Class 7 Science Chapter 5 Notes Physical and Chemical Changes
→ Chemical change : A change in which one or more new substances are formed is called a chemical change.
→ Chemical reaction: A chemical change is also called a chemical reaction. In fact,the process involving a chemical change is accompanied by a chemical reaction.
→ Crystallisation : It is the process of separating pure crystals of a substance from supersaturated solution on cooling.
→ Galvanisation : The process of depositing a layer of zinc metal on iron is known as galvanisation.
→ Physical change : A change in which a substance undergoes a change in its physical properties is known as physical change.
→ Rusting : A process in which a layer of rust covers the surface of the iron is called rusting.
→ We see many changes taking place around us every day. Broadly, changes are of two kinds – physical changes and chemical changes.
→ Change in which a substance undergoes a change in its physical properties is called a physical change. Properties such as shape, size, colour and state of a substance are called physical properties.
→ Characteristics of a physical change :
- A change where no new substances are formed.
- Properties of the substance remain same even if the physical state changes.
- There may or may not be evolution of heat.
- Properties like appearance, texture, size, shape, colour, smell, state, etc. change.
- Chemical properties of the substance do not change.
- A physical change is generally reversible and temporary.
- Some of the physical changes are permanent and irreversible.
→ Change in which a substance undergoes a change in its chemical properties is called a chemical change.
→ Characteristics of a chemical change :
- One or more new substances are formed during the change.
- New substances have different properties from the original substance.
- Heat or light or both may be produced or absorbed during the change.
- Chemical changes are usually irreversible and permanent.
- During a chemical reaction sound may be produced. A gas can be formed. A change in colour or smell may also take place.
→ A chemical change is the result of a chemical reaction between two or more substances. Chemical change is also called a chemical reaction.
→ Anything that occupies space and has mass is called matter.
→ Element is a substance which cannot be split into simpler substances. Chemical symbol is the representation of an element. It represents one atom of an element.
→ Atom is the smallest particle of an element.
→ Molecule is the smallest particle of an element or a compound.
→ Compound is a substance made from a chemical combination of two or more elements in a definite proportion.
→ Chemical formula is the representation of a molecule of an element or a compound.
→ A chemical equation is the representation of a chemical reaction through symbols and chemical formulae.
Balancing of the chemical equation :
- Write down the symbols and formulae of the reactants on the left hand side and those of the products on the right hand side of an arrow.
- Count the atoms of each kind on both sides of the arrow.
- Make the number of atoms of each kind, equal on both sides by using proper coefficients.
→ The mass of the substances taken together before and after a chemical change remains the same.
Types of chemical reactions:
- In a combination or synthesis reaction, two or more reactants combine to form a product.
- In a decomposition reaction, single reactant breaks down into two or more simpler products.
- In a displacement reaction, one element displaces another element from a compound and takes its place. A more reactive element displaces a less reactive element from a compound in solution form.
- In a double displacement reaction, two compounds react by exchanging their radicals.
- In precipitation reaction, two compounds react in their aqueous state to form an insoluble product, which appears in the form of a precipitate.
- In neutralisation reaction, an acid reacts with a base and the radicals are exchanged to form salt and water.
→ Oxidation reaction involves addition of oxygen or removal of hydrogen from a substance.
→ Reduction reaction involves addition of hydrogen or removal of oxygen from a substance.
→ An oxidizing agent oxidizes other substance by providing oxygen to them or by removing hydrogen from them.
→ A reducing agent reduces other substance by providing hydrogen to them or by removing oxygen from them.