Which of the following is not a mixture?

The correct answer is option 4. Methane.

Letus break down each substance to determine whether it is a mixture or not:

(a) Blood:  Blood is a mixture. It consists of various components such as red blood cells, white blood cells, platelets, plasma, and dissolved substances. These components exist in a heterogeneous mixture, and blood exhibits different properties throughout.

(b) Air: Air is a mixture. It is a gaseous combination of nitrogen, oxygen, carbon dioxide, and trace gases. Despite being a mixture, air is homogeneous, with a relatively uniform composition and properties throughout.

(c) Soil: Soil is a mixture. It is a complex combination of minerals, organic matter, water, and air. Soil exhibits heterogeneity, with different regions having varying compositions and properties.

(d) Methane:  Methane is not a mixture. Methane is a simple chemical compound with a molecular formula of CH₄. It consists of carbon and hydrogen atoms bonded together in a specific ratio. Unlike mixtures, which involve the physical combination of different substances, methane is a pure compound.

In summary:

Mixtures: Blood (a), Air (b), and Soil (c)

Not a mixture: Methane (d)

Therefore, methane is not a mixture. It is a compound with a specific and well-defined chemical composition