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7.1 When Substances Change (Identifying chemical change (A gas is produced…
7.1 When Substances Change
Identify physical change
A substance undergoes physical change if it changes shape or form, if it expands or contracts, if it changes state or if it mixes with another substance
The physical properties of substance may change but the substance before and after the change is exactly the same, this is why physical changes can often be reversed
The key feature of a physical change is that no new substance is produced during the change,
Changing shape or form
When you squash an aluminium can, it's shape changes but the can is still aluminium, changes can be reversed relatively easily
Expansion and contraction
When expansion occurs, the volume of the object increases and its density decreases but no new substances are formed
The reverse of expansion is contraction, when all the states are cooled, it contracts - the volume of the substance decreases and it's density increases
Physical changes also occur when the temperature of a substance increases or decreases, when all states are heated they expand and take up more space
Thermometers use expansion and contraction of liquids such as ethanol and mercury to measure temperature
A force is applied to break, bend, stretch, crush, twist or compress the object, theses changes do not produce a new substance making it physical change
Changes of state
If enough heat is applied to a substance, then it will change state from a solid to a liquid (melting) or from a liquid to a gas (evaporation)
Few substances change directly from a solid to a gas (sublimation) or from a gas to solid (deposition)
When a substance is cooled the change of state is reversed from a gas to a liquid (condensation), or from a liquid to a solid (freezing or solidification)
Melting
When a solid is heated enough it will melt and change into a liquid. The physical properties of a solid and the liquid may be very different, but they are both made from the same substance making melting a physical change
Freezing
When a liquid is cooled enough, it will form a solid. Freezing converts liquids into solids, so it is the reverse of melting
Evaporation
Evaporation occurs when a liquid is warmed and becomes a gas. Evaporation occurs at the surface of a liquid at all temperatures, but it becomes faster as the temperature of the liquid increases. If the liquid is heated above a certain temperature known as the boiling point, then bubbles of gas will form within the liquid and the liquid boils
Condensation
Condensation (the opposite of evaporation) is when a gas cools and becomes a liquid. When you leave a cold can of soft drink out of the fridge, it develops moisture on the outside because water vapour in air condenses on the cold outside of the can to form liquid water
Mixing
The mixing of two substances is another type of physical change
A bucket of different-coloured balls is an example of a mixture that can be seen by the eye, the balls mix but no new ball are formed
Dissolving
The particles of the solute mix and spread with the particles of the solvent
Although the solution looks and acts differently to the solute and solvent, no new substance is formed and for this reason the process of dissolving is a physical change
When a solid is dissolved in a liquid, they form a mixture, the solid is known as the solute and the liquid is the solvent, the mixture formed is referred to a solution
In a solution, the solute seems to disappear when in reality the solute is broken down into such small particles that they are invisible
When the solvent is removed by boiling or evaporation, the solute particles can join together again to form crystals this is known as crystallisation
Identifying chemical change
If a new substance is produced it is chemical change
You can detect the presence of a new substance by: seeing a change in colour, smelling a gas or seeing bubbles, seeing a new solid forming in a clear solution (precipitate), seeing light or feeling a change in temperature
To distinguish between a chemical change and a physical change, you need to determine whether a new substance has formed as a result of the change
Identifying chemical change also requires a good understanding of physical changes because there can be exceptions to these rules
Sometimes both types of change can happen at roughly the same time
A colour change occurs
Burning a match , paper or wood produces a black powder, which is carbon
Sometimes a physical change can also cause a colour change, for example when snow melts to for a liquid it goes from white to colourless but it is not a chemical change because snow and water are both made from water
If you see something changing colour, then it is very likely that a chemical change has occurred, this happens because the new substance produced by the chemical change has a different colour to what you started with
A gas is produced
Carbon dioxide can be formed by the chemical change in soft drink to create fizz or when antacid tablets are dissolved in water
Rotten eggs are well known for the pungent smell, the smell is the gas hydrogen sulfide (H2S) which is produced when eggs rot
A production of a gas usually indicates a chemical change, a gas produced during a chemical change may be observed as bubbles, a new smell, or perhaps smoke
Rotting is another example of chemical change since it produces gas (smell)
However, nor all gases indicate a chemical change, sometimes physical changes give off gases too
A precipitate forms
Precipitation occurs in pipes and drains when scale or lime deposits inside
Crystals can also precipitate inside the body, some collect in the kidneys and can cause very painful kidney stones
Occasionally, a new solid is produced when two solutions are mixed, this indicates a chemical change has occurred
A precipitate can also form when a gas is bubbled through a liquid, this type of precipitation is the basis of a simple test for carbon dioxide gas, when carbon dioxide gas is bubbled through a solution of calcium hydroxide (limewater), it precipitates solid calcium carbonate, turning the clear solution milky white
Energy is produced or absorbed
A change in temoerature or a flash or glow of light can be a sign that a chemical change has occured
The light, heat and sound from bushfires, fireworks and sparklers are easily observed, in contrast some chemical changes give out light but with little or no heat (chemical changes that happen in glow sticks and glow worms)
Energy plays and important role in chemical changes, some chemical changes release energy in the form of heat, light or both
Other chemical changes absorb heat, causing their surroundings to cool for example, an ice pack uses a chemical change that absorbs a lot of heat energy and so feels icy cold
Some physical changes can also cause a change in temperature, when you sweat you feel cooler because the moisture evaporates and absorbs some heat from your skin, in contrast steam can cause serious burns as it condenses on your skin and releases its heat energy