science

science 2.1
pure substances
matter that always has the exactly the same composition is classified as pure substance or simply a substance. table salt are two examples of pure subsatances. every pinch of salt tastes equally salty. and the same for sugar *every sample of a given substance has the same properties because a substance has a fixed uniform
composition. substances can be classified into two categories elements and compounds.
*an element is a substance that cannot be broken down into simpler substances. an atoms is the smallest particle of an element an atom has a fixed composition because it contains only two types of atoms.*
compounds
a compound is a substance that is made from two or more simpler substances and can be broken down into  simpler substances
*a compound always contains two or more elements joined in a fixed proportion *
mixtures tend to retain some properties of their individual substances. but the properties of a mixture are less constant than the property of the substance *the properties of a mixture can vary because of the composition of a mixture is not fixed *
heterogenous mixture
in a heterogeneous mixture are noticeably different from another.
homogenous the substances are so evenly mixed you cannot distinguish one mixture in the substance from another.
solutions suspensions and colloids
*based on the size of its largest particles , a mixture can be classified as a solution suspension or colloid
solutions
when substances dissolve and form a homogenous mixture that forms is called a solution
suspensions
a suspension is a heterogeneous mixture that separates into two layers over time.
colloids
colloids contains some particles that are intermediate in size between the smaller particle and in a solution and larger particles in a suspension

2.2
examples of physical propetries
a physical property is any characteristics of a material that can be observed or measured without changing the composition of the substances in the material *viscosity conductivity and malleability hardness melting point boiling point and density are examples of physical properties.
viscosity
the tendency of a liquid to keep from flowing its resistance to flowing is called viscosity
the greater the viscosity the slower the liquid moves thinner liquids like vinegar have slower viscosity
conductivity
a materials ability to allow heat to flow is conductivity
malleability
malleability is the ability of a solid to be hammered multiple times with out breaking gold is malleable however glass is not malleable
using physical properties
*physical properties are used to identify a material to choose a material for a specific purpose or to separate a substances in a mixture*
using properties to separate mixtures
*filtration and distillation are two common methods*
filtration
is a process of separation materials based on the size of their particles
distillation
is the process that separates the substances in a solution based on their boiling points
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

2.3
observing chemical properties
chemical properties is any abillity to produce a change in the composition of matter
*chemical properties can be obsered only when the substances in a sample of matter are changing in the composition of matter*
chemical properties can be observed only when the substances in a sample of matter are changing into different substances.
flammability is a material that has the ability to burn at the presence of oxygen
reactivity
the property that describes how readily a substance combines chemically with other substances is reactivity. nitrogen has extremely low reactivity
recognizing chemical changes
a chemical change occurs when a substance reacts and forms one or more new substances
*three common types of evidence for a chemical change in color the production of gas and the formation of precipitate
precipitate
any solids that forms and separates from a liquid mixture is called a precipitate
is a chemical change physical
*when matter undergoes chemical change the composition of the matter change when matter undergoes a physical change the composition of the matter remains the same.