---
title: "In a water-treatment materials study, a student adds solid \\(\\text{Na}_2\\text{O}_2\\text{(s)}\\) to excess \\(\\text{H}_2\\text{O(l)}\\) at \\(25^\\circ\\text{C}\\). The solid disappears, and chemical analyses give the results shown.  | Stage | Species identified in the liquid | |—|—| | Before addition | \\(\\text{H}_2\\text{O(l)}\\) | | After the solid disappears | \\(\\text{Na}^+\\text{(aq)}\\), \\(\\text{OH}^-\\text{(aq)}\\), and \\(\\text{H}_2\\text{O}_2\\text{(aq)}\\); no \\(\\text{O}_2^{2-}\\text{(aq)}\\) is detected |  The oxidation state of oxygen is \\(-1\\) in both \\(\\text{O}_2^{2-}\\) and \\(\\text{H}_2\\text{O}_2\\). Which statement best classifies the process and justifies the classification?"
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url: "https://nerd-notes.com/ubq/120065/"
date_modified: "2026-08-21T08:41:08+00:00"
---

# In a water-treatment materials study, a student adds solid \(\text{Na}_2\text{O}_2\text{(s)}\) to excess \(\text{H}_2\text{O(l)}\) at \(25^\circ\text{C}\). The solid disappears, and chemical analyses give the results shown.

| Stage | Species identified in the liquid |
|—|—|
| Before addition | \(\text{H}_2\text{O(l)}\) |
| After the solid disappears | \(\text{Na}^+\text{(aq)}\), \(\text{OH}^-\text{(aq)}\), and \(\text{H}_2\text{O}_2\text{(aq)}\); no \(\text{O}_2^{2-}\text{(aq)}\) is detected |

The oxidation state of oxygen is \(-1\) in both \(\text{O}_2^{2-}\) and \(\text{H}_2\text{O}_2\). Which statement best classifies the process and justifies the classification?

In a water-treatment materials study, a student adds solid \(\text{Na}_2\text{O}_2\text{(s)}\) to excess \(\text{H}_2\text{O(l)}\) at \(25^\circ\text{C}\). The solid disappears, and chemical analyses give the results shown.

| Stage | Species identified in the liquid |
|---|---|
| Before addition | \(\text{H}_2\text{O(l)}\) |
| After the solid disappears | \(\text{Na}^+\text{(aq)}\), \(\text{OH}^-\text{(aq)}\), and \(\text{H}_2\text{O}_2\text{(aq)}\); no \(\text{O}_2^{2-}\text{(aq)}\) is detected |

The oxidation state of oxygen is \(-1\) in both \(\text{O}_2^{2-}\) and \(\text{H}_2\text{O}_2\). Which statement best classifies the process and justifies the classification?

- **A.** The overall process includes a chemical reaction, because separating the ionic lattice into hydrated ions creates new substances according to \[ \text{Na}_2\text{O}_2\text{(s)}\rightarrow 2\text{Na}^+\text{(aq)}+\text{O}_2^{2-}\text{(aq)} \]
- **B.** The overall process includes a chemical reaction, because proton transfer from water to peroxide produces \(\text{H}_2\text{O}_2\text{(aq)}\) and \(\text{OH}^-\text{(aq)}\), consistent with \[ \text{Na}_2\text{O}_2\text{(s)}+2\text{H}_2\text{O(l)}\rightarrow 2\text{Na}^+\text{(aq)}+2\text{OH}^-\text{(aq)}+\text{H}_2\text{O}_2\text{(aq)} \]
- **C.** The process is only physical dissolution, because oxygen remains in the \(-1\) oxidation state and a chemical reaction must involve electron transfer.
- **D.** The process is only physical dissolution, because an \(\text{O}-\text{O}\) bond is present in both \(\text{O}_2^{2-}\) and \(\text{H}_2\text{O}_2\), so the identity of the peroxide species is unchanged.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120065/*