Which one of the following is not a mixture?

In chemistry, matter is classified into pure substances and mixtures. A mixture consists of two or more substances physically combined where each retains its chemical identity [c2, t2]. Air is a homogeneous mixture of gases like nitrogen and oxygen [t2, t4]. Milk is a complex mixture, specifically a colloid or emulsion containing water, fats, and proteins [c3, t5, t6]. Cement is a mixture of various silicates and aluminates, often forming a heterogeneous mixture when combined with other materials like sand or gravel [c5, t4]. In contrast, Mercury is a chemical element (symbol Hg) [t4]. Elements are pure substances consisting of only one type of atom and cannot be broken down into simpler substances by chemical means [c2, t4]. Therefore, Mercury is not a mixture but a pure substance.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > Snapshots > p. 130
2. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > A step further > p. 121
3. [3] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.4 How Do We Use Elements, Compounds, and Mixtures? > p. 129