Physical & Chemical Changes

Physical properties include many other aspects of a substance. The following are (but not limited to) physical properties.

• Luster

• Malleability

• Ability to be drawn into a thin wire

• Density

• Viscosity

• Solubility

• Mass

• Volume

**Any change in these physical properties is referred to as a physical change.

Examples of Physical Changes

No new chemical species forms in a physical change. Changing the state of a pure substance between solid, liquid, or gas phase is a physical changes since the identity of the matter does not change. A physical change involves changes in physical properties, but not chemical properties. For example, physical properties change during tempering steel, crystallization, and melting. Here are examples of physical changes:

• Crumpling a sheet of aluminum foil

• Melting an ice cube

• Casting silver in a mold

• Breaking a bottle

• Boiling water

• Evaporating alcohol

• Shredding paper

• Sublimation of dry ice into carbon dioxide vapor

• Carbon changing from graphite into a diamond

Examples of Chemical Changes

A new compound (product) results from a chemical change as the atoms rearrange themselves to form new chemical bonds. A chemical change always involves a chemical reaction. The starting materials and final product are chemically different from one another. Here are some examples of chemical changes:

• Burning wood

• Souring milk

• Mixing acid and base

• Digesting food

• Cooking an egg

• Heating sugar to form caramel

• Baking a cake

• Rusting of iron

Differences Between Physical and Chemical Change

How to Tell Whether It's a Physical or Chemical Change?

Look for an indication that a chemical change occurred. Signs of a chemical change include the following:

• Gas is produced. In liquids, bubbles may form.

• An odor is produced.

• The substance changes color.

• Sound is produced.

• There is a temperature change. The surroundings become either hot or cold.

• Light is produced.

• A precipitate forms.

• The change is difficult or possible to reverse.

A chemical change might not display all of these signs. If you don't see any of these indications, a physical change likely occurred. Be aware a physical change may produce a dramatic change in the appearance of a substance. Every sign of a physical change can be produced by a physical change. This doesn't mean a chemical reaction occurred. The only way to know for certain whether a change is chemical or physical is a chemical analysis of the starting and ending materials.

In some cases, it may be hard to tell whether a chemical or physical change occurred. For example, when you dissolve sugar in water, a physical change occurs. The form of the sugar changes, but it remains the same chemically (sucrose molecules). However, when you dissolve the salt in water the salt dissociates into its ions (from NaCl into Na+ and Cl-) so a chemical change occurs. In both cases, a white solid dissolves into a clear liquid and in both cases, you can recover the starting material by removing the water, yet the processes are not the same.

Chemical change

Metallic Oxides

Formation of metal oxides are examples of chemical changes. They are formed by the reaction of oxygen in air.

– Burning of Magnesium ribbon:

– 2Mg + O2 → 2MgO

– The product formed is the oxide of magnesium, which is in the form of ash. It does not look anything like the magnesium ribbon used for burning.

Reaction of metallic oxides with water

• Reaction of metal oxides with water form metal hydroxides.

• Example dissolving Magnesium oxide in water, by stirring the ash very well with water.

• MgO + H2O → Mg(OH)2

• The product formed is basic in nature and turns red litmus paper → blue

Reaction between baking soda and vinegar

When a pinch of baking soda is added to vinegar we hear a hissing sound and observe the formation of bubbles.

– Vinegar (Acetic Acid) + Baking Soda (Sodium bicarbonate) → CO2 (Carbon dioxide) + Other products

The carbon dioxide produced during the reaction of Vinegar and baking soda, when passed through lime water gives calcium carbonate, as follows:

– CO2 + Ca(OH)2 (lime water) → Calcium Carbonate (CaCO3) + H2O

– The calcium carbonate turns lime water milky.

Observations that indicate a chemical change

• Heat or light is absorbed or given out during a chemical reaction.

• Production of sound

• Production of gases or precipitates

• Production of smell

• A colour change may occur

Rusting

• When substances made of Iron are exposed to oxygen and moisture in the atmosphere, it forms a red layer, which is called rust.

• The formation of rust can be represented by the following reaction:

• 4Fe + 3O2 → 2Fe2O3. The chemical formula for rust is Fe2O3.nH2O. More the moisture in the air, quicker the formation of rust.

Galvanization

– The process of depositing zinc on the surface of Iron to prevent rusting is called galvanisation.

– Example: Iron water pipes are galvanized. Ships are made out of iron which is galvanized. Due to the presence of salts in seawater, the process of rusting is hastened. Hence ships need to replace their iron body every year.

Crystallisation

– The process of separation of salts from their solution is called crystallization. It is a purification technique that purifies seawater or separates crystals from impure samples. It is a physical change.