Structure decides property
A property is what you can observe a material do: conduct, dissolve, bend, melt. The structure is what is inside: which particles, what charge they have, how they are packed and how strongly they pull each other.
We call the tiny picture the micro structure and what we see and measure the macro property. Chemistry links the two. To predict a property, ask four questions:
- What are the particles (atoms, ions or molecules)?
- Do they carry charge?
- Can they move?
- How strong are the pulls between them, and between them and water?
Conductivity: metals, salts and sugar
Electric current is a flow of charges. A material conducts only if it has charges that are free to move.
- Metals: the atoms hold on to their inner electrons, but outer electrons are free. They flow, so metals conduct, even as solids.
- Solid salt: it has ions, but each ion is stuck in the crystal. No flow, so no conduction.
- Melted or dissolved salt: the ions can move, so it conducts. The ions carry the current, not electrons.
- Sugar: it is made of neutral molecules. Even when dissolved, there is no charge to carry, so it does not conduct.
Solubility and water binding
Water molecules are slightly charged at the two ends (one end a little positive, the other a little negative). So water likes particles that also have charge or charged spots.
- Salt dissolves: water molecules surround each ion and pull it out of the crystal.
- Sugar dissolves: it has many OH groups that attract water molecules.
- Oil or wax does not dissolve: its molecules have no charged spots. Like dissolves like.
Water binding: a polymer with many OH (or similar) groups holds water around its chains. The chains spread apart and the material swells into a gel. This is how the gel in a nappy works.
Plastics and biodegradable polymers
A polymer is a very long chain made by joining many small units. Ordinary plastics have chains held together by strong carbon-carbon bonds. Bacteria cannot easily break them, so the plastic stays for years.
A biodegradable polymer has chains with weak links, for example ester links, that enzymes of bacteria and fungi can cut. The chain falls into small pieces that microbes use as food.
Chemists design new materials, medicines and food by changing the structure. For example, adding OH groups makes a polymer take up water, and adding ester links makes it biodegradable. Spotting such a change in a product is how you recognise a chemical innovation.
Predicting a property: a method
Use this short plan on any new material:
- Draw or imagine the particles.
- Charged and free to move? Then it conducts.
- Charged spots or OH groups? Then it likes water (dissolves or binds water).
- Long chain with weak links? Then it can be broken down by microbes.
Always say why in one sentence that joins the micro picture to the property.
Key formulas and definitions
- Conducts = has charges + charges can move
- Metals: free electrons carry current
- Salt solutions and melts: free ions carry current
- Neutral molecules (sugar, alcohol): no current
- Like dissolves like: charged or OH-rich particles dissolve in water
- Polymer with weak links = biodegradable
Worked examples
1. Why does copper conduct electricity but dry sodium chloride does not?
Copper has free electrons that can flow. Solid sodium chloride has ions, but they are locked in the crystal and cannot move. So copper conducts and dry salt does not.
2. Explain why molten sodium chloride conducts.
On melting, the crystal pattern breaks and the Na⁺ and Cl⁻ ions can move. Moving charges carry current, so the melt conducts.
3. A solution of sugar in water does not conduct, but a solution of salt does. Why?
Sugar dissolves as neutral molecules, so no charges are present. Salt dissolves as free ions, which carry charge.
4. Cooking oil does not mix with water but ethanol does. Link this to structure.
Ethanol has an OH group that attracts water molecules, so it mixes. Oil molecules have no such charged spots, so water does not pull them in.
5. A company says its new bag rots in compost within months. What must be true about the structure of the polymer?
The chains must contain weak links (such as ester links) that enzymes of microbes can cut, so the long chains break into small pieces that microbes can use.
6. A new polymer for nappies must hold 100 times its weight in water. Which feature would you give its chains and why?
Many OH (or other water-loving) groups along the chains. They attract water molecules and hold them between the chains, so the polymer swells into a gel.
Common mistakes
- Saying electrons carry current in salt solution. In solutions and melts the moving ions carry it.
- Thinking every solid that dissolves conducts. Sugar dissolves but has no ions, so it does not conduct.
- Saying "salt has no charges" when it does not conduct as a solid. It has ions; they just cannot move.
- Thinking biodegradable means the plastic vanishes at once. It needs the right conditions (microbes, warmth, moisture) and takes time.