Composition of Matter Class 8 Science Notes Maharashtra State Board
We saw in the previous standards that all the objects that we see around us and also those that cannot be seen are made of the same or other matter.
Take some mustard seeds in a transparent plastic jar. Thread a long thread at the center of a big ballon using a needle and tie it tight. Stretch this rubber diaphragm and fix it on the mouth of the jar using a rubber band. Pull the diaphragm up and down with the help of the thread first slowly, then with moderate force, and then vigorously. In the above experiment, we give energy, less or more, to the mustard seeds by pulling the diaphragm up and down, making them move differently. The particles in the solid, liquid, and gaseous states of matter have movement somewhat similar to that.
An intermolecular force of attraction acts between the particles (atoms or molecules) of matter. The extent of the particle movement is determined by the strength of this force. The particles of solid are very close to each other and vibrate at their fixed positions. Due to this, solids get properties like definite shape and volume and also high density and non-compressibility. The strength of the intermolecular force is moderate in the liquid state. Though it is not strong enough to fix the particles in a definite position, it is strong enough to hold them together.
As a result, liquids have a definite volume. However, they have fluidity and their shape is not definite but changes with the container. The intermolecular force is very weak in gases. Therefore the constituent particles of gases move freely and occupy all the available space. Consequently, gases have neither definite shape nor definite volume. The figure shows a schematic representation of a submicroscopic picture of the physical states of matter and the table shows the characteristics of the states of matter.
This is the second method of classification of matter. In this method, the criterion used for the classification of matter is the chemical composition of matter. We have seen in the previous standard that matter is classified into three types ‘element’, ‘compound’, and ‘mixture’ by considering whether the smallest particles of matter are similar or different and what are they made of. All the smallest particles (atoms/molecules) in an element or a compound are alike, however, the smallest particles in a mixture are of two or more types.
The smallest particles of an element contain identical atoms. For example, each molecule of oxygen contains two oxygen atoms in a bonded state. The smallest particles (molecules) of a compound are formed by joining two or more types of atoms to each other. For example, each molecule of water contains two hydrogen atoms joined to one atom of oxygen. The smallest particles of a mixture are atoms/molecules of two or more elements/compounds. For example, the main constituent molecules of the mixture namely, air are N2, O2, Ar, H2O, and CO2. Similarly, the mixture (an alloy) of brass contains atoms of copper (Cu) and zinc (Zn), while bronze contains atoms of the elements copper (Cu) and tin (Sn). The figure shows a schematic submicroscopic picture of the types of matter, namely, element, compound, and mixture, and also their characteristics.
Water: A Compound
Pure water is a compound formed by a chemical combination of the elements hydrogen and oxygen. Whatever may be the source of water, the proportion of its constituent elements oxygen and hydrogen by weight is always 8 : 1. Hydrogen is an inflammable gas while oxygen gas supports combustion. However, the compound water formed by the chemical combination of the gaseous elements hydrogen and oxygen is a liquid. It is neither inflammable nor does it support combustion. On the contrary, it helps to extinguish the fire.
Milk: A Mixture
Milk is a mixture of water, lactose, fats, protein, and a few more natural substances. The proportion of various ingredients of milk is different as per its source. The proportion of fats in cow milk is 3-5 %, while it is 6-9 % in buffalo milk. The ingredient water is naturally present in large proportions in milk. Therefore milk exists in a liquid state. The sweetness of milk is due to the ingredient called lactose. In other words, the properties of the constituent substances are retained in milk.
Types of Element
Take the following objects: an iron nail/sheet, copper wire, aluminum wire, and a piece of coal. Rub each object on a fresh surface of sandpaper and observe. Hammer each object with force. (Take care not to hurt yourself.) The objects in the above activity are made of the elements iron (Fe), copper (Cu), aluminum (Al), and carbon (C) respectively. Fill the following table based on the observations obtained by doing the above two tests on each of the objects.
You noticed that element has different physical properties like lustre/paleness, and malleability/brittleness. According to that elements are classified. In early times, elements were classified into ‘Metals’ and ‘Nonmetals’. After the invention of new elements a new type ‘Metalloid’ is discovered. A detailed study of these types of elements will be made in the chapter Metals and Non-metals.
Types of Compound
Apparatus: Evaporating dish, tripod stand, burner, etc.
