The world around you is constantly changing. Whether it is the passing of the seasons, a cool breeze or ice blocks melting in a drink, changes are occurring. most of these can be classified as physical or chemical changes depending on whether any new substances are produced.

Identifying physical change.

The key feature of a physical change is that no new substance is produced during the change. Physical changes include:

• A change in shape or form,
• Expansion or contraction
• a change of state (solid, liquid or gas)
• Or mixing the substance before and after the change is exactly the same. For this reason, physical changes can often be easily reversed.

The physical properties of the substance may change but the substance before and after the change is exactly the same. For this reason, physical changes can often be easily reversed.

Changing shape or form

The simplest example of a physical change is one that changes the shape or form of a substance. In this case a force, is applied to break, bend, stretch, crush, twist or compress the object. These changes do not produce new substances. For example, when you crush an aluninium can, although its shape changes, it is still aluminium.

Expansion and contraction

Physical changes also occur when the temperature of a substance increases or decreases. When solids, liquids and gases are heated, they expand and take up more space. 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 a solid, liquid or gas is cooled, it contracts-the volume of the substance decreases and its density increases.

Thermometers use expansion and contraction of liquids such as ethanol and mercury to measure temperature. When the temperature increases, the liquid in the thermometer expands and moves up the thermometer . When the temperature decreases, the liquid contracts and moves down.

Changes of state

If enough heat is applied to a substance, then it will not expand but will also change state from a solid to a liquid (melting) or from a liquid to a gas (evaporation). These changes are easily reversed. When a substance is cooled enough, it will not just contract but change state from a gas to a liquid (condensation) or from a liquid to a solid (freezing or solidification). A substances change directly from a solid to a gas (sublimation) or a gas to a solid (deposition).

Melting

When a solid is heated enough, it will melt and change into a liquid. For example, when you place an ice cube in your mouth, the heat from your mouth melts it into liquid water. Likewise, when butter is spread on hot toast, the butter melts and soaks into the bread. the physical properties of the solid and liquid may be very different, but they are both made from the same substance. Therefore melting is a physical change.

Freezing

Freezing is the reverse of melting. When a liquid is cooled enough, it will form a solid. This is what happens when liquid water is put in the freezer to form ice cubes. Freezing can also be seen when drops of liquid candle wax a cool and become solid again.

Evaporation

Evaporation occurs when a liquid is warmed and becomes a gas. Evaporation occurs at the surface of a liquid at all temperatures but becomes faster as the temperature of the liquid increases. This is the reason that puddles of water dry faster on a hot da . It is also why tumble driers heat clothes to dry them more quickly. If the liquid is heated above a certain temperature, known as the boiling point, then bubbles of ags will form within the liquid and the liquid is said to boil.

Condensation

The opposite of evaporation is condensation. Condensation 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. this is because water vapor in the air condenses on cold outside of the can to form liquid water. Similarly, when you breath out on a cold day, ‘fog; may come out of your mouth. This ‘fog’ is water vapor in your breath condensing to form a mist of tiny liquid water droplets.

Mixing

Mixing two substances represent another type of physical change.

Dissolving

When a solid (solute) is dissolved in a liquid (solvent they form a solution. A solution is a mixture.

Liquids and gases can also be dissolved. For example, liquid food coloring dissolves in water. In a solution, the solute seems to disappear. In reality, the solute is broken down into such small particles that they are invisible. If the solvent is removed by boiling or evaporation, then the solute particles can join together again to form crystals. This process is known as crystallization.

When sugar is dissolving in water, the sugar particles spread out evenly throughout the water to form a sugar solution. However, the sugar particles remain sugar particles and the water particles- a physical change has occurred. If the water is boiled off or left to evaporate, then the dissolving process is reversed and sugar is left behind.

Identifying chemical change

To distinguish between a chemical change and a physical change, you need to determine if a new substance has formed as a result. If a new substance has been produced, then a chemical change has occurred.

The new substance formed in a chemical change could be a solid, liquid or gas. You can detect the presence of a new substance by:

• Seeing a change in color

• smelling a gas or seeing bubbles

• seeing a new solid (known as a precipitate) forming in a clear solution

• Seeing light or feeling a change in temperature

  

Permanent color change

If you see something changing color then it is very likely that a chemical change occurred. This happens because the new substance produced by the chemical change has a different colorful to what you started with. Burning a match, paper or wood produces a black powder, which is carbon. When iron is left in the air, it turns orange-brown because iron oxide (rust) is produced.

Sometimes physical change can also cause a color change. For example, when snow melts to form liquid water it turns from white to colorless. This is not a chemical change because snow and liquid water are both made of water (chemical formula H2O)

Mixing paint is another example of

a physical change that causes a color change. They produce green paint. however, no new substances are formed, the yellow and blue particles are mixed so closely together they give off the illusion of green paint.

A gas is given off

The production of a gas usually indicates a chemical change may be observed as bubbles, as new smell or perhaps smoke. When vinegar is added to bicarbonate of soda, many bubbles of carbon dioxide (CO2) are formed. Carbon dioxide is also formed by chemical changes in soft drink to create fizz or when antacid tablets are dissolved in water.

An exception of this rule is the case of boiling or sublimation. In these cases, a solid or liquid becomes a gas, but no new substance is produced.

A precipitate forms

Occasionally, a new solid is produced when two solutions are mixed. This indicates that a chemical change has occurred. the solid precipitate (falls) out of the solution and sinks to the bottom. the new solid is know as precipitate. 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.

A precipitate can also form when a gas is bubbled through a liquid. This type of precipitation is the basic of a simple test for carbon dioxide gas. When carbon dioxide gas is bubbled through a solution of calcium hydroxide (lime water), it precipitates solid Calcium carbonate that turns the clear solution milky white.

Light or a change in temperature

Energy plays an important role in chemical changes. Some chemical changes release energy int he form of heat, light or both. As a result, a change in temperature or a flash or glow of light can be a sign that a chemical change has occurred. Likewise, the light, heat and sound from bush fires, fireworks and sparklers are easily seen, felt and heard from far away. In contrast, some chemical changes give out light but with littler or no heat, such as glow sticks and in glow worms.

Photosynthesis in plant leaves is another example of a chemical change that absorbs some heat from your skin. On the other hand, steam can causes serious burns as it condenses on your skin. On the other hand, steam can cause serious burns as it condenses on your skin and releases its heat energy.