Mada za sehemu hiiDemonstrate mastery of concepts, theories and principles in ChemistryMada 6
- Describe the concept of Chemistry (meaning, branches, applications and relationships with other disciplines)
- Discuss the contribution of Chemistry in the development of modern society
- Describe the concept of matter (physical and chemical changes of matter)
- Describe the concepts of chemical elements (criteria for assigning chemical symbols), compounds (binary compound) and mixtures (types of solution and separation of mixtures)
- Compare the properties of compounds with those of constituent elements
- Explain the concept of the periodic table (1st 20 elements in the Periodic Table; metals and non-metals)
When two or more different elements combine chemically, they form a compound. The key idea is this: the elements that form a compound lose their original properties and the compound gains completely new properties that are different from those of the original elements. This is what makes compounds different from mixtures.
When elements react to form a compound:
- The original properties of the individual elements disappear
- New properties appear in the compound
- The elements can only be separated by chemical methods, not physical ones
- The proportion of elements in a compound is fixed (always the same)
Example 1: Water (H₂O)
Water is a compound made from two elements: hydrogen and oxygen.
| Property | Hydrogen (element) | Oxygen (element) | Water (compound) |
|---|---|---|---|
| State at room temperature | Gas | Gas | Liquid |
| Color | Colorless | Colorless | Colorless (but looks blue in large amounts) |
| Flammability | Highly flammable | Supports combustion | Does not burn |
| Taste | Tasteless | Tasteless | Tasteless |
Notice how water has completely different properties from both hydrogen and oxygen. Hydrogen is a flammable gas, oxygen supports burning, but water extinguishes fires!
Example 2: Common Salt (Sodium Chloride, NaCl)
Salt is formed from the elements sodium and chlorine.
| Property | Sodium (element) | Chlorine (element) | Salt (compound) |
|---|---|---|---|
| State at room temperature | Solid (soft metal) | Greenish-yellow gas | Solid (white crystals) |
| Reactivity | Reacts violently with water | Toxic, corrosive gas | Stable, safe for food |
| Color | Silvery-white | Greenish-yellow | White |
Sodium is a reactive metal that explodes with water, and chlorine is a poisonous gas—but when they combine, they form table salt, which we safely use in cooking!
Example 3: Iron Sulphide (FeS)
When iron filings and sulphur powder are heated together, they form iron sulphide.
| Property | Iron (element) | Sulphur (element) | Iron sulphide (compound) |
|---|---|---|---|
| Magnetism | Magnetic | Not magnetic | Not magnetic |
| Color | Grey-black | Yellow | Dark grey |
| Reaction with acid | Produces hydrogen gas | No reaction | Produces hydrogen sulphide (rotten egg smell) |
The compound iron sulphide behaves completely differently from both iron and sulphur.
- A compound has new properties that are different from the elements that form it
- The properties of a compound are not a simple mixture of the properties of its elements
- Elements in a compound are joined by chemical bonds
- The ratio of elements in a compound is always fixed (e.g., water is always H₂O)
In everyday life in Tanzania, this concept is important when cooking. Table salt (NaCl) is a compound made from sodium (a highly reactive metal) and chlorine (a poisonous gas). But when combined, they create something safe and essential for seasoning food. Similarly, when baking, the baking powder or soda you buy from shops in Dar es Salaam or Dodoma releases carbon dioxide gas through a chemical reaction—showing how compounds behave differently from the individual ingredients used to make them.
Swali
What happens to the original properties of elements when they chemically combine to form a compound?
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