Measurement of Matter Class 9 Science Notes Maharashtra Board

Measurement of Matter Class 9 Science Notes Maharashtra State Board

Laws of Chemical Combination
The composition of a substance changes during a chemical change. The fundamental experiments in this regard were performed by scientists in the 18th and 19th centuries. While doing this, they measured accurately, the substances used and formed. The scientists Dalton, Thomson, and Rutherford studied the structure of substances and the atom and thus discovered the laws of chemical combination. Scientists could then write the molecular formulae of various compounds based on Dalton’s atomic theory and the laws of chemical combination. Here we shall verify the laws of chemical combination using known molecular formulae.

Apparatus: Conical flask, test tubes, balance, etc.
Chemicals: Calcium Chloride (CaCl₂), Sodium Sulphate (Na₂SO₄), Calcium Oxide (CaO), Water (H₂O)

Law of Conservation of Matter
In the above activities, the mass of the original matter and the mass of the matter newly formed as a result of the chemical change are equal. In 1785, the French Scientist Antoine Lavoisier inferred from his research that ‘there is no rise or drop in the weight of the matter during a chemical reaction.’ In a chemical reaction, the total weight of the reactants is the same as the total weight of the products formed due to the chemical reactions and this is called the law of conservation of matter.

Law of Constant Proportion
In 1794 the French scientist J.L. Proust stated the law of constant proportions as “The proportion by weight of the constituent elements in the various samples of a compound is fixed,” e.g., the proportion by weight of hydrogen and oxygen in water is 1:8. This means that 9 g water is formed by chemical combination of 1 g hydrogen and 8 g oxygen. Similarly, the proportion by weight of carbon and oxygen in carbon dioxide is 3 : 8. For example, in 44 g of carbon dioxide there is 12 g of carbon and 32 g of oxygen so the proportion by weight of carbon and oxygen is 3 : 8.

Antoine Lavoisier (1743-94)
French scientist, father of modern chemistry, and substantial contribution in the fields of chemistry, biology, and economics.

• Nomenclature of oxygen and hydrogen.
• Showed that matter combines with oxygen during combustion.
• Was the first to use accurate weighing techniques to weigh reactants and products in a chemical reaction.
• Discovered that water is made up of hydrogen and oxygen.
• Assigned systematic names to the compounds, e.g. sulphuric acid, sulphate, sulphite.
• Author of the first book on modern chemistry.
• Studied the elements such as oxygen, hydrogen, nitrogen, phosphorus, mercury, zinc, and sulphur.
• First writer of the law of conservation of mass, in a chemical reaction.

Verification of the Law of Constant Proportion
Many compounds can be made by different methods. For example, two samples of the compound copper oxide, CuO, were obtained, one by the decomposition of copper carbonate, CuCO₃, and another by the decomposition of copper nitrate, Cu(NO₃)₂. From each of these samples, a mass of 8g of copper oxide was taken and each was treated independently with hydrogen gas. Both gave 6.4 g copper and 1.8 g water. Let us see how this is a verification of the law of constant proportion.

The reaction of copper oxide with hydrogen yielded two known substances, namely, the compound water and the element copper. It is known that, in the compound water, H₂O, the elements H and O are in the proportion 1:8 by weight. This means that in 9g of water, there are 8g of the element oxygen. Therefore, 1.8g of water contains (8 × 1.8/9 = 1.6g) oxygen. This oxygen comes from 8g of copper oxide. It means that 8g of both the samples of copper oxide contained 6.4g copper and 1.6g oxygen; and the proportion by weight of copper and oxygen in it is 6.4 : 1.6, that is, 4 : 1. Thus, the experiment showed that the proportion by weight of the constituent elements in different samples of a compound is constant.

Now let us see what the expected proportion by weight of the constituent elements of copper oxide would be from its known molecular formula CuO. To find out this, we need to use the known atomic masses of the elements. The atomic masses of Cu and O are 63.5 and 16 respectively. This means that the proportion by weight of the constituent elements Cu and O in the compound CuO is 63.5 : 16 which is 3.968:1, or approximately 4:1. The experimental value of proportion by weight of the constituent elements matched with the expected proportion calculated from the molecular formula. Thus, the law of constant proportion is verified.

Atom: Size, Mass and Valency
We have learned that at the center of an atom is the nucleus and that there are moving electrons in the extra-nuclear part. The electrons are negatively charged elementary particles while the elementary particles that make up the nucleus are positively charged protons and electrically neutral neutrons. Look at the image of an atom obtained with a field ion microscope.

