Test: Physical and Chemical Changes

A chemical reaction expresses a chemical change. For example, one chemical property of hydrogen is that it will react with oxygen to make water. We can write that as follows:

hydrogen reacts with oxygen to make water

We can represent this chemical change more succinctly as

hydrogen + oxygen → water

where the + sign means that the two substances interact chemically with each other and the → symbol implies that a chemical reaction takes place. But substances can also be represented by chemical formulas. Remembering that hydrogen and oxygen both exist as diatomic molecules, we can rewrite our chemical change as

H₂ + O₂ → H₂O

This is an example of a chemical equation, which is a concise way of representing a chemical reaction. The initial substances are called reactants, and the final substances are called products.

Changes of state are physical changes in matter. They are reversible changes that do not involve changes in matter’s chemical makeup or chemical properties. Common changes of state include melting, freezing, sublimation, deposition, condensation, and vaporization.

In a physical change, no new substance is formed. The change is temporary and can be easily reversed. There is no change in mass of the substance as there is a change in its form and not in the bulk. There is no loss or gain of energy because the exact same amount of energy is released back during the reverse process.

Burning of candle is both physical and chemical change. When the candle burns, the wax slowly melts which freezes after some time (Physical change).Burning of wick is a chemical change as it involves combustion.

Heating of sugar is a chemical change. Sugar molecules break in to small groups and they further break down to carbon while the gases given off.

Apparently charring will happen if sugar crystals are heated. Charring is carbon and fumes seen are let out gases with moisture.

A physical change can affect the size, shape or color of a substance but does not affect its composition or mass. The substances may be changed to another phase (i.e. gas, liquid, solid) or separated or combined.

Examples:

when ice melts
when sulfur is mixed with iron filings. 
breaking a glass 
dissolving sugar in water

A chemical reaction involves the breaking and forming of chemical bonds. In the methane reaction above, bonds are broken in methane and oxygen, and bonds are formed in carbon dioxide and water. A reaction like this, in which a compound or element burns in oxygen, is called a combustion reaction. 
CH_₄[gas] + 2O_₂[gas] → CO_₂[gas] + 2H_₂O[steam] + 891kJ

Examples of Chemical Changes
A new compound (product) results from a chemical change as the atoms rearrange themselves to form new chemical bonds.

• Burning wood
• Souring milk
• Mixing acid and base
• Digesting food
• Cooking an egg
• Heating sugar to form caramel
• Baking a cake
• Rusting of iron

Changes of state are physical changes. They occur when matter absorbs or loses energy. Processes in which matter changes between liquid and solid states are freezing and melting. Processes in which matter changes between liquid and gaseous state are vapourization, evaporation and condensation.

Because sodium hydroxide is a type of chemical and when we add it to water than this will be a chemical activity.