Which of the following is a pair of physical and chemical property respectively of a substance

The correct option is density and acidity.

To understand why this is the correct pair, let's break down the definitions of physical and chemical properties and analyze the options provided.

1. Physical Properties:
A physical property is a characteristic of a substance that can be observed or measured without changing the chemical identity of the substance. Examples include color, density, melting point, boiling point, and solubility. Observing these properties does not alter the arrangement of atoms in the substance.

2. Chemical Properties:
A chemical property describes the ability of a substance to undergo a specific chemical change, which transforms it into one or more new substances. Examples include acidity, basicity, combustibility, flammability, and reactivity with water or acids. Observing a chemical property always involves a chemical reaction, which changes the chemical structure of the substance.

Now, let's evaluate each option to find the pair containing a physical property first, followed by a chemical property:

• basicity and colour: Basicity is a chemical property (measures reactivity/ability to accept protons), and colour is a physical property. This is a chemical/physical pair, which is the reverse of the requested order.

• density and acidity: Density is a physical property (mass per unit volume, measured without changing the substance's identity). Acidity is a chemical property (measures the ability to donate protons or react with bases to form new substances). This matches the requested physical and chemical property order respectively.

• colour and density: Both colour and density are physical properties.

• acidity and combustibility: Both acidity and combustibility (ability to burn in the presence of oxygen) are chemical properties.

Therefore, the pair that represents a physical property and a chemical property, in that exact order, is density and acidity.