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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

•  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 •  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

•  The chisel is made of metal which is harder than the wood so it is used to carve into the wood

Tip of drill is coated in diamond

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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) Mel$ng  and  Boiling  Points  of  Some  Substances

Substance Mel$ng  point  (solid  to  liquid) Boiling  point  (liquid  to  gas)

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

Ace$c  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

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•  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)