Let's Start Learning:

We've learned about elements as the basic forms of matter that cannot be broken down further by chemical means. Now, let's explore compounds, another category of pure substances that form when elements unite in a very specific way.

A compound is a substance composed of two or more elements that are chemically combined with one another in a fixed proportion. This chemical combination is what fundamentally differentiates a compound from a mixture.

What Happens When Elements Form a Compound?

When elements combine chemically to form a compound, a profound transformation occurs: the new substance formed (the compound) has totally different properties compared to its constituent elements. This is a key distinguishing feature from mixtures, where the components largely retain their individual properties.

Let's illustrate this with an intriguing activity (Activity 2.4):

Imagine dividing a class into two groups. Both groups are given 5 g of iron filings and 3 g of sulphur powder in a china dish – the same starting materials.

• Group I: Simply mixes and crushes the iron filings and sulphur powder. This group performs a physical change.
• Group II: Mixes and crushes the iron filings and sulphur powder, but then heats this mixture strongly until it is red hot. After cooling, this group has undergone a chemical change.

Now, let's compare the results:

1. Magnetism:
   • Group I's material (iron and sulphur mixture) will still be attracted to a magnet because the iron remains iron, simply mixed with sulphur. The components can be separated fairly easily by physical methods, like using a magnet to pull out the iron.
   • Group II's material (the compound formed by heating iron and sulphur, which is iron sulphide) will not be attracted to a magnet. The iron has chemically reacted and lost its individual magnetic property.
2. Appearance and Texture:
   • Group I's mixture will show distinct particles of iron (greyish) and sulphur (yellowish), and its texture might be non-uniform. The properties of the mixture are the same as that of its constituents.
   • Group II's compound will have a uniform texture and color throughout (often a dark, brittle solid), demonstrating a new substance with consistent properties. The composition of the compound is the same throughout.
3. Reaction with Acid:
   • If Group I's mixture reacts with dilute sulphuric acid or dilute hydrochloric acid, the iron will react to produce hydrogen gas, which is colorless, odorless, and combustible.
   • If Group II's compound reacts with the same acids, it will produce hydrogen sulphide gas, which is a colorless gas with the distinct smell of rotten eggs. This clearly indicates a new chemical entity has formed, reacting differently from its original components.

These observations starkly highlight the differences: Group I created a mixture, where elements just mix together and no new compound is formed, and the mixture shows the properties of its constituents. Group II created a compound, where elements react to form a new substance with totally different properties.

Key Differences Between Mixtures and Compounds:

In essence, compounds represent a higher level of organization in matter, where elements forge new identities through chemical bonds, leading to a vast array of substances with unique and fascinating properties.

Question for You: Water (H?O) is a compound. How does its chemical property of extinguishing fire differ from the properties of its constituent elements, hydrogen (flammable) and oxygen (supports combustion)?