Chemical Changes

Vocab List

Atoms: The smallest particle of a chemical element that can exist.
Electrons: A stable subatomic particle with a charge of negative electricity, found in all atoms and acting as the primary carrier of electricity in solids.
Protons: A stable subatomic particle occurring in all atomic nuclei with a positive electric charge equal in magnitude to that of an electron.
Neutrons: A subatomic particle of about the same mass as a proton but without an electrical charge, present in all atomic nuclei except those of ordinary hydrogen.
Molecules: A group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.
Matter: A physical substance which occupies space and possesses rest mass, especially as distinct from energy.
Elements: Substances that cannot be chemically interconverted or broken down into simpler substances.
Compounds: A substance formed from two or more elements chemically united in fixed proportions.
Mixtures: A substance made by mixing other substances together
Solid: Form and stable in shape, not liquid or fluid.
Liquid: A substance that flows freely but is of constant volume, having a consistency like that of water or oil.
Gas: A substance or matter in a state in which it will expand freely to fill the whole of a container, having no fixed shape.
Reversible: Able to be turned the other way around.
Irreversible: Not able to be undone or altered.
Reactants: A substance that takes part in and undergoes change during a reaction.
Products: An article or substance during a natural, chemical or manufacturing process.
Photosynthesis: The process by which plants and some other organisms use sunlight to synthesise nutrients from carbon dioxide and water.
Respiration: A process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide.
Endothermic: A reaction or process accompanied by or requiring the absorption of heat.
Exothermic: A reaction or process accompanied by the release of heat.

Chemical Changes Intro

Chemistry is the study of matter and energy and the interactions between them. The elements are the building blocks of all types of matter in the universe. Each element is made up of only one type of atom, each with its separate number of protons known as its atomic number.

Matter is made up of atoms. An atom is the smallest neutral particle that makes up matter. The type of atom and the way these are arranged and connected to each other determines the type of matter — and therefore the physical and chemical properties of it.

The Ancient Greeks

Much of our civilisation’s foundations such as government, democracy, citizenship, drama, law, medicine etc. can be traced back to the Greek civilisation which flourished over 2000 years ago.

One of the most influential thinker of the time was Aristotle. He was one of the first people (that we know of) to try to answer the question “What is everything made of?”. He decided that everything was made up of just 4 basic constituents, or “elements”; earth, water, air and fire.

About 1000 years later, some great thinkers in the Islamic culture carried on developments in Mathematics and Science. Among other things, they invented Alchemy.

Alchemy in the Middle ages

Alchemy was partly practical experimenting and partly mystical magic. The basic aim of alchemy was to “transmute” common metals into gold, and to find chemicals which would make someone immortal. From the alchemists we get our legends of sorcerers like Merlin the Magician.

Many alchemists were crooks who used “magical” tricks to fool people into giving them money. From this, alchemy got a very bad name. However, the alchemists did discover many facts about solids, liquids, and gases. They invented processes like distillation, filtration and crystallisation and discovered new dyes and other useful substances. One of the most important processes they developed was decomposition. This means to break a substance down into smaller, more simpler parts.

Alchemy becomes chemistry

Some substances could never be decomposed any further, no matter what was done to them. These became known as ‘chemical elements’, the most basic substances of matter. For example, when electricity was found, it was discovered that water (one of Aristotle’s elements) could be further decomposed into different substances.

By about 1800, Alchemy had become the modern science of chemistry. Chemistry is based on the idea that there are certain substances which are the simplest and most basic. These elements can be understood scientifically in terms of particles, forces and energy, and chemical reactions.

We now know that about 90 elements occur naturally on Earth. 20 or so can be made artificially in nuclear reactors. Of these elements, many are very rare. All the familiar substances on Earth are composed of only about 20-30 of the most common elements.

An element is a pure substance which cannot be decomposed into anything simpler.
An element is a substance made up entirely of identical particles.
Each element is composed of atoms which have the same number of electrons.
The number of electrons is equal to the element’s atomic number.

Physical and Chemical Change

Physical Change

A physical change occurs when a substance changes, but no new substance is formed. Physical changes occur when the state of a substance changes or a substance is crushed, ground or cut into smaller pieces. The following are examples of a physical change:

Ice cream melting
Freezing water
Water evaporating

Chemical change

A chemical change or chemical reaction occurs whenever a new substance forms. When a substance forms that looks different or acts different than what it was before, the chemical change is very obvious.Sometimes, however, the change is more difficult to observe. The only indication may be a change in colour, the production of heat or light, or a drop in the temperature of the material.