Chemicals: Camphor, washing soda, blue vitriol, sugar, glucose, urea.
Keep the evaporating dish on the tripod stand. Take some camphor in the evaporating dish. Heat the camphor in the dish strongly with the help of a burner. Find out what remains behind in the evaporating dish. Repeat the above procedure using limestone, washing soda, blue vitriol, sugar, glucose, and urea instead of camphor. Record your observation in the following table. (Do this activity carefully under the supervision of your teacher, as some of the powders may catch fire.)
You saw in the above activity that on heating strongly some compounds give residue while others do not give any residue or give a blackish residue. The black residue is mainly made of carbon. Moreover, when such compounds are strongly heated in air, combine with oxygen to form some gaseous substances. In case their combustion is not complete, black-colored carbon remains behind as residue. These compounds are called organic compounds or carbon compounds. For example, materials like carbohydrates, proteins, and hydrocarbons (for example, petrol, and cooking gas) are made of organic compounds. The camphor, sugar, glucose, and urea used in the above activity are organic compounds.
On the other hand, the compounds that decompose on strong heating to leave a residue behind are inorganic compounds. Common salt, soda, rust, blue vitriol, and limestone are inorganic compounds. In addition, there is one more type of compound, namely complex compounds. The molecules of compounds have a complex structure formed by many atoms and in the center of this structure metal atoms are also included. Chlorophyll which contains magnesium, hemoglobin iron, and cyanocobalamine (vitamin B-12) which contains cobalt are some examples of complex compounds. Various atoms in the molecules of compounds are joined by chemical bonds. We are going to look at that later.
Types of Mixtures
Take three beakers. Take a little sand and water in the first beaker. Take some crystals of blue vitriol and water in the second beaker. Take some blue vitriol and sand in the third beaker. Stir the materials in all three beakers and observe. Record your observation in the following table. The part of matter having uniform composition is called phase. Write the third column of the table the number of phases seen in each of the beakers after stirring. When all the components of a mixture form one phase, it is called a homogeneous mixture. When the components of a mixture are distributed into two or more phases it is called a heterogeneous mixture.
All the particles of a solid that stay together (or are in the same container) constitute a single phase. (eg., a heap of stones.) A liquid substance along with all the soluble substances dissolved in it together constitutes a single phase. (eg., seawater) A liquid or all its drops present together or in the same container constitute a single phase. (eg., raindrops) The liquids present together or in the same container, but not mixed, constitute a separate phase. (eg., oil and water) All the gases present together constitute a single phase. (eg., air)
Take three beakers. Take 10 g common salt in the first beaker, 10 g sawdust in the second beaker, and 10 ml milk in the third beaker. Add 100 ml water to all the three beakers and stir. which of the mixtures shows a separate water phase? Place the three beakers in front of a vertically held paper and pass a laser beam through the beakers from the opposite side. (use the laser beam under the guidance of a teacher.) At the same time observe what appears on the paper in front of the beaker. Also, look at the beaker from the side. Arrange three filtration assemblies using a conical flask, funnel, and filter paper for filtration. Stir the mixtures in the three beakers and carry out filtration. Record all the observations in the following table.
Solution:
A homogeneous mixture of two or more substances is called a solution. In the first beaker in the above activity, a homogeneous mixture of water and salt is formed. It is called a salt solution. That component of a solution that is present in the largest proportion is called the solvent. The other components which are in less proportion than the solvent are called solutes. The process of forming a solution by mixing solutes in a solvent is called dissolution. According to the states of the components, solutions can be of many types. Solutions such as seawater, blue vitriol dissolved in water, salt dissolved in water, and sugar syrup are of the type solid in liquid.
In addition to this, the solution can also be of the types liquid in liquid (for example, vinegar, dilute sulphuric acid), gas in gas (for example, air) solid in solid’ (for example, alloys like brass, steel, stainless steel, etc), gas in liquid (for example, chlorinated water, hydrochloric acid). The composition of a homogeneous mixture, that is to say, solution, is uniform throughout the bulk. If solvent is a transparent liquid, the solution is also transparent and it passes through a filter paper.
