Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Physical and Chemical Properties
Physical and Chemical Properties
Physical and Chemical Properties
Every substance possesses a set of properties that are unique to it — these are what allow us to identify one substance from another. These characteristic properties fall into two broad categories: physical properties and chemical properties.
Physical properties are those that can be measured or observed without changing the identity or the composition of the substance. Colour, odour, melting point, boiling point, and density are all physical properties. When you measure the melting point of ice, the water remains water — no new substance is formed.
Chemical properties, on the other hand, can only be observed when a chemical change occurs — that is, when the substance itself is transformed into something else. Composition, combustibility, and reactivity with acids or bases are chemical properties. To know that hydrogen is combustible, you must burn it; to know that sodium reacts violently with water, you must let the reaction happen. In each case, the original substance is consumed and a new substance appears.
A common mistake is to confuse a physical change with a chemical one. If the identity of the substance remains the same, the change is physical. If new substances are formed, the change is chemical. Melting wax is physical; burning wax is chemical.
Chemists describe, interpret, and predict the behaviour of substances based on knowledge of these properties. That knowledge is not guessed — it is determined by careful measurement and experimentation.
Key Distinction: Physical vs Chemical Properties
| Aspect | Physical Properties | Chemical Properties |
|---|---|---|
| What is observed | Colour, odour, melting point, boiling point, density | Composition, combustibility, reactivity with acids/bases |
| Does identity change? | No — the substance remains the same | Yes — a new substance is formed |
| Does a chemical change occur? | No | Yes |
| How are they measured? | By direct observation or measurement | By allowing a chemical reaction to occur |