Types of chemical reactions

There are various types of chemical reactions, but in all cases the substances present before the chemical reaction had occurred are called the reactants. The chemicals formed by the reaction are called products. You can represent a simple chemical reaction using a simple work equation, in which reactants are written on the left and an arrow points to the products formed. Chemical reactions can be classified into their different types.

Combination reaction

In all combination reactions, the two or more reactants join together to form a new substance. Combination reactions are also called synthesis reactions.

Decomposition reaction

Chemical reactions do not always need two reactants. Sometimes, one is enough. A single reactant can break down or decompose to form two or more new substances. For example, light breaks down silver chloride to form two substances — silver and chlorine.

Precipitation reaction

Sometimes, a solid forms when two solutions are mixed together. This solid is called a precipitate. Because precipitates are insoluble, this type of reaction is a useful way of removing certain chemicals from a solution. Some precipitates are very colourful and are often used as paint pigments.

Combustion reaction

Combustion reactions happen whenever something burns or explodes. They involve a substance reacting with oxygen, usually from the air around it. New (often gaseous) substances form, accompanied by the release of heat and/or light, sometimes as a flame or an explosive ash.

Speeding up reactions

When fireworks explode, the rate of reaction is fast: the firework chemicals are used almost all at once. In contrast, iron rusts at a very slow rate.The rate of a reaction can be determined by observing how quickly products are produced pr how quickly reactants disappear. If you examine the factors that affect reactions rates, you will see that controlling reaction rates is possible.

Reaction rates can be affected by:

The amount of concentration of reactants. The reason for this is that more molecules are available to take part in the reaction, so products are produced more quickly
Temperature. The reason for this is that reactants move more quickly at higher temperatures and have more energy available to break bonds between atoms to allow the formation of new substances.
Surface area. This is because a lot of small pieces of a substance have a greater surface area than one big block, allowing more atoms of the substance to be exposed to another.

Enzymes

Special types of catalysts called enzymes are found in our bodies. Digestive enzymes help break down large molecules such as starch into smaller molecules such as glucose. Think of enzymes as a pair of scissors and the starch molecule as a string of beads being cut by the enzyme into small beads (the glucose). This allows the starch to be more easily digested and ultimately be used by body cells. Like all catalysts, digestive enzymes do not combine with other atoms or molecules; they simply help the chemical reaction to occur more quickly.

Indicators of Chemical Reactions

Matter can change. When matter changes, some or all of its properties may change. There are two types of changes matter can undergo: physical or chemical.

A physical change is any change that does not involve a change in a substance’s chemical makeup. Therefore, no new substance is produced. A physical change is usually reversible. A change in an object’s size or shape is a physical change. A phase change is a physical change as well. Although the arrangement of atoms in matter changes with freezing, melting, evaporation or condensation, the chemical makeup of matter is not altered.

A chemical change involves a change in a substance’s physical and chemical properties. Unlike a physical change, a chemical change produces a new substance and a chemical change is usually not reversible. Burning, cooking and corrosion are types of chemical changes. These processes change the properties of a substance and produce a new substance.

There are ways to tell whether a physical change or chemical change has taken place. If a chemical change has taken place, you may observe a new substance produced, light or spark, bubbles, colour change, new smell or temperature change. If you do not observe any of these things, a physical change most likely took place.

Q1) What is a physical change?

A physical change is any change that does not involve a change in a substance’s chemical makeup. Therefore, no new substance is produced. A physical change is usually reversed.

Q2) What is a chemical change?

A chemical change involves a change in a substance’s physical and chemical properties. Unlike a physical change, a chemical change produces a new substance and is usually not reversible.

Q3) How can you tell whether a physical or chemical change occurs?

If a chemical change has taken place, you may observe a new substance produced, light or spark, bubbles, colour change, new smell or temperature change. If you do not observe any of these things, a physical change most likely took place.

Q4) Is burning a candle a physical and chemical change? Justify your answer.

Burning a candle is both a physical and chemical change. The burning of the wick is an example of a chemical change — the wick is burned and chemically changed. However, the melting of the wax is a physical change. The wax melts, and phase change is a physical change.