# The Ocean in a Tide Pool

How salt, sugar and oxygen dissolve in water, from solute and solvent to saturation, temperature effects, electrolytes and crystals forming as seawater evaporates\.

By Numa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/9O1x6DZK5g23)

## Transcript

### 1. A Piece of the Ocean

At low tide, this rock pool is a small piece of the ocean\. Seawater is a solution: a mixture so even that every drop is the same\. The liquid doing the dissolving, water, is the solvent\. What it dissolves, mostly salt, is the solute\. The brass refractometer reads 35 grams of salt per kilogram of seawater\.

Spread over the land, all the salt in the oceans would make a layer about 150 metres thick\.

### 2. Water Pulls Salt Apart

Drag the scoop and tip salt into the jar\. Water is polar, with a slightly negative oxygen end and slightly positive hydrogen ends\. So it tugs sodium and chloride ions out of the crystal and surrounds them, shown magnified and not to scale\. The readout shows molarity, the moles of dissolved salt per litre\. It rises with every scoop\.

Seawater holds about 0\.6 moles per litre, counted as if all its salt were sodium chloride\.

### 3. Saturated, Then Warmed

While more salt could dissolve, the solution was unsaturated\. Now crystals stay on the bottom: it is saturated, holding all the salt it can at 18 degrees\. Drag the jar into the sunshine, shown here sped up\. Only a little more dissolves: 100 grams of water holds 35\.9 grams of salt at 18 degrees and 36\.4 at 40\.

Hot tea dissolves far more sugar than iced tea, which is why sweet tea is made hot\. Salt does not follow the same rule\.

### 4. More Than It Can Hold

The warm jar is clear: all the salt has dissolved\. Cooled slowly in the shade, shown here sped up, it keeps 1\.4 grams more salt than it can normally hold\. The solution is now supersaturated\. Drop in a seed crystal and the extra salt comes out in seconds\.

Reusable hand warmers use this trick\. Click the disc and crystals race through the pack, making heat\.

### 5. Sugar, Stone and Current

Drag the sugar cube into the big jar, then drag the pebble in too\. The sugar dissolves but the pebble does not: not every solid is soluble\. When the sugar has gone, press Test current\. The probe tries the small glass of salty pool water first and the bulb lights, because salt gives free ions\. Salt is an electrolyte\. Then it tries the sugar water and the bulb stays dark, because whole sugar molecules carry no charge\.

Your nerves and muscles run on electrolytes, which is why sports drinks contain salts\.

### 6. Oxygen in Warm Water

The sculpins breathe oxygen dissolved in the pool\. Slide the pool temperature up and watch the fish\. Gases dissolve less in warm water, the opposite of most solids\. So oxygen leaves as bubbles and the fish breathe hard near the surface\. The pool holds 8\.8 milligrams of oxygen per litre at 10 degrees but only 6\.1 at 30\.

A warm fizzy drink goes flat faster for the same reason\.

### 7. Like Dissolves Like

Drag the bottle of oil over the pool and tip it in\. Water molecules are polar and cling to each other\. Oil molecules are nonpolar, so the water will not make room for them, and the oil floats as a thin film\. The lens shows both, not to scale\. Polar solvents dissolve polar and ionic solutes\. Nonpolar solvents dissolve nonpolar ones\. Like dissolves like\.

Soap works because one end of its molecule is polar and the other is not\. Never pour oil into a real rock pool\.

### 8. Salt from the Sea

On salt flats, the sun does the work\. In this time lapse only the water evaporates, so the salty water left behind, the brine, grows more concentrated until it is saturated\. Then cubic crystals grow, because sodium and chloride ions stack in a cubic lattice, a repeating grid\. How much dissolves depends on the solute, the solvent and the temperature\.

The Dead Sea is about 34 percent salt, so dense that people float in it with ease\.