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6 1 When Substances Change PS8

0 In a physical change no new substance is
0 It is a change in:
1. shape or form
2. expansion or contraction
3. change of state (solid, liquid, gas)
4. mixing substances together
0 Physical changes are reversible
1. Changing shape or form
0 A force is applied to break, bend, stretch,
crush, twist or compress the object
0 No new substance is formed
0 Example:
0 crushing a can of coke
0 crushing a tablet into powder
2. Expansion or contraction
0 When substances are heated they expand
0 Expansion means the volume increases (it takes
up more space)
0 Their density decreases
0 When substances are cooled they contract
0 The volume decreases (take up less space) and it
become more dense
0 Example:
0 Thermometers – ethanol and mercury.
When the liquid is heated it expands and
moves up the thermometer
0 Hot-air balloons – the hot air inside the
balloon is heated and is less dense than the
cooler air outside the balloon so the balloon
3. Change of State
0 When heat is applied, a substance can also
change state:
0 solid to liquid = melting (at melting point)
0 liquid to gas = evaporation (at boiling point
we see “bubbles” in a boiling liquid)
0 (solid to gas = sublimation)
0 If a substance is cooled, it can change state too:
0 liquid to solid = freezing or solidification
0 gas to liquid = condensation
0 (gas to solid = deposition)
4. Mixing
0 Example: a mixture of Lego blocks(but no new
blocks are formed)
0 When a solid (solute) is dissolved in a liquid
(solvent) a mixture is made (solution)
0 Sugar in water:
0sugar = solute
0water = solvent
0sugar water = solution
0 In this case we think the solute disappears but
it is broken down into such small particles
which are invisible
0 The water may later on be boiled off or left to
evaporate leaving behind sugar crystals
0 In a chemical change, a new substance forms.
0 The new substance may be identified by:
1. seeing a change in colour
2. smelling a gas or seeing bubbles
3. seeing a new solid (precipitate)
4. observing energy produced or absorbed
(heat or light or sound)
1. Permanent colour change
0 The change of colour is because the new substance
has a different colour to what you started with
0 Examples:
0 Burning a match, paper or toast produces a
black solid which is carbon
0 Rusting of iron (gray to orange-brown)
0 Ripening of fruits (green apples turning red)
0 Sometimes a physical change can produce
a change in colour (mixing paint colours)
but no new substance is formed
2. A gas is given off
0 A gas produced during a chemical reaction may
be observed as:
0 bubbles
0 a new smell
0 smoke
0 Example:
0 vinegar and bicarbonate soda produces
bubbles that are the gas carbon dioxide (CO2)
0 Rotting eggs (produces hydrogen sulfide gas)
3. A precipitate forms
0 When two solutions are mixed they may
produce a solid known as a precipitate
0 Example:
0 solids forming in pipes and drains
4. Energy is produced or
0 Energies include heat, light or sound
0 Energy is given off (or released)
0 Examples include fireworks, firing a bullet,
lighting a match, explosions and fires
0 Energy is absorbed from the surroundings
0 Examples include chemical ice packs – heat is
absorbed making the surroundings (the ice pack)
0 Physical changes can also be endothermic or
exothermic but no new substance is formed.
0 Evaporation is an endothermic physical
0 when sweat evaporates from your skin it
makes you feel cooler (takes heat away
from your skin)
0 Condensation is an exothermic physical
0 Steam is dangerous, it condenses on your
skin and releases heat energy