6. Composition of matter

Stretch this rubber diaphragm and fix it on the mouth of the jar by means of a rubber band. Pull the diagphragm up and down with the help of the thread first slowly, then with moderate force and then vigorously. Record your observation in the following table.

On 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 some what similar to that.

An intermolecular force of attraction acts between the particles (atom 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 intermolecular force is moderate in the liquid state. Though it is not strong enough to fix the particles in definite position, it is strong enough to hold them together. As a result, liquids have definite volume. However they have fludity and their shape is not definite but changes in accordance 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. Figure 6.2 shows schematic representation of submicroscopic picture of the physical states of matter and the table 6.3 shows the characteristics of the states of matter.

This is the second method of classification of matter. In this method the criterion used for classification of matter is 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 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 contain 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, H2 O, CO2 . Similarly, the mixture (an alloy) brass contain atoms of copper (Cu) and zinc (Zn), while bronze contain atoms of the elements copper (Cu) and tin (Sn).

The figure 6.4 shows a schematic submicroscopic picture of the types of matter, namely, element, compound and mixture and also their characteristics.

Types of element

The objects in the above activity are made of the elements iron (Fe), copper (Cu), aluminium (Al) and carbon (C) respectively. Fill the following table on the basis of the observations obtained on doing the above two tests on each of the objects.

You noticed that element has different physical properties like lustre/paleness, malleability/ brittleness. According to that elements are classified. In early times, elements were classified into ‘Metals’ and ‘Non metals’. After invention of new elements a new type ‘Metalloid’ is discovered. The detail study of these type elements will be made in 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 (fig 6.4). 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 coloured carbon remain behind as residue. These compounds are

called organic compounds or carbon compounds. For example materials like, carbohydrates, proteins, hydrocarbons (for example, petrol, cooking gas) are made of organic compounds. The comphor, 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. Comman salt, soda,

rust, blue vitriol, limestone are inorganic compounds. In addition there is one more type of compounds, namely complex compounds. The molecules of compounds have a complex structure formed by many atoms and in the centre of this structure metal atoms are also included. Chlorophyll that contains magnesium, hemoglobin that contain iron, cyanocobalamine (vitamin B-12) that contain 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 10 g common salt in the first beaker, 10 g saw dust 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 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 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 conical flask, funnel and filter paper for doing 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 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 which is present in the largest proportion is called 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 solution can be of many types. The solution such as sea water, blue vitriol dissolved in water, salt dissolved in water, 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 saw dust was formed. It is a mixture of a liquid and a solid. 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. More over 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 transluscent. 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 milk phase in this heterogeneous mixture are dispered evenly in water phase, and the diameter of these particles is around 10-5m. Such a heterogeneous mixture is called a colloid. However as the pores of an ordinary filter paper are larger than 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 around 10-5m 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 compound and mixture it is learnt 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.

In the above activity, on testing the matter obtained by mixing iron filings and sulphur with horse shoe 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 towards magnet was unchanged. In other words the components iron and sulphur were in free state in that matter. On the contary when iron filings and sulphur were heated together and cooled there was no effect of magnet and the characteristic yellow colour 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 atom of iron and sulphur became joined by chemical bond to form molecules of a new compound.

Molecular formula and valency : There is a definite proportion of the constituent elements in a compound. Certain number of the atom of the constituent elements are joined to each other in a molecule of a compound. Molecular formula indicates the number of atom of each of the constituent elements present in one molecule of a compound. A molecular formula includes the information regarding the symbols of the all the constituent elements and their respective number as subscripts.

We have seen the relationship between a molecular formula and the number of atom of various elements in 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 valency of that atom. An atom forms as many chemical bonds with other atom as its valency. Generally valency of an element remain constant in its various compounds.

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.

Pairs of elements and their valencies are given in the following table. Use them logically to deduce the molecular formulae of the compounds formed from the pairs and write them in the last column.