Suspension:
In the second beaker in the above activity, a heterogeneous mixture of water and sawdust was formed. It is a mixture of a liquid and a solid. A heterogeneous mixture of a liquid and a solid is called a suspension. The diameter of the solid particles in a suspension is larger than 10-4 m. Therefore light cannot transmit through it. Moreover, these solid particles remain on an ordinary filter paper as residue and therefore the liquid and solid components of a suspension get separated by filtration.
Colloid:
The mixture of water and milk in the third beaker in the above activity is translucent. It means that, when light is incident on the surface of this mixture, it is partly transmitted and partly scattered. This is because the tiny particles of the milk phase in this heterogeneous mixture are dispersed evenly in the water phase, and the diameter of these particles is around 10-5 m. Such a heterogeneous mixture is called a colloid. However as the pores of an ordinary filter paper are larger than the colloid, the heterogeneous mixture, cannot be separated by filtration. Milk is itself a colloid. In it, the solid and liquid particles of proteins, fats, etc. having a diameter of around 10-5 m are dispersed in the aqueous medium. Apart from this, there are some more types of colloids such as ‘solid in gas’ (for example, smoke), ‘liquid in gas’ (for example, fog, cloud), etc.
Let us Understand Compounds:
While studying matter we have seen that element is a type of matter having the simplest composition. On inspection of the composition of the types of compound and mixture, it is learned that they are formed from two or more units. Whether these units are in a joined state with each other or separate decides whether the matter is a compound or a mixture.
Take two evaporating dishes. Take 7 g iron filings in the first dish and 4 g sulphur in the second. Take a horseshoe magnet near the matter in both dishes and observe. Transfer the entire iron filings from the first dish to the second, stir with a glass rod, and observe by taking the horseshoe magnet near the matter. Also, observe the color of the matter. Now heat the matter in the second dish for a while and let it cool. Observe the colour change, if any in the matter and observe whether there is any effect of the horseshoe magnet on it.
In the above activity, on testing the matter obtained by mixing iron filings and sulphur with a horseshoe magnet (step 3) it was found that the resulting matter was a mixture of iron and sulphur and possessed properties of both the components. Some particles were yellow. They were of sulphur. Some particles were black. They were iron particles. The property of iron particles to get attracted toward magnets was unchanged. In other words, the components iron and sulphur were in a free state in that matter. On the contrary, when iron filings and sulphur were heated together and cooled there was no effect of the magnet and the characteristic yellow color of sulphur also disappeared. From this, we understand that the matter formed in the above activity is different from the original components. A chemical combination took place between iron and sulphur due to heating in the above activity. The atoms of iron and sulphur became joined by chemical bonds to form molecules of a new compound.
Molecular Formula and Valency:
There is a definite proportion of the constituent elements in a compound. A certain number of the atoms of the constituent elements are joined to each other in a molecule of a compound. The molecular formula indicates the number of atoms of each of the constituent elements present in one molecule of a compound. A molecular formula includes information regarding the symbols of all the constituent elements and their respective number as subscripts.
We have seen the relationship between a molecular formula and the number of atoms of various elements in a molecule. The ability of joining to another atom with a chemical bond is a chemical property of each atom. This ability is indicated by a number and this number is called the valency of that atom. An atom forms as many chemical bonds with another atom as its valency. Generally, the valency of an element remains constant in its various compounds. Scientists performed many experiments regarding the composition of compounds during the 18th and 19th centuries and from that they deduced the valencies of elements.
Valencies of the constituent elements can be deduced from the known molecular formula of a compound. The basis for this is the univalency of hydrogen. On the other hand, the molecular formula of a compound can be written from the known valencies of the constituent elements by the method of cross-multiplication.
Cross-multiplication method for writing the molecular formula of simple compounds
Step 1: To write symbols of constituent elements.
C O
Step 2: Write the valency below the respective element.
C O
4 2
Step 3: Cross multiply to obtain the number of atoms of the constituent elements in the molecule of the compound.
Step 4: Write the formula of the compound obtained by cross multiplication.
C2O4
Step 5: Write the final molecular formula of the compound. The number of constituent atoms in the final molecular formula should be the smallest possible whole number. To get this, divide the formula obtained in step 4 by a suitable number. The formula obtained by cross multiplication: C2O4
The final molecular formula is obtained by dividing by ‘2’: CO2
Maharashtra State Board Class 8 Science Notes Composition of Matter can be used for revisiting and reinforcing previously learned content.