating metal the anode, and use a solution containing ions of the plating metal.

In every chemical reaction, energy is transferred. The reaction is either exothermic or endothermic, depending on which way the energy moves.

• An exothermic reaction 放热反应 gives out thermal energy 热能 to the surroundings 环境. So the temperature of the surroundings goes up.
• An endothermic reaction 吸热反应 takes in thermal energy from the surroundings. So the temperature of the surroundings goes down.

Examples of exothermic reactions are combustion 燃烧 (burning a fuel) and neutralisation 中和 (an acid reacting with an alkali). An example of an endothermic reaction is the thermal decomposition 分解 of a compound (breaking it down using heat).

The amount of thermal energy transferred in a reaction is called the enthalpy change 焓变. It is written as $\Delta H$ and has these signs:

• $\Delta H$ is negative for an exothermic reaction, because energy leaves the chemicals.
• $\Delta H$ is positive for an endothermic reaction, because energy is taken in.

Particles do not react every time they meet. The activation energy 活化能 ($E_a$) is the smallest amount of energy that colliding 碰撞 particles 粒子 must have before they can react. It is like a hill the particles must get over before the reaction can happen.

A reaction pathway diagram 反应进程图 shows how the energy changes as a reaction happens. Energy is on the vertical axis, and the progress of the reaction is on the horizontal axis.

• The line starts at the energy level of the reactants 反应物.
• It rises over a 'hill' — the height of this hill is the activation energy $E_a$.
• It then falls or rises to the energy level of the products 生成物.
• The gap between the reactant level and the product level is the enthalpy change $\Delta H$.

For an exothermic reaction, the products are lower than the reactants, so energy is given out and $\Delta H$ is negative.

For an endothermic reaction, the products are higher than the reactants, so energy is taken in and $\Delta H$ is positive.

A reaction involves breaking the bonds in the reactants and making new bonds in the products.

• Bond breaking 断键 takes in energy, so it is an endothermic step.
• Bond making 成键 gives out energy, so it is an exothermic step.

The bond energy 键能 is the energy needed to break one mole of a particular bond. You can use bond energies to find the enthalpy change:

If more energy is given out making bonds than is taken in breaking bonds, the reaction is exothermic ($\Delta H$ negative). If less energy is given out, it is endothermic ($\Delta H$ positive).

Worked example

For the reaction $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$, use these bond energies (in kJ/mol): H–H $= 436$, Cl–Cl $= 242$, H–Cl $= 431$.

Bonds broken: one H–H and one Cl–Cl $= 436 + 242 = 678$.

Bonds made: two H–Cl $= 2 \times 431 = 862$.

The answer is negative, so this reaction is exothermic.

• Exothermic gives out heat, so the surroundings warm up and $\Delta H$ is negative; endothermic takes in heat, so the surroundings cool and $\Delta H$ is positive.
• Bond breaking takes energy in (endothermic); bond making gives energy out (exothermic). Use $\Delta H = (\text{energy to break bonds}) - (\text{energy released making bonds})$.
• If more energy is released making bonds than is used breaking them, the reaction is exothermic. Always check the sign of your final answer.
• On a reaction pathway diagram, an exothermic reaction ends lower than it starts; an endothermic one ends higher. The height of the hill is the activation energy.

An acid 酸 is a substance that forms hydrogen ions ($\text{H}^{+}$) when dissolved in water. Acids have three typical reactions:

• with metals 金属: acid + metal → a salt 盐 + hydrogen 氢气
• with bases: acid + base 碱 → a salt + water (this is neutralisation 中和)
• with carbonates 碳酸盐: acid + carbonate → a salt + water + carbon dioxide 二氧化碳

Describing what you see

Naming the products is only half of what the exam wants. "State the observations" asks what you would see with your eyes, and it carries its own marks - so learn the words:

So for magnesium added to hydrochloric acid, a full answer is: effervescence, and the solid dissolves (and the tube gets warm). For copper(II) oxide added to warm sulfuric acid: the black solid dissolves and the solution turns blue.

Two rules save most of the lost marks. Never write "hydrogen is given off" as an observation - you cannot see hydrogen, you can only see the bubbles, so write effervescence. And do not write a name you cannot see: "a salt forms" is a conclusion, not an observation.

Indicators

An indicator 指示剂 is a dye that changes colour to show whether a solution is acidic or alkaline.

Aqueous solutions of acids contain hydrogen ions 离子 ($\text{H}^{+}$). Aqueous solutions of alkalis contain hydroxide ions ($\text{OH}^{-}$).

