Properties of Matter Chapter 2, Section3

Chemical & Physical Properties Chemical properties are about how a substance reacts with another substance to form new substances Chemical reactions describe how one substance combines and rearranges atoms in order to change into another substance Reactivity is how a substance changes Physical properties describe a pure substance. Physical properties stay the same as long as the substance is the same

Chemical Changes Chemical Changes are changes in the composition of the substance Atoms rearrange Becomes a new substance with new properties Identifying Chemical Changes New substance formed Change in energy (up or down) Smells Color change

Physical Changes Physical Changes Do Not Change Composition of the Substance Changes the way it looks Make smaller/larger particles Changes from solid/liquid/gas Properties of the substance do not change Dissolving is a PHYSICAL Change Molecules of salt/sugar haven’t changed Molecules are just surrounded by water molecules

Physical Properties of Matter Extrinsic Properties – depend upon how much of the substance is present Mass, weight, volume Intrinsic Properties are the same no matter how much of the substance you have: Density (mass/volume) Melting point Boiling point Strength Hardness Conductivity Magnetism Physical Properties can all be MEASURED

Density Density is mass/volume When we say light, we frequently really mean less dense. When we say heavy, we frequently mean more dense Compare density by weighting equal volumes of two substances: Less dense weighs less More dense weighs more

What’s So Important About Density? Density allows you to know if something will float or sink: A less dense item will ‘float’ on a more dense item (especially liquids & gasses) Gasses bubble to the top of a liquid because they are much less dense Oil floats on water because it is less dense Buoyancy is the tendency of a less dense substance to rise and float on top of a more dense liquid Usually water

Properties Let Us Determine Uses Why is copper used for electrical wires? It has a high measure of conductivity It is cheaper than the next best alternative (silver) Why do we use antifreeze in our cars? It freezes at a much lower temperature than water It boils at a higher temperature than water

Properties of Liquids Boiling point Melting point Diffusion – thorough mixing without stirring Very little compressibility Buoyancy – the buoyant force on an immersed body is equal to the weight of the liquid displaced by the body Viscosity – resistance to flow

Buoyancy Buoyancy is the relative density, or weight per unit of volume, of the body compared to the fluid determines the buoyant force. If the body is less dense than the fluid, it will float or, in the case of a balloon, it will rise. If the body is denser than the fluid, it will sink. Relative density also determines the proportion of a floating body that will be submerged in a fluid. If the body is two thirds as dense as the fluid, then two thirds of its volume will be submerged, displacing in the process a volume of fluid whose weight is equal to the entire weight of the body. In the case of a submerged body, the apparent weight of the body is equal to its weight in air less the weight of an equal volume of fluid. The fluid most often encountered in applications of Archimedes' principle is water, and the specific gravity of a substance is a convenient measure of its relative density compared to water. In calculating the buoyant force on a body, however, one must also take into account the shape and position of the body.

Viscosity Viscosity is a RESISTANCE to flowing More viscous flows slower Less viscous flows faster Viscosity is determined by attraction between particles in a fluid Which is more viscous – water, syrup, motor oil? Syrup, then motor oil, then water

Tasks for Today Make Sure Everyone has Set Up a User ID Complete the Density – Slice & Dice Lab Everyone take the Quiz at the end under your UserID Complete Density Via Comparison Gizmo Everyone take the quiz Work on the Buoyancy Lab (Archimedes Principal) Take the Quiz