Compounds Have New Properties

Introduction

Imagine two substances that are dangerous in completely different ways. One is a soft metal that fizzes and can burst into flame when it touches water. The other is a choking, poisonous gas that was once used as a weapon. Now imagine that when these two substances combine, the result is something you sprinkle on your chips every day. That is exactly what happens with sodium, chlorine and sodium chloride. It is the most striking example of a key idea in chemistry: a compound has properties of its own.

Core explanation

The elements.

- Sodium, Na , is a Group 1 metal. It is silvery when freshly cut but quickly tarnishes in air. It is soft enough to cut with a knife, melts at about 98 °C and is less dense than water. It reacts vigorously with water, releasing hydrogen gas and heat. It must be stored under oil to keep it away from air and moisture. - Chlorine, Cl₂ , is a Group 17 non-metal. It is a yellow-green gas at room temperature, boiling at about −34 °C. It has a sharp, choking smell, is toxic to breathe, and is a powerful bleach.

The compound. When sodium and chlorine react, a vigorous reaction gives out a great deal of energy and forms sodium chloride, NaCl . Its properties are completely different:

Property Sodium Chlorine Sodium chloride --- --- --- --- State at 25 °C solid gas solid Appearance silvery metal yellow-green gas white crystals Melting point about 98 °C about −101 °C about 801 °C Reaction with water vigorous, gives hydrogen dissolves, forms a bleaching solution dissolves quietly Hazard burns skin, reacts with moisture toxic gas safe in normal amounts in food

The general principle. A compound is a new substance, not a blend of its elements. Its properties cannot be worked out by "averaging" the properties of the elements. This is one of the clearest ways to tell a compound from a mixture: a mixture keeps the properties of its components, but a compound does not.

More examples.

- Water, H₂O: hydrogen is a flammable gas and oxygen supports burning, yet water is a liquid used to put out many fires. - Carbon dioxide, CO₂: carbon is a black solid and oxygen is needed for burning, yet carbon dioxide is a colourless gas used in some fire extinguishers. - Iron(II) sulfide, FeS: unlike iron, it is not strongly magnetic and is not shiny; unlike sulfur, it is not yellow.

Step-by-step reasoning

To compare a compound with its elements:

1. List the elements present, using the formula. 2. Describe the key properties of each element: state, colour, reactivity, hazards. 3. Describe the same properties for the compound. 4. Point out which properties have changed, and conclude that the compound is a new substance.

Visual explanation

Picture three sealed jars side by side. The first holds grey-silver lumps of sodium under oil. The second is filled with a faint yellow-green gas. The third holds a heap of white cubic crystals. Nothing in the third jar looks, smells or behaves like the contents of the first two, even though it is made of exactly those two elements.

Real-world analogy

When hydrogen and oxygen form water, it is like two musicians forming a band. The band has its own sound, which is not simply one musician's sound added to the other's. You cannot hear the individual players in the same way once they are playing as one group.

Real-world example

Sodium chloride is essential to the human body: it helps control water balance and allows nerves to send signals. It is also used to preserve food and to grit icy roads. Pure sodium and chlorine, by contrast, are handled in industry only with special equipment and strict safety controls because of their hazards.

Why?

Why are the properties so different? In sodium chloride, sodium atoms have each given an electron to chlorine atoms, forming charged particles called ions: Na⁺ and Cl⁻. These ions are much more stable than the original atoms and are held in a strong, regular lattice. The particles themselves have changed, so the properties change too.

Common misconception

"Salt contains sodium, so eating salt means eating the reactive metal." The sodium in salt is present as sodium ions, which are stable and do not react with water. The properties of the element sodium do not carry over into the compound.

Worked example

Question: Hydrogen is a flammable gas and oxygen is a gas that supports combustion. Predict whether water, H₂O, can be expected to be flammable, and explain.

Reasoning: A compound's properties cannot be predicted from those of its elements, so we must look at water itself. Water is already the product of hydrogen combining with oxygen and does not burn.

Answer: Water is not flammable; its properties are different from those of hydrogen and oxygen.

Quick check

1. What is the colour and state of chlorine at room temperature? Answer: A yellow-green gas.

Exam focus

Examiners often ask you to compare a compound with its elements. Use a table or a point-by-point comparison covering state, appearance and reactivity or hazard. Finish with a clear statement: "the compound has different properties from its elements, so it is a new substance".

Advanced insight

The large energy released when sodium and chlorine combine reflects how much more stable the ions in the salt lattice are than the separate elements. Getting sodium and chlorine back out of salt requires this energy to be supplied again, which is done industrially by passing electricity through molten sodium chloride or salt solution.

Summary

Compounds have properties that are different from those of the elements they contain. Sodium is a soft, highly reactive metal and chlorine is a toxic yellow-green gas, but sodium chloride is a stable white crystalline solid with a high melting point that is safe to eat. Water and carbon dioxide show the same principle. New properties are strong evidence that a new substance has formed.

Practice questions

1. Give two ways in which sodium chloride differs from sodium. Answer: Sodium chloride is a white crystalline solid that dissolves quietly in water; sodium is a silvery metal that reacts vigorously with water. 2. Why is sodium stored under oil? Answer: To keep it away from air and moisture, with which it reacts. 3. Explain why the properties of a mixture of iron and sulfur are similar to those of the elements, but the properties of iron(II) sulfide are not. Answer: In the mixture, the elements are not bonded and keep their properties; in the compound, they are chemically combined to form a new substance. 4. Carbon dioxide is used in some fire extinguishers even though it contains oxygen. Explain why this is not a contradiction. Answer: Carbon dioxide is a compound with its own properties; it does not support burning as oxygen gas does.