2-7 What affects the solubility of a substance in water? (Cambridge AS & A Level Biology)

Summary

This video explains the relationship between polarity and solubility, focusing on why some substances dissolve in water while others do not. Water, a polar molecule with partially positive and negative charges, can dissolve polar substances like salt through electrostatic interactions, making them hydrophilic. In contrast, nonpolar substances like oil lack charges, preventing interaction with water molecules. As water forms hydrogen bonds with itself, it pushes oil aside, which then forms hydrophobic interactions and floats due to lower density. The video also highlights examples, such as sugars and amino acids being polar and soluble, while oils and starch are nonpolar and insoluble, providing a clear understanding of solubility in biological contexts.

Takeaways

• 💧 Polarity refers to the presence of charges in a molecule, where polar molecules have partial positive and negative charges, and nonpolar molecules do not.
• 🌊 Water is a polar molecule with a partially positive hydrogen and partially negative oxygen, making it a dipole.
• 🧂 Solubility is the ability of a substance to dissolve in a solvent, and it is closely linked to polarity.
• ⚗️ Water is an important solvent in biological systems, which is why solubility in water is often studied.
• 🧂 Salt (sodium chloride) dissolves in water because both salt and water are polar, allowing charge interactions.
• 🔗 Sodium ions (Na⁺) interact with the negative end of water molecules, while chloride ions (Cl⁻) interact with the positive end.
• 🛢️ Oil does not dissolve in water because it is nonpolar and lacks charges, so it cannot interact with polar water molecules.
• 💦 Water molecules form hydrogen bonds with each other, which pushes nonpolar molecules like oil aside, creating separation.
• 🌱 Polar molecules are hydrophilic (water-loving) and dissolve easily in water, while nonpolar molecules are hydrophobic (water-fearing) and do not dissolve.
• 🍬 Examples of polar molecules that dissolve in water include sugars, amino acids, and some salts, while oils, fats, and polysaccharides like starch are nonpolar and insoluble.
• 🔬 Molecular structure matters: alpha-glucose is soluble in water, but when linked into starch (amylose), it becomes insoluble.

Q & A

What is polarity, and how does it relate to solubility?

Polarity refers to the presence of partial positive and negative charges in a molecule. Polar molecules can dissolve in polar solvents like water, because the partial charges in the molecule interact with the charges in the solvent. This is key to understanding solubility.

Why is water considered a polar molecule?

Water is considered polar because it has a partially positive charge on the hydrogen atoms (δ⁺) and a partially negative charge on the oxygen atom (δ⁻), making it a dipole.

What does it mean when a substance is hydrophilic?

Hydrophilic means 'water-loving.' A hydrophilic substance is one that interacts with water molecules due to its polarity, allowing it to dissolve in water. Examples include salts and sugars.

Why does salt dissolve in water but oil does not?

Salt dissolves in water because it is polar and interacts with water molecules, which are also polar. Oil, on the other hand, is nonpolar and cannot interact with water molecules, so it does not dissolve in water.

What happens when salt (NaCl) is added to water?

When salt is added to water, the positive sodium ions (Na⁺) interact with the partially negative oxygen atoms of water, and the negative chloride ions (Cl⁻) interact with the partially positive hydrogen atoms. This interaction causes the salt to break apart and dissolve.

What does it mean for a molecule to be nonpolar?

A nonpolar molecule has no charge separation, meaning there are no partial positive or negative charges within the molecule. Nonpolar molecules, like oil, do not interact with polar solvents like water.

How does the concept of 'like dissolves like' apply to solubility?

'Like dissolves like' means that polar substances tend to dissolve in polar solvents (like water), and nonpolar substances tend to dissolve in nonpolar solvents. Water, being polar, dissolves other polar substances but not nonpolar ones like oil.

What is a hydrophobic interaction?

A hydrophobic interaction occurs when nonpolar molecules, like oil, aggregate together because they are repelled by the polar water molecules. This results in the oil floating on top of water.

Why does oil float on top of water?

Oil floats on top of water because it is less dense than water and does not dissolve in water due to its nonpolar nature. The water molecules form hydrogen bonds with each other, pushing the oil molecules away.

Why is starch (amylose) insoluble in water despite being made of glucose molecules?

Although starch is made of glucose, when glucose molecules are linked together in the form of starch (amylose), their structure changes, making it insoluble in water. Alpha glucose is soluble in water, but starch is not.