MYP Chemistry Topic Neutral Solutions Revision Notes

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MYP Chemistry Topic Neutral Solutions Revision Notes | RevisionDojo

What Does It Mean for a Solution to Be Neutral?

Pure Water and Neutral Solutions

In pure water at $25^{\circ} \mathrm{C}(298 \mathrm{~K})$:

A few water molecules ionise very slightly into hydrogen ions and hydroxide ions: $$\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$$
At this temperature, in a neutral solution: $$\left[\mathrm{H}^{+}\right]=\left[\mathrm{OH}^{-}\right]=1.0 \times 10^{-7} \mathrm{~mol} \mathrm{dm}^{-3}$$
The product of these concentrations is constant at $25^{\circ} \mathrm{C}$: $$\left[\mathrm{H}^{+}\right]\left[\mathrm{OH}^{-}\right]=1.0 \times 10^{-14}$$

This constant is called $K_w$, the ionic product of water.

Definition

Neutral solution (at 25 °C)

A solution is neutral if $\left[\mathrm{H}^{+}\right]=\left[\mathrm{OH}^{-}\right]$ and its pH = 7.

Tip

If $\left[\mathrm{H}^{+}\right]>\left[\mathrm{OH}^{-}\right]$ → acidic (pH < 7)
If $\left[\mathrm{H}^{+}\right]<\left[\mathrm{OH}^{-}\right]$ → basic (pH > 7)
If $\left[\mathrm{H}^{+}\right]=\left[\mathrm{OH}^{-}\right]$→ neutral (pH = 7 at 25 °C)

Water as Both Acid and Base: Amphiprotic

As discussed in the previous article, in the Brønsted–Lowry model:
1. An acid is a proton (H⁺) donor.
2. A base is a proton (H⁺) acceptor.
Water can do both, depending on what it is reacting with:
1. With a strong base like NaOH, water can donate a proton (acting as an acid).
2. With a strong acid like HCl, water can accept a proton (acting as a base).

Note

A substance that can both donate and accept H⁺ is called amphiprotic.
Meaning, water is amphiprotic.

How Can We Test if a Solution Is Neutral in the Lab?

We never taste solutions to check if they are acidic or basic. Instead, we can:

Use universal indicator
1. Neutral solution → green (around pH 7).
2. Acidic solution → red/orange/yellow.
3. Basic solution → blue/purple.
Use litmus paper
1. Blue litmus in neutral solution → stays blue.
2. Red litmus in neutral solution → stays red.

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MYP Chemistry Neutral Solutions Notes

What Does It Mean for a Solution to Be Neutral?

Pure Water and Neutral Solutions

Water as Both Acid and Base: Amphiprotic

How Can We Test if a Solution Is Neutral in the Lab?

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What Does It Mean for a Solution to Be Neutral?

Pure Water and Neutral Solutions

Water as Both Acid and Base: Amphiprotic

How Can We Test if a Solution Is Neutral in the Lab?

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Periodic table5 subtopics

Matter5 subtopics

Types of chemical reaction18 subtopics

International Union of Pure and Applied Chemistry (IUPAC)4 subtopics

Pure and impure substances2 subtopics

The atmosphere2 subtopics

Bonding4 subtopics

MYP Chemistry Neutral Solutions Notes

Periodic table5 subtopics

Matter5 subtopics

Types of chemical reaction18 subtopics

International Union of Pure and Applied Chemistry (IUPAC)4 subtopics

Pure and impure substances2 subtopics

The atmosphere2 subtopics

Bonding4 subtopics

What Does It Mean for a Solution to Be Neutral?

Pure Water and Neutral Solutions

Water as Both Acid and Base: Amphiprotic

How Can We Test if a Solution Is Neutral in the Lab?

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What Does It Mean for a Solution to Be Neutral?

Pure Water and Neutral Solutions

Water as Both Acid and Base: Amphiprotic

How Can We Test if a Solution Is Neutral in the Lab?

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