Grade 7 Science Unit 2

After a physical change, the substance is still the same substance Changes in state are physical changes The substance may have some different properties after a physical change

Chemical changes are also known as chemical reactions A change that produces one or more new substances is a chemical change or a chemical reaction

Electrolysis of water is an example of a chemical change that water undergoes Water is broken down into hydrogen and oxygen using an electric current to break the bonds

Physical changes – no new substances are formed; the water changed state and became ice; the water froze! Chemical changes – a new substance is formed; water chemically changed by breaking and rearranging bonds into hydrogen peroxide Comparing Chemical and Physical Changes

Quick Check for Understanding Compare and contrast chemical and physical changes. Give an example of a chemical and physical change from your daily life.

A chemical reaction produces new substances by changing the way in which the atoms are arranged New arrangements of atoms form different substances

Reactants are the substances present at the beginning of a chemical reaction Products are the substances formed by a chemical reaction present at the end of a chemical reaction

1. Color change 2. Formation of a precipitate (a solid product) 3. Formation of a gas (sometimes seen as bubbles) 4. Temperature change

Quick Check for Understanding How do reactants differ from products? How can you tell that a chemical reaction has occurred?

Rates of chemical reactions can vary Many factors affect reaction rates such as: concentration, surface area, temperature, and the presence of a catalyst

Concentration measures the number of particles present in a certain volume A high concentration of reactants means that there is a large number of particles that collide and react during a chemical reaction

Surface area affects the rate of chemical reactions Breaking a large piece of material into smaller parts increases the surface area of the material which increases the reaction rate

Reaction rate can be increased by making the particles move faster Faster moving particles have more collisions with greater force Adding energy to the reactants raises the temperature and causes reaction rate to increase

A catalyst is a substance that increases the rate of the chemical reaction but is not consumed in the chemical reaction

Quick Check for Understanding What factors impact reaction rate? How does surface area affect the rate of a reaction? What is a catalyst?