Contents: 7 sections
Cambridge IGCSE Chemistry 0620 · Core and Extended
Syllabus points
- Compare the general physical properties of metals and non-metals, including thermal conductivity, electrical conductivity, malleability, ductility, and melting and boiling points.
- Describe the general chemical properties of metals, including their reactions with dilute acids and with oxygen.
- Describe the reactions of metals with cold water and with steam, and name the products.
- Identify a substance as a metal or a non-metal from given physical data.
- Explain the physical properties of metals in terms of the metallic lattice and its delocalised electrons.
Everything in 9.1 is Core. There is no Extended-only content in this subtopic.
Physical properties
| Property | Metals | Non-metals |
|---|---|---|
| Electrical conductivity | Good, as solid and as liquid | Poor, except graphite |
| Thermal conductivity | Good | Poor |
| Malleable, can be hammered into shape | Yes | No, brittle when solid |
| Ductile, can be drawn into wire | Yes | No |
| Melting and boiling points | Usually high | Usually low |
| Density | Usually high | Usually low |
| Appearance | Shiny when freshly cut | Dull |
| Sound when struck | Sonorous | Not sonorous |
The word "usually" is doing real work in that table, and Cambridge tests the exceptions:
- Mercury is a metal and a liquid at room temperature.
- The Group I metals have low densities and low melting points, and sodium melts at 98 °C.
- Graphite is a non-metal that conducts electricity, because each carbon atom has one delocalised electron.
- Diamond is a non-metal with a melting point above 3500 °C.
So a single property never proves the classification. Two or three together do.
Worked example. A shiny solid conducts electricity, is malleable, and 10 cm<sup>3</sup> of it has a mass of 89 g.
density = 89 / 10 = 8.9 g per cm<sup>3</sup>
That is a metal on every count, and the density identifies it as copper.
Why metals behave that way
Metallic bonding, from chapter 2, explains the whole table. A metal is a lattice of positive ions surrounded by a sea of delocalised electrons.
- Electrical conductivity: the delocalised electrons are free to move through the whole structure, so they carry charge.
- Thermal conductivity: those same moving electrons also carry energy quickly from the hot end to the cold end.
- Malleability and ductility: the layers of positive ions can slide over one another without breaking the bonding, because the electron sea holds the lattice together whichever way the ions move. An ionic solid shatters instead, because sliding brings like charges together.
- High melting point: the attraction between the positive ions and the delocalised electrons is strong, so a lot of energy is needed to separate them.
Chemical properties
Metals lose electrons to form positive ions, and every chemical property below follows from that.
With oxygen, giving a basic oxide:
2Mg + O<sub>2</sub> → 2MgO, a white solid, burning with a brilliant white flame
2Cu + O<sub>2</sub> → 2CuO, a black solid, formed on gentle heating without a flame
Worked example. 6.4 g of copper is heated in air until there is no further gain in mass. The relative atomic mass of copper is 64 and the relative formula mass of copper(II) oxide is 80.
mass of copper(II) oxide = 6.4 × 80 / 64 = 8.0 g
With dilute acid, giving a salt and hydrogen, but only for metals above hydrogen in the reactivity series:
Mg + 2HCl → MgCl<sub>2</sub> + H<sub>2</sub>
Zn + H<sub>2</sub>SO<sub>4</sub> → ZnSO<sub>4</sub> + H<sub>2</sub>
Copper, silver and gold give no reaction with dilute acids at all.
With cold water, giving a hydroxide and hydrogen. Only the most reactive metals do this:
2Na + 2H<sub>2</sub>O → 2NaOH + H<sub>2</sub>
Ca + 2H<sub>2</sub>O → Ca(OH)<sub>2</sub> + H<sub>2</sub>
With steam, giving an oxide and hydrogen. Metals in the middle of the series react with steam but not with cold water:
Mg + H<sub>2</sub>O → MgO + H<sub>2</sub>
3Fe + 4H<sub>2</sub>O → Fe<sub>3</sub>O<sub>4</sub> + 4H<sub>2</sub>
The difference between those last two pairs is worth memorising as a single line: cold water gives a hydroxide, steam gives an oxide, and both give hydrogen. Writing sodium hydroxide as a product of a steam reaction, or magnesium oxide as a product of a cold water reaction, is a common way to lose the mark.
Metal oxides are basic
Because metal oxides are basic, they neutralise acids:
CuO + H<sub>2</sub>SO<sub>4</sub> → CuSO<sub>4</sub> + H<sub>2</sub>O
The soluble ones, from Group I and calcium, dissolve to give alkalis. That link ties 9.1 to 7.2 and to the salt preparations in 7.3, where an insoluble metal oxide is exactly the reagent used in excess.
Telling a metal from a non-metal in data
Given a table of substances with melting points, conductivity and appearance, work through in this order:
- Does it conduct as a solid? Almost only metals and graphite do.
- Is it malleable or brittle? Metals bend, non-metals shatter.
- What kind of oxide does it form? A basic oxide means a metal, an acidic oxide means a non-metal.
The third test is the most reliable of the three, because it has fewer exceptions than the physical properties do.
Common mistakes
- Saying all metals have high melting points, forgetting mercury and the alkali metals.
- Saying no non-metal conducts electricity, forgetting graphite.
- Giving a hydroxide as the product of a metal reacting with steam.
- Writing that copper reacts with dilute hydrochloric acid to give hydrogen.
- Explaining conductivity by "free ions" rather than delocalised electrons.
- Explaining malleability without saying the layers of ions slide over one another.
- Classifying a substance as a metal from its shininess alone, when iodine is a shiny grey-black non-metal.