Bases are oxides 氧化物 or hydroxides 氢氧化物 of metals. An alkali 可溶性碱 is a base that dissolves in water (a soluble base).

Bases have two typical reactions:

• with acids: base + acid → a salt + water (neutralisation again)
• with ammonium salts 铵盐: this releases ammonia gas.

An acid is a proton 质子 donor — it gives away $\text{H}^{+}$ ions (a hydrogen ion is just a proton). A base is a proton acceptor.

How strong an acid is depends on how much of it splits into ions in water:

• A strong acid 强酸 is completely dissociated 电离 (fully split into ions) in water. Hydrochloric acid 盐酸 is strong:

• A weak acid 弱酸 is only partly dissociated. Ethanoic acid 乙酸 is weak, so the equation uses the reversible arrow:

Note: 'strong' and 'weak' are about dissociation, not about being dilute or concentrated.

The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. A higher hydrogen ion concentration means a lower pH.

You can find pH using universal indicator 通用指示剂, which turns different colours:

When an acid and an alkali react, the $\text{H}^{+}$ and $\text{OH}^{-}$ ions join to make water:

Oxides can be sorted by how they behave:

• Acidic oxides are oxides of non-metals, such as $\text{SO}_2$ and $\text{CO}_2$.
• Basic oxides are oxides of metals, such as $\text{CuO}$ and $\text{CaO}$.
• Amphoteric 两性 oxides react with both acids and bases to make a salt and water. $\text{Al}_2\text{O}_3$ and $\text{ZnO}$ are amphoteric.

So metal oxides tend to be basic and non-metal oxides tend to be acidic.

Whether a salt can be made by a certain method depends on whether it is soluble 可溶 (dissolves) or insoluble 不溶 (does not dissolve).

Solubility rules

Making a soluble salt

You react an acid with one of these:

• an alkali, using titration 滴定 (since both are solutions, you must measure the exact volumes);
• an excess 过量 of a metal, an insoluble base, or an insoluble carbonate.

When you use an excess of a solid, you then filter 过滤 to remove the leftover solid. Finally you evaporate 蒸发 some water and let the solution crystallise 结晶 to get the salt.

Worked example. Which method would you use to prepare (a) copper(II) sulfate and (b) barium sulfate? Check the solubility rules first. (a) Copper(II) sulfate is soluble (only barium, calcium and lead sulfates are not), so make it from an insoluble solid and an acid: add excess copper(II) oxide to warm dilute sulfuric acid, filter off the leftover solid, then evaporate some water and crystallise. (b) Barium sulfate is insoluble, so make it by precipitation: mix two solutions that each carry one of the needed ions, then filter, wash and dry the solid. The solubility of the salt you want is what decides the method - consult the rules before you choose.

Making an insoluble salt

An insoluble salt is made by precipitation 沉淀: mix two solutions that each contain one of the needed ions, and the insoluble salt forms as a solid. You then filter, wash and dry it.

Water in salts

• A hydrated 水合 substance is chemically joined with water.
• An anhydrous 无水 substance contains no water.

The water molecules inside hydrated crystals are called the water of crystallisation 结晶水. For example, $\text{CuSO}_4 \bullet 5\text{H}_2\text{O}$ has five water molecules for each formula unit.

• Learn the three acid reactions: acid + metal → salt + hydrogen; acid + base → salt + water (neutralisation); acid + carbonate → salt + water + carbon dioxide.
• Strong and weak describe how much the acid splits into ions (dissociation), not how concentrated it is. A strong acid is fully dissociated; a weak acid only partly.
• A lower pH means a higher hydrogen-ion concentration. pH 7 is neutral, below 7 is acidic, above 7 is alkaline.
• Choose the salt method by solubility: a soluble salt is made from an acid + excess solid (then filter, evaporate, crystallise), or by titration with an alkali; an insoluble salt is made by precipitation (mix two solutions, then filter).
• Learn the solubility rules — all sodium, potassium, ammonium and nitrate salts are soluble.

The Periodic Table 周期表 arranges all the elements 元素 in order of increasing proton number 质子数 (the atomic number 原子序数). The horizontal rows are called periods 周期 and the vertical columns are called groups 族.

A few key patterns:

• Across a period, the elements change from metals 金属 on the left to non-metals 非金属 on the right.
• The group number tells you the charge of the ions 离子 that the elements form. Group I forms $+1$ ions, Group II forms $+2$ ions, and Group VII forms $-1$ ions.
• Elements in the same group have similar chemical properties. This is because they have the same number of outer-shell electrons 电子 (the same outer electronic configuration 电子排布).

Because of these patterns, you can use an element's position to predict its properties.