Changes to Matter

The re-arrangement of existing particles in a substance or material.

New substances are NOT formed.

(Composition remains the same)

• Cutting

• Chopping

• Crushing (powderizing)

• Grinding

• Breaking

• smaller pieces = more surface area, but not new substance(s)

(phase changes)

• melting (s) --> (l)

• vaporizing, (l) --> (g)

• subliming, (s) --> (g)

• condensing, (g) --> (l)

• freezing, (l) --> (s)

• depositing, (g) --> (s)

​Water on a hot plate

At the particle level

Changing phase from liquid to gas does not change a water molecule into a new substance. (Still H₂O)


H₂O (l) --> H₂O (g)

Ex: dissolving solid NaCl in water

NaCl _(s) ---> NaCl <sub>(aq)


NaCl (s) can be recovered using physical methods of separation</sub>

A new substance (CO₂) is formed that has different composition than the original substances (NaHCO₃).

A chemical reaction occurred.

(a.k.a. a chemical change)

The formation of new substances with physical and chemical properties different than the starting substances.

Chemical composition has changed (chemical formula is different now)

• Most chemical reactions release heat

• (though a few absorb heat)

• If Temp goes up, heat was released. (exothermic)

• If Temp goes down, heat was absorbed. (endothermic)

​combustion of a fuel (such as propane)

• Example: Some compounds change color based on acidity of the solution

• some chemical reactions release energy in the form of light

• PbCl_{2 (aq)} + 2NaI _(aq) ---> PbI_{2 (s)} + 2NaCl _(aq)

• Ex: Alka Seltzer tablet in water

• production of CO_{2 (g)}

• Cooking causes chemical changes in foods that are observed as new smells or new flavors

• Examples: carmelizing onions, baking bread

Physical changes alter the arrangement of particles, but do NOT change the composition:

• cutting, crushing, powderizing (more pieces and surface area)

• phase changes NaCl (s) --> NaCl (l)

• dissolving NaCl (s) --> NaCl (aq)

• Chemical changes result in new substance(s) with different properties than the starting materials.

• Ex: 2Na _(s) + Cl_{2 (g)} ---> 2NaCl _(s)

• Sodium metal and chlorine gas are the "reactants" (starting materials)

• Sodium chloride is the "product" and has different properties than the reactants

Signs of a chemical change include:

• Spontaneous formation of a gas (bubbling), solid precipitate, or new odor/taste

• release of light

• Temperature change (Heat change)

• Color change