Chemical Reactions

A physical change involves alterations like state or shape without forming new substances.

Physical changes are typically reversible as they do not alter the molecular composition.

Chemical changes involve the formation of new substances due to changes at the molecular level.

Energy changes are always involved in chemical changes, often as heat, light, or sound.

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

During a chemical reaction, the total mass of the reactants equals the total mass of the products.

Exothermic reactions release heat energy into their surroundings.

Endothermic reactions absorb heat from their surroundings.

In a decomposition reaction, a single compound breaks down into two or more simpler substances.

During a chemical reaction, atoms are rearranged to form new substances.

When coal burns in oxygen, carbon dioxide is one of the primary products.

The burning of coal is an irreversible chemical change as coal cannot be recovered once burned.

Rusting of iron is a chemical reaction where iron reacts with oxygen in the presence of water to form rust.

When methane burns, it undergoes a combustion reaction producing carbon dioxide and water.

The formula for photosynthesis is \( \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Glucose} + \text{O}_2 \).

Oxygen is produced during photosynthesis as a byproduct.

Chlorofluorocarbons (CFCs) are known to damage the ozone layer.

The ozone layer is important because it blocks harmful ultraviolet (UV) rays from the sun.

Chemical equations must be balanced to follow the law of conservation of mass, ensuring the number of atoms for each element is the same on both sides.

The correct balanced form is \( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \).

Balancing chemical equations is often done by trial and error, adjusting coefficients to ensure the same number of atoms on both sides.

Two HCl molecules are needed to balance the equation \( \text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2 \).

Antoine Lavoisier proposed the law of conservation of mass.

According to the law of conservation of mass, the total mass remains the same before and after a reaction.

There are 4 hydrogen atoms in \( 2\text{H}_2 \).

Addition reactions are also known as combination reactions.

In an addition reaction, two or more substances combine to form a new single product.

The reaction of carbon with oxygen to form carbon dioxide is an example of an addition reaction.

In a decomposition reaction, one product breaks down into two or more simpler substances.

The reaction \( 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2 \) is an example of decomposition.

Heat is the form of energy most commonly involved in chemical reactions.

The study of heat changes in chemical reactions is called thermochemistry.

Endothermic reactions absorb heat energy from their surroundings.

Exothermic reactions release heat energy into their surroundings.

The thermite reaction is used to repair cracks in railway tracks due to the high heat it produces.

Respiration helps maintain body temperature in cold environments by releasing energy.