Maharashtra Board Class 7 Science Chapter 14 Elements Compounds and Mixtures PDF Download

14. Elements, Compounds And Mixtures

Let's Recall

1. How many different states of matter are there? Name them.

2. What brings about a change of state of matter?

3. What are the properties of matter?

4. Do all substances have the same properties?

Classify the following substances according to their properties.

Water, thermocol, soil, iron, coal, paper, rubber, copper, coir, plastic.

Can You Tell?

1. What are objects made of?

2. What are these articles of everyday use made of – electric wire, kitchen utensils, nails, tables and chairs, window panes, soil, salt, sugar?

Matter

We say that an object is made of a certain substance. The term matter is also used as a synonym of substance. In scientific language, however, a single term is used for a single concept, and that which an object is made of is called matter.

The Particulate Nature Of Matter And Properties Of Matter

Try This

1. Take a piece of chalk and keep on dividing it into smaller pieces. What will happen?

2. Wipe a drop of ink with a handkerchief. What effect does it have on the cloth of the handkerchief?

3. What happens when the lid of a bottle of perfume is opened?

It is the matter present in various things in solid, liquid or gaseous states that is responsible for their properties. Even though they are divided into small particles their properties due to the matter present in them, remain the same. The properties such as the white colour of a chalk, the blue colour of ink, the fragrance of a perfume are the properties of the matter of which each of them is made.

Use Your Brain Power!

1. In day-to-day life, we come across many things in our surroundings. We touch them, we study their properties. Are all these things made from only one kind of matter or from more than one kind of matter?

2. Classify the following according to the nature of matter in them – whether it is made from one kind of matter or from more than one kind of matter; and whether it is in solid, liquid or gaseous state: an engraved idol, gold, milk, water, a plank, concrete, salt, soil, coal, smoke, sherbet, cooked khichadi, steam.

Teacher's Note

Matter is anything that takes up space and has weight. In India, we see matter all around us - the chair you sit on, the water you drink, and the air you breathe are all matter.

Exam Trick

Remember: Matter = anything that has mass and volume. Just like your body has weight and takes up space, all matter does the same.

Points To Remember

Matter is made up of very tiny particles.
Objects are made of different substances.
Properties of matter help us identify what things are made of.
Matter can be solid, liquid, or gas.
Even small pieces of matter keep the same properties.

Try This

1. Fill a glass of water upto the brim. Drop a small stone in it. What happens?

2. Take a balance. Place a small stone in one pan and a big stone in the other. Which pan goes down? Why?

Which properties of matter can you tell from the above activities?

Objects have mass and mass can be measured with the help of devices like the common balance. Also, they occupy space. They acquire both these properties from the matter that they are made of. In other words, mass and volume are two important properties of matter.

Many kinds of matter found in nature are in pure form, that is, they contain only one constituent. In scientific language, matter made of only one constituent is called substance, for example, gold, diamond, water, chalk. Other kinds of matter are made of two or more substances. They are called mixtures.

Use Your Brain Power!

Which of the following are mixtures – water, sherbet, iron, steel, coal, air, salt, copper, brass, soil.

Teacher's Note

A substance is pure and made of only one type of material. For example, pure gold or pure water. In India, we often buy "branded" milk which is a pure substance compared to milk mixed with water.

Exam Trick

Remember: Substance = pure. Mixture = mixed. Think of Aadhaar - one person has one Aadhaar number (pure), but a classroom has many students (mixture).

Points To Remember

Substances are made of only one type of matter.
Mixtures are made of two or more substances.
Mass and volume are properties of all matter.
Pure substances have fixed properties.
Mixtures do not have fixed properties.

Elements

Try This

1. Take water in a teapot and cover it. Heat the water to a boil. What do you see on the inside of the lid?

2. Fill water in a spray pump, spray the water and observe the spray.

The water droplets collected on the inside of the lid of the teapot are formed by condensation of the vapour from the boiling water. Water in the form of vapour is composed of extremely tiny particles and, therefore, we cannot even see them. You will see that the spray is also composed of small particles of water. Similarly, all substances are made of extremely tiny particles. The smallest particles of substances are molecules. A substance whose molecules are made of one or more atoms which are exactly alike, is called an element.

We do not get different substances by the decomposition of an element. The smallest particles of elements are made of only one type of atoms. We cannot see atoms with the naked eye, but when crores of atoms come together, their total volume is large enough to be visible to our eyes. The mass and volume of atoms of different elements are different.

Do You Know?

To date, scientists have discovered 118 elements. Of these, 92 elements occur in nature, while the remaining are man-made. Hydrogen, oxygen, nitrogen, carbon, iron, mercury, copper are a few of the important natural elements. More new elements are being discovered through research work.

Teacher's Note

Elements are the building blocks of everything. In India, iron is a very important element used to make tools, machines, and buildings.

Exam Trick

Remember: Element = only one type of atom. Just like your name is unique, each element has one unique type of atom.

Points To Remember

Elements are made of only one type of atom.
There are 118 known elements in the world.
92 elements are found naturally in nature.
Atoms are the smallest parts of elements.
We cannot see atoms with our eyes.

Great Scientists

Democritus named the small particles of elements 'atom' because in the Greek language atomos means indivisible.

In 1803, John Dalton proposed his theory stating that atoms cannot be created or divided into smaller particles or destroyed.

He used certain symbols to represent elements.

For example: Cu Copper, S Sulphur, H Hydrogen.

Oxygen occurs in nature in the gaseous state. Two atoms of oxygen are joined to form a molecule of oxygen, which has an independent existence. Oxygen in air is always in molecular state. Just like atoms, molecules also cannot be seen with naked eyes.

Use Your Brain Power!

1. Which elements are present in air?

2. Is carbon dioxide an element?

3. What are the properties of elements due to?

4. Are the atoms of different elements similar or dissimilar?

Can You Tell?

What do the short forms Dr, H.M., AC, Adv., C.M., DC stand for?

In day-to-day life, we use short forms in many places. A similar method is used to indicate elements.

The scientist, Berzelius, was the first to use the present method of using symbols for elements. The symbol of an element is written in the English script and is the short form of its name.

Some elements and their symbols are listed in the table alongside. When the initial letter in the names of two or more elements is the same, a pair of letters is used to write the symbol. For example, we write C for carbon and Cl for chlorine.

From the Internet or reference books, obtain information about elements and prepare a table according to the format given below.

Teacher's Note

Each element has a special symbol to make it easy to write and remember. In India, students learn element symbols just like they learn the Hindi alphabet.

Exam Trick

Remember: Element symbol = short name. Like how we write "Dr." for Doctor, we write "O" for Oxygen.

Points To Remember

Elements are represented by one or two letter symbols.
The symbol is the short form of the element's name.
First letter is always capital, second letter is small.
There are 118 known elements with different symbols.
Symbols help us write chemical names quickly.

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