Revision: Elements, Compounds and Mixtures Chemistry ICSE ICSE Class 8 CISCE

Atom — An atom is the smallest part of an element that takes place in a chemical reaction.

Chemical properties of matter tell us how a substance changes when it interacts with other substances. These properties describe how matter reacts and forms new substances. When a chemical change happens, the matter changes into something new. The atoms in a substance rearrange themselves, and a new substance is formed. For example:

• When wood burns, it turns into ash and smoke.
• When iron is exposed to air and water, it forms rust.

Examples of Chemical Properties:

• Flammability: This describes if a substance can burn. For example, wood is flammable because it can catch fire.
• Reactivity: This tells us how a substance reacts when mixed with others. For example, if you mix vinegar with baking soda, they react to create bubbles.
• Rusting: Some metals, like iron, will form rust when they come in contact with water and air.
• Acidity and Basicity: Some substances are acidic (like lemon juice), while others are basic (like soap). Acids and bases can react with each other to form new substances.

Matter—Anything that has mass and occupies space is called matter.

Anything that has mass and occupies space is called matter.

An emulsion is a colloid in which minute droplets of one liquid are dispersed in another liquid which is not miscible with it. Examples are milk and butter.

• Matter is defined based on its physical and chemical structure. It occupies space and has mass, particularly as opposed to energy.
• Atoms and molecules are the building blocks of matter, consisting of positively charged protons, neutral neutrons, and negatively charged electrons, respectively.

The matter is defined as anything that has mass and takes up space. The matter is found in solid, liquid and gas.

Atom: An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have an independent existence. An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have an independent existence. 

Element is a substance which cannot be broken further into simpler substances and has a definite set of properties. Elements are made up of only one kind of atoms.

Formula: Formula is a short way of representing the molecule of an element or a compound

Compounds are pure substances composed of two or more elements in definite proportion by mass and has properties, entirely different from those of its constituents elements.
Compound, are made up of different types of atoms combined chemically.

Elements: An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process. 

“Mixtures can be defined as. a kind of matter which is formed by mixing two or more pure substances (elements and compounds) in any proportion, such that they do not undergo any chemical change and retain their individual properties. Therefore they are impure substances.

The number of atoms in a molecule of an element is called its atomicity. 

Element is a substance which cannot be split up into two or more simple substances by usual chemical methods of applying heat, light or electric energy; for example, hydrogen, oxygen and chlorine.

Molecule: A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has an independent existence. They are divisible into atoms.

Certain elements which show properties of both metals and non metals are called metalloids or semi-metals.

Non-metals, except hydrogen, are the elements which form negative ions by the gain of electrons.

Metal is a solid material which conduct heat & electricity ,hard, shiny, malleable, fusible, and ductile. Nonmetal is a substance that does not exhibit properties of metals such as hardness, luster, malleability ,ductility and the ability to conduct electricity.

Compounds: Compounds are pure substances composed of two or more elements in definite proportion by mass and has a definite set of properties. The compound is made up of only one kind of molecules

Mixtures are formed when two or more substances are mixed together without participating in a chemical change. A mixture has a variable composition. A mixture shows the properties of the constituent substances. Physical methods can easily separate the constituents.

• Atoms/molecules of a mixture are of two or more types.
• The constituent molecules of a mixture are different from each other and are not joined by chemical bonds.
• Components in a mixture retain their original properties because they are not chemically combined.
• The proportion of constituent substances in a mixture can change.
• The properties of constituent substances are retained in the mixture. 
• Unlike compounds, mixtures do not have a fixed ratio of their components.

For Examples,

Air is a mixture containing nitrogen (N₂), oxygen (O₂), and other gases.

Types of mixture

1. Homogeneous Mixtures: Homo means same. A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. For example, salt in water. It has a uniform composition, and it can’t be separated out physically.
2. Heterogeneous Mixture: Hetero means different. A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. For example, vegetable soup. It has a non-uniform composition and can be separated out physically.

Alloy: “A homogeneous solid mixture of two or more metals or a metal and a non-metal is called an alloy.”

Miscible liquids : Homogeneous liquid-liquid mixtures are called miscible liquids.

Immiscible liquids : Heterogeneous liquid-liquid mixtures are called immiscible liquids.

Pure substance: “Pure Substance is either element or compound. It contains the same kind of atom or molecules and has a definite set of physical and chemical properties.”

Impure substance: “A substance in which some other substances are also present in smaller or larger amounts is called an impure substance. Mixtures are impure substances. An example of an impure substance is air.

Solution: “The homogeneous mixture of water (or any other solvent) and a substance soluble in it is called a solution.”

Heterogeneous mixture: “A mixture in which the components are not uniformly distributed through its volume and can be easily seen separately is called a heterogeneous mixture.”

Homogeneous mixture: “A mixture in which its constituents are uniformly distributed throughout its volume and cannot be seen separately is called a homogenous mixture.”

The change of state from solid to gas directly is called sublimation.

The change of a solid substance directly into a gas or vapour without first changing into a liquid is called sublimation.

OR

The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime.

Evaporation: Evaporation is a process which is used to separate a solid substance dissolved in liquid. It is based on the fact that liquids vaporize easily whereas solids do not.

Evaporation : Is the process ~of converting a liquid into its vapours state either by exposing it to air or by heating.

Evaporation Is the process of converting a liquid into its vapour state either by exposing it to air or by heating.

Stationary phase is a solid or a liquid supported on a solid which remains fixed in a place and on which different solutes are adsorbed to a different extent.