Describing Matter

Describing Matter Pages 11-19

How do you identify an object? • Descriptions: • Color • Smell • Touch • Taste • Hear • Others?

Physical Properties • Physical property – something that can be observed or measured without changing the identity of matter • Ex) Rip a piece of paper

Physical Properties • Physical properties help you with everyday things, like determining if your socks are dirty (odor), can fit books into your locker (volume), and matching your clothes (color). • Other physical properties include: • Thermal conductivity • State • Malleability • Ductility • Solubility • Density

Density is… • The amount of matter in a given space • D = m / v • Units for density = g/cm3, or g/ mL • Density can be used to identify substances for two reasons: • 1. density stays the same for a substance at a given pressure and temperature • 2. density of a substance is unique

Densities

Determining Substances • How can we determine if the blocks below are plastic or rubber? They might look the same, but they aren’t. • We use density to know for sure

Liquid Layers • Have you ever seen a bottle of salad dressing have two layers to it? • The reason for the two layers is density. Density causes a separation between the layers that are more dense(bottom) and the those that are less dense (top). • If you shake the salad dressing it will mix, but not for long. Once it settles, the dressing separates again.

Chemical Properties • Chemical properties – describe a substance based on its ability to change into a new substance with different properties • Ex) Wood has the chemical property of flammability or ability to burn. • Other examples include reactivity with oxygen, reactivity with acid, and reactivity with water.

Observing chemical properties • You can observe chemical properties with your senses, but not as easily as physical properties • You can observe a piece of wood burning, but the wood still has the property of flammability when it is not burning.

Rust • When an object rusts, it has a reactivity with oxygen • The wrench is made of steel which comes from iron • Iron is strong and malleable, but it can react with oxygen and rust.

Physical vs. Chemical Properties

Physical change • Physical change is a change that affects one or more physical properties of a substance. • Ex) break a piece of chalk, but its still chalk • Examples of physical changes: • Melting • Dissolving • Freezing • Sanding • Cutting • Crushing • Bending • Mixing

Physical Changes • Physical changes do not change the identity of a substance. • They are often easy to undo • Ex) clay can be remolded, aluminum cans can be uncrushed, etc

Chemical Changes • Chemical changes – occurs when one or more substances are changed into entirely new substances with different properties. • It is different from chemical properties. • A property is the ability to undergo a chemical change, while the change is the actual process • Ex) bake a cake, sour milk, Statue of Liberty, effervescent tablets.

How do you know when it’s a chemical change? • There are clues to knowing when a chemical change has occurred. • Color changes • Fizzing or foaming • Heat given off • Sound • Light • odor

Can you reverse a chemical change? • Chemical changes cannot be reversed. • You can’t unbake a cake into eggs, flour and sugar. • Exception: • There are a few exceptions to chemical changes • One is to reverse the forming of water from the gases hydrogen and oxygen. • It can be done using an electric current

Describing Matter