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(Video) Liquids: Crash Course Chemistry #26

by CrashCourse

In this video, Hank from Crash Course tells you all about what makes liquids so unique. A lot of their properties are the result of the types of bonds that form between molecules. London dispersion forces, dipole-dipole interactions, and hydrogen bonds play a big role in how liquids act. These bonds allow liquids to have properties like:

• cohesion
• adhesion
• viscosity
• capillary action
• surface tension
• water's ability to act as a universal solvent

There are three states of matter: solids, liquids, and gasses. On the periodic table, liquids are the rarest. Only two elements are liquids at standard temperature and pressure(STP): the halogen bromine ($Br$) and the transition metal mercury ($Hg$). The bottom half of the alkali metals, rubidium ($Rb$) to francium ($Fr$), have low melting points. So, they are liquids just above STP. A similar element is the metal gallium. Gallium is famous for the disappearing spoon trick. To perform the deception, give someone a hot drink. Offer them a gallium spoon for their cream or sugar. As they stir, it disappears! The best part, gallium is not poisonous. So, if the person does not notice it vanished, no big deal!

Despite the rarity of liquids on the periodic table, we are very familiar with them. Liquid water covers seventy percent of our planet! We drink them every day. Even if you do not drink water, you may drink milk, soda, coffee, or tea. We use liquid oils in cooking.

Let's recall some of their properties. Envision pouring water into a square container. The water is square. Now, imagine pouring that same water into a drinking glass. Now the water is cylindrical. Liquids take the shape of their containers. But, they have fixed volumes. Let's say you poured 500.0 ml of water into the square glass. That means you poured 500.0 ml into the drinking glass. Why can a liquid be so flexible? Let's look at the chemistry!

The Chemistry of Liquids

What causes a liquid's chemical properties? Van Der Waal's forces! There are three types: dispersion, dipole-dipole, and hydrogen bonding. All are weaker than ionic and covalent bonds. In liquids, the atoms or molecules can move past one another. Yet, they still experience intermolecular attraction. So they stay near each other.

Van Der Waal's forces create the unique physical properties of liquids. Let's explore these!

Unique Properties of Liquids

Cohesion: Cohesion is a liquid's attraction to itself. To imagine cohesion, think of a drop of water sitting on plastic. The water will form a dome shape. It is holding itself together by hydrogen bonding. Water is more attracted to itself than to plastic. So we say water has strong cohesion or a powerful, cohesive force.

Adhesion: Adhesion is a liquid's attraction to its container or surface. It results from London dispersion forces, and all substances experience some adhesion. Let's return to our water drop on a piece of plastic. We know cohesion keeps the water together. But, adhesion keeps the water drop from moving. Imagine slowly tilting the plastic. Once the gravitational force exceeds adhesion, then the water runs off.

Viscosity: A liquid's viscosity is its resistance to flow. A larger viscosity means less flow. In everyday life, we talk about viscosity as thickness. For example, pancake syrup is thicker than water. Syrup is very viscous because of the hydrogen bonding between glucose and fructose. Glucose and fructose are two allotropes of sugar.

Surface Tension: Surface tension is a weird concept. It has units of newtons per meter. A force divided by length, why is that? It is the force needed to keep the surface in place. The surface breaks if anywhere experiences too large a force. Liquids will form the shape that maximizes surface tension. For example, in space, water forms into a sphere. As we saw earlier, a water droplet on plastic will form a dome.

But why does surface tension happen? Let's imagine the surface tension of a droplet of water. At its center, the molecules surround one another. So, water hydrogen bonds in every direction. The net force is zero. But, on the droplet's surface, the molecules are not surrounded. They are only hydrogen bonding with the inner molecules. So, there is a force toward the center of the droplet. The inward cohesive force is strong. The adhesion with the air is much weaker. So, the surface stays together.

Capillary Action: Think about dipping the corner of a paper towel in water. What will you see? The area above the water will become wet. The water is crawling up the paper towel. What you are seeing is capillary action. The adhesive force between the water and the paper towel is larger than gravity. When a large enough mass of water rises, the flow stops. The gravitational force has equaled the adhesion.

Water as a Solvent: Water is often called the universal solvent. More substances are soluble in water than any other liquid. Water can even break some ionic compounds like sodium chloride. Water interacts with sodium and chloride ions via ion-dipole forces. So, water balances the charges.