Which of the following materials is least dense?

• A. Air
• B. Water (H_2O)
• C. Aluminum (Al)
• D. Gold (Au)

Answer: (A)

Verified step by step guidance

1. Understand that density is defined as mass per unit volume, given by the formula \(\text{density} = \frac{\text{mass}}{\text{volume}}\).
2. Recall or look up the typical densities of the given materials: water, aluminum, gold, and air. These values are usually expressed in units like grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³).
3. Compare the densities of the materials: water has a density around 1 g/cm³, aluminum about 2.7 g/cm³, gold approximately 19.3 g/cm³, and air roughly 0.0012 g/cm³ at standard conditions.
4. Identify which material has the smallest density value, meaning it has the least mass per unit volume and is therefore the least dense.
5. Conclude that air, having a much lower density compared to solids and liquids listed, is the least dense material among the options.