The compound formed, which turns \(\mathrm{NaOH}\) solution yellow: (a) \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) (b) \(\mathrm{Na}_{2} \mathrm{~S}\) (c) \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (d) NaI

Chromate ions are known for their distinctive yellow color in solution, which can be a key indicator in chemical analysis and reactions. The color arises because the chromate ion, \( \mathrm{CrO}_{4}^{2-} \), absorbs certain wavelengths of light, which gives the remaining transmitted light a yellow hue. This unique property is not only visually useful for identification purposes but also forms the basis for various qualitative and quantitative analytical techniques in chemistry.

It's important to note that chromate ions exist in equilibrium with dichromate ions \( \mathrm{Cr}_{2}\mathrm{O}_{7}^{2-} \) in solution. The equilibrium can shift from yellow chromate to orange dichromate or vice versa depending on the pH level of the solution. In a basic environment like with \( \mathrm{NaOH} \) present, the equilibrium is pushed towards the formation of the yellow chromate ions.

Sodium hydroxide \( \mathrm{NaOH} \) is a highly reactive base that plays a significant role in many chemical reactions. Its ability to alter the pH of a solution makes it a useful reagent for driving reactions toward the production of certain ions or compounds. For example, when \( \mathrm{NaOH} \) is added to a solution containing dichromate ions, it can prompt the conversion to chromate ions, manifesting as a color change from orange to yellow.

Additionally, \( \mathrm{NaOH} \) reacts with various types of acids, forming water and the corresponding salt. Such reactions are fundamental in neutralization processes. The reactivity of \( \mathrm{NaOH} \) extends to its interactions with acidic oxides, certain non-metals, and many organic compounds, making it an indispensable substance in laboratories and industries.

Identifying ions in a solution is a crucial skill in chemistry, allowing the determination of the composition of unknown samples. Chromate ions \( \mathrm{CrO}_{4}^{2-} \) are identifiable by their yellow color in solution, while the orange color of dichromate \( \mathrm{Cr}_{2}\mathrm{O}_{7}^{2-} \) ions can also serve as a visual cue. To distinguish between these similar ions, chemists often manipulate the pH of the solution since chromate is favored in basic conditions whereas dichromate is stable in acidic environments.

Other common tests for ion identification involve precipitation reactions, flame tests, and even advanced spectroscopic methods. These techniques aid in confirming the presence of specific ions, which is essential for understanding chemical behavior and for conducting accurate analytical measurements.

Sodium compounds exhibit a wide variety of chemical properties due to the reactivity of sodium ions \( \mathrm{Na}^{+} \). In the context of chromate and dichromate ions, sodium salts like \( \mathrm{Na}_{2}\mathrm{CrO}_{4} \) and \( \mathrm{Na}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7} \) display distinct solubility and color characteristics. Sodium chromate is highly soluble in water and imparts a yellow color, owing to the presence of chromate ions.

Furthermore, sodium compounds react with other reagents such as \( \mathrm{NaOH} \) to form new substances, revealing the chemical dynamics between different species. They also take part in redox reactions, reflecting the change of oxidation states in transition metals found in those compounds. Understanding the chemical properties of sodium compounds is vital for predicting reaction products and for the synthesis of new materials.