Biology · The Chemistry of Life

Water, solutions, ions and biological environments

Imagine two labelled containers, each holding the same dissolved amount of a substance. One contains twice the final solution volume of the other. The substance has not vanished from the larger container, but it is less concentrated. That distinction matters whenever we describe a cell's chemical surroundings.

You can begin without a completed chemistry course. Each lesson introduces its essential symbols. The earlier chapter's distinction between a system and its surroundings is useful background. The Chemistry map offers optional depth on atoms, bonding and solutions; those links are support rather than a requirement to finish another subject first.

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By the end, you can

Explain water's interactions with solutes and calculate concentration without confusing it with total amount.

Water changes the surroundings of dissolved particles

Water is polar: its oxygen side is partially negative and its hydrogen sides partially positive. Around a positive ion, oxygen sides tend to face inward; around a negative ion, hydrogen sides tend to face inward. The hydration sketch represents this orientation, not a fixed shell that freezes every molecule in place.

Solute names what is dissolved; solvent names the surrounding medium. A solution is a mixture with uniform composition at the scale at which we describe it. Dissolved particles can remain present even when we cannot see them as separate pieces.

Water dissolves many ionic and polar substances, but not everything. An oil-rich phase can remain separate. Hydrophilic and hydrophobic describe behaviour in relation to water, not feelings or a simple ranking of useful and useless molecules.

Water faces positive and negative ions differently

Full ion charges and partial water charges are different. The δ labels mark partial charges; the surrounding water molecules are not covalently bonded to the ion.

Read the diagram in words

The partially negative oxygen side tends to orient towards a positive ion. The partially positive hydrogen side tends to orient towards a negative ion. Both panels represent moving water molecules and full ion charges.

Na⁺ (+1): Oxygen, δ−
Cl⁻ (−1): Hydrogen, δ+

Ask 'how much?' and 'in how much solution?'

Amount of substance counts specified entities using the mole, mol. One mole contains exactly 6.02214076 × 10²³ entities. A millimole, mmol, is one thousandth of a mole. Naming the entities matters: moles of glucose molecules and moles of sodium ions are different records.

Amount concentration is amount divided by final solution volume: c = n/V. Here n is dissolved amount and V solution volume. A concentration of 20 mmol L⁻¹ means twenty millimoles per litre, not twenty millimoles in every container regardless of size.

The constructed comparison contains 4 mmol in 200 mL, which is 0.200 L. Its concentration is 4/0.200 = 20 mmol L⁻¹. With the same amount in a final volume of 0.400 L, concentration is 10 mmol L⁻¹. Doubling the volume halves concentration under the stated conservation assumptions.

Same amount, larger volume, lower concentration

The two-dimensional areas represent a volume ratio for this teaching example. The same twelve solute tokens remain in both panels; vessel shape and molecular spacing are schematic.

Read the diagram in words

The solute amount is unchanged at 4 mmol. Final solution volume doubles from 200 to 400 mL, so concentration halves from 20 to 10 mmol L⁻¹.

Same amount, larger volume, lower concentration
Description	mmol	mL	mmol L⁻¹	Count · ●
Before	4	200	20	12
After dilution	4	400	10	12

A cell's water is a chemical environment

Water participates in reactions as well as providing a medium. It has a relatively high specific heat capacity: a given mass requires substantial transferred heat for a given temperature rise compared with many other materials. This can moderate temperature changes, but it does not keep water or cells at one unchanging temperature.

A record of a cellular environment should identify the solutes, their concentrations and relevant conditions. We will later ask how membranes maintain different environments. For now, recognising the difference does not require explaining every transport mechanism.

Worked example

Dilute without losing solute

Begin with the supplied 4 mmol in 0.200 L. Add solvent until the final solution volume is 0.400 L, with no solute loss or reaction. The amount remains 4 mmol, while c changes from 20 to 10 mmol L⁻¹. Check the inventory backwards: 10 mmol L⁻¹ × 0.400 L = 4 mmol. A statement such as 'half the substance remains' would confuse a lower concentration with a smaller total amount.

Check your understanding

Four millimoles are dissolved in a final volume of 100 mL. What is the amount concentration?
Show reasoning
100 mL is 0.100 L, so 4 mmol/0.100 L = 40 mmol L⁻¹.
If you double solution volume without adding or losing solute, what happens to its concentration?
Show reasoning
It halves, assuming the substance remains dissolved and is not chemically transformed. Its total amount stays the same.
Why does the negative oxygen side of water tend to face a positive ion?
Show reasoning
The opposite charges favour that orientation. Water molecules still move and rearrange; the sketch is not a rigid arrangement.

What to carry forward

Describe both the dissolved entities and their amount per final solution volume.

References

Resources supporting this lesson’s explanations and examples.