matter is anything that has mass

and occupies space. it can have many forms. we will commonly divide matter into

three phases

solid liquid and gas. we are probably familiar with these terms

let's define them anyway. a solid has fixed

shape and volume, the particles are touching and do not move

a liquid has fixed volume but no fixed shape

it will mold to its container. the particles are still touching

but they do move fluidly. a gas has no fixed volume

or fixed shape. it will completely fill its container. the particles are far apart

moving freely, and seldom touch. matter will frequently change forms

and describing these changes is at the heart of chemistry. a physical change

is one in which the chemical composition of the substance is

unchanged. when ice melts water is going from the solid phase

to the liquid phase. the way the water molecules are arranged

changes but they are still water molecules. a chemical change

is one in which the chemical composition of the substance does change

chemical bonds between atoms must break and form

to generate completely new substances. here

hydrogen gas and oxygen gas combine to form water

notice that hydrogen and oxygen used to be connected to themselves

and are now connected to each other. new bonds

new substance, chemistry has occurred

so things like a substance changing from one phase to another

are always physical changes. but if we make an entirely new substance with new

chemical bonds

thats a chemical change. a pure substance

is one that can't be separated into other materials

by any physical process. water whether you boil

or freeze it will still be water. it's a type of molecule

which is just multiple connected atoms. an element can't be broken down into

smaller parts by physical

or chemical means. for example water is not an element

it's a compound because it is made up two or more

elements, in this case oxygen and hydrogen. by a chemical process we can

divide water into these two elements

but that's as far as we can go with chemistry. these are now molecules

of a given element. a mixture is made of two or more

pure substances and these can be separated by physical processes

if you boil salt water the water will evaporate

and the salt won't so the substances will have separated

a mixture can be homogeneous meaning all the substances

are distributed evenly and every section looks the same

like sugar and water, or they can be heterogeneous

meaning they are not distributed evenly and zooming in on different sections

would look differently like oil and water.

we've got pure substances which can be elements made of only one type of atom

or compounds which are made of different types of atoms

but are still just one type of molecule.

or we can have mixtures which are made of multiple types of molecules

and are arranged in either homogeneous or heterogeneous fashion. let's check

comprehension

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