Lecture 70 – Lecture 71 – Lecture 72 Liquids and Solids Ozgur Unal NIS – CHEMISTRY Lecture 70 – Lecture 71 – Lecture 72 Liquids and Solids Ozgur Unal

Liquids Can take the shape of its container Have a fixed volume Are much denser than gases Are considered incompressible, unlike gases Kinetic-molecular theory can also be applied to liquids. The particles in a liquid also moves randomly, but this motion is limited to smaller space compared to gases. The forces of attraction between particles (interparticle forces) play an important role.

Liquid Fluidity Gases and liquids are classified as fluids because they can flow and diffuse. Liquids usually diffuse more slowly than gases at the asme temperature, because intermolecular attractions interfere with the flow. Liquids are less fluid than gases. Example: Water and natural gas in pipes

Liquid Viscosity Viscosity is a measure of the resistance of a liquid to flow. The particles in a liquid are close enough for attractive forces to slow their movement as they flow past one another. There are three factors that affect the viscosity of a liquid: Intermolecular forces Size and shape of the particles Temperature

Viscosity Attractive forces: In typical liquids, the stronger the intermolecular attractive forces, the higher the viscosity. Particle size and shape: Recall kinetic energy (KE)  depends on mass and speed Consider 2 different liquids A and B at the same temperature If the molecules in A are heavier, then liquid A is more viscous than B.

How do you think temperature changes viscosity? Viscosity decreases with temperature. Example: The oil in a frying pan starts flowing easily as you heat it.

Surface Tension How do you think this spider is able to stand on the water? Do you think intermolecular forces have anything to do with this?

Surface Tension Particles in the midlle of a liquid can be attracted from particles above and below them. The particles at the surface are attracted by the particles below. In order to increase the surface area of a liquid, the particles below the surface has to overcome these intermolecular attraction forces. The energy required to increase the surface area of a liquid by a given amount is called surface tension.

Surface Tension In general, the stronger the attractions between particles, the greater the surface tension. Water’s high surface tension is what allows the spider to walk on the surface of the pond. Why do water droplets form a sphere? Why do we use soaps and detergents in water to remove dirt from dishes and clothes? Compounds that lower the surface tension of water are called surfactants.

Cohesion and Adhesion How can you explain the surface shape (called meniscus) of this liquid in the container? Cohesion describes the force of attraction between identical molecules. Adhesion describes the force of attraction between molecules that are different. If a cylinder is extremely narrow, a thin film of water will be drawn upward  capillary action

Solids In solids, the intermolecular attractive forces are dominant  particles are tightly packed and they vibrate at their position. Fixed volume and shape  not a fluid! Generally solids have higher density than most liquids. Incompressible Some solids become less dense when they melt  benzene Some solids become more dense when they melt  water

Crystalline Solids In most solids, including ice, the molecules in the solid are packed together in a predictable way. A crystalline solid is a solid whose atoms, ions or molecules are arranged in an orderly, geometric structure. Check out Figure 12.19 for the three ways that particles in a crystal lattice can be arranged to form a cube.

Crystalline Solids A unit cell is the smallest arrangement of atoms in a crystal lattice that has the same symmetry as the whole crystal. The unit cell can be thought of as a building block whose shape determines the shape of the crystal. Check out Table 12.4 for the seven categories of crystals based on shape.

Categories of Crystalline Solids Crystalline solids can be classified into five categories based on: the types of particles they contain how those particles are bonded together. These categories are: Atomic solids Molecular solids Covalent network solids Ionic solids Metallic solids The only atomic solids are noble gases  their properties reflect the weak dispersion forces between the atoms

Categories of Crystalline Solids Molecular solids: In molecular solids, molecules are held together by dispersion forces, dipole-dipole forces or hydrogen bonds. Poor conductor of electricity. Most molecular compounds are not solids at room temperature. Example: Water Molecular compounds with high molar mass are solid at room temperature. Example: Sugar

Categories of Crystalline Solids Covalent network solids: Atoms such as C and Si, which can form multiple covalent bonds, are able to form covalent network. Example: Quartz (contains Si) Ionic solids: Each ion in an ionic solid is surrounded by ions of opposite charge. The type of ions and the ratio of ions determine the structure of the lattice and the shape of the crystal. Example: Salt

Categories of Crystalline Solids Metallic solids: Remember that metallic solids consist of positive metal ions surrounded by a sea of mobile electrons. As the mobile electrons increase, the metal becomes a better conductor, more malleable and ductile. Amorphous solids: An amorphous solid is one in which the particles are not arranged in a regular pattern  no crystals It forms when a molten material cools too quickly to allow enough time for crystals to form. Example: Rubber and plastics