Physical Properties (Section 2.2)

Physical Properties Is any characteristic of a material that can be observed or measured without changing the composition of the substances in the material Viscosity, conductivity, malleability, hardness, melting point, boiling point, and density are examples of physical properties

Viscosity Viscosity is the tendency of a liquid to keep from flowing (its resistance to flowing) The greater the viscosity, the slower the liquid moves Thick liquids like corn syrup and honey have a high viscosity Thin liquids like vinegar and water have a low viscosity The viscosity of a liquid usually decreases when it is heated

Conductivity A material’s ability to allow heat to flow Materials that have a high conductivity, such as metals, are called good conductors If a material is a good conductor of heat it is usually also a good conductor of electricity Wood is not a good conductor of heat

Malleability Is the ability of a solid to be hammered without shattering Most metals are malleable Solids that shatter when struck are brittle

Hardness Tip of drill is coated in diamond One way to compare the hardness of two materials is to see which of the materials can scratch the other Diamond is the hardest known material

Melting & Boiling Points The temperature at which a substance changes from a solid to a liquid is its melting point The temperature at which a substance boils is its boiling point (liquid to gas) Melting and Boiling Points of Some Substances Substance Melting point Boiling point Hydrogen -259.30C -252.90C Nitrogen -210.00C -195.80C Ammonia -77.70C -33.30C Octane (found in gasoline) -56.80C 125.60C Water 0.00C 100.00C Acetic Acid (found in vinegar) 16.60C 117.90C Table Salt 800.70C 14650C Gold 1064.20C 28560C

Density Is the ratio of the mass of a substance to its volume Density = Mass Volume Can be used to test the purity of a substance Silver has a density of 10.5 g/cm3 at room temperature but if you have a coin with a density of 9.9 g/cm3 at room temperature it must not be pure

Using Properties to Identify Materials Step 1: decide which properties to test Step 2: do tests on a sample of the unknown Step 3: compare the results with the data reported for the known materials When a car is involved in a hit-and-run accident detectives use paint chips from the scene of the crime to identify the make and model of the vehicle using a database of the different paint mixtures

Using Properties to Choose Materials People don’t consider just one property when choosing a material for a particular application Example You wouldn’t want shoelaces made of wood or steel Shoelaces need to be flexible, durable, and easy to secure

Separating Mixtures Some properties can be used to separate mixtures Filtration Uses a porous barrier to separate a solid from a liquid Good for heterogeneous mixtures

Distillation Uses differences in the boiling points of the substances involved Crystallization Results in the formation of pure solid particles of a substance from a solution containing the dissolved substance

Chromatography Parts of a mixture are separated based on how fast they travel through a medium

Recognizing Physical Changes A physical change occurs when some of the properties of a material change, but the substances in the material remain the same Change in the state of matter

Physical Changes Crumpling a piece of paper changes the size and shape of the paper and slicing a tomato changes the size and shape of the tomato but it does not change their composition Some physical changes can be reversed such as melting ice and then freezing it again Some cannot be reversed (You can’t put the sliced tomato back together and make it whole)