The size of an atom is determined by its radius. The atomic radius of an isolated atom is the distance between the nucleus of an atom and its outermost orbit. Atomic radius is expressed in nanometres.

Atoms are very very tiny. Modern instruments like the electron microscope, field ion microscope, and scanning tunneling microscope can show enlarged images of the atom. The atomic size depends on the number of electron orbits in the atom. The greater the number of orbits the larger the size. For example, an atom of K is bigger than an atom of Na. If two atoms have the same outermost orbit, then the atom having the larger number of electrons in the outermost orbit is smaller than the one having fewer electrons in the same outermost orbit. For example, an atom of Mg is smaller than an atom of Na.

The Mass of an Atom
The mass of an atom is concentrated in its nucleus and it is due to the protons (p) and neutrons (n) in it. The number (p+n) in the atomic nucleus is called the atomic mass number. Protons and neutrons are together called nucleons. An atom is very very tiny. Then how do we determine its mass? Scientists too, struggled with this question. Scientists of the 19th century couldn’t measure atomic mass accurately. Therefore, the concept of ‘relative mass of an atom’ was put forth. A reference atom was required for expressing the relative mass of an atom. The hydrogen atom being the lightest was initially chosen as the reference atom. The relative mass of a hydrogen atom which has only one proton in its nucleus was accepted as one (1). Therefore, the magnitude of the relative atomic masses of various atoms became equal to their atomic mass number (p+n).

Let us see how to express the relative mass of a nitrogen atom, having accepted the relative atomic mass of hydrogen as 1. The mass of one nitrogen atom is fourteen (14) times that of a hydrogen atom. Therefore, the relative mass of a nitrogen atom is 14. This is how the relative atomic masses of various elements were determined. On this scale, the relative atomic masses of many elements came out to be fractional. Therefore, over time, different atoms were chosen as reference atoms. Finally, in 1961, the carbon atom was selected as the reference atom. In this scale, the relative mass of a carbon atom was accepted as 12. The relative atomic mass of one hydrogen atom compared to the carbon atom becomes 12 × 1/12, that is 1. The mass of one proton and one neutron on the scale of relative atomic masses is approximately one.

Today, we have highly accurate methods for measuring the mass of an atom directly. Hence, instead of relative mass, Unified Mass has now been accepted as the unit of atomic mass. It is called Dalton. Its symbol is ‘u’.
1u = 1.66053904 × 10^-27 kg

Chemical Symbols of Elements
Dalton used certain signs to represent elements. For example dot for hydrogen, copyright for copper. Today we use the symbols determined by IUPAC (International Union of Pure and Applied Chemistry). These are official names and symbols and are used all over the world. The current method of choosing chemical symbols is based on the method invented by Berzelius. According to this method, the symbol of an element is either the first letter or the first and second/another specific letter in its name. Of the two letters, the first is a capital letter and the second is small.

Molecules of Elements and Compounds
Atoms of some elements such as helium, and neon have independent existence. It means that these elements are in a mono-atomic molecular state. Sometimes, two or more atoms of an element combine to form molecules of that element. Such elements are in a polyatomic molecular state. For example, the elements oxygen, and nitrogen are in a diatomic molecular state as O₂ and N₂ respectively. When atoms of different elements combine, the molecules of compounds are formed. In other words, compounds are formed by chemical attraction between different elements.

Molecular Mass and the Concept of Mole

Molecular Mass
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a single molecule of that substance. Like atomic mass, molecular mass is also expressed in the unit Dalton (u).
How to deduce the molecular mass of H₂O?

Following are the atomic masses of a few elements in Daltons and the molecular formulae of some compounds. Deduce the molecular masses of those compounds.
Atomic masses → H(1), O(16), N(14), C(12), K(39), S(32) Ca(40), Na(23), Cl(35.5), Mg(24), Al(27)
Molecular formulae → NaCl, MgCl₂, KNO₃, H₂O₂, AlCl₃, Ca(OH)₂, MgO, H₂SO₄, HNO₃, NaOH

Mole
When elements and compounds take part in chemical reactions, it is their atoms and molecules that react with each other, and therefore it is necessary to know the numbers of their atoms and molecules. However, while carrying out a chemical reaction it is convenient to measure out quantities that can be handled instead of counting the numbers of atoms and molecules. The concept of ‘mole’ is useful for this purpose. A mole is the quantity of a substance whose mass in grams is equal in magnitude to the molecular mass of that substance in Daltons. Thus, the molecular mass of oxygen is 32u, and therefore 32g oxygen is 1mole of oxygen. The molecular mass of water is 18u. Therefore, 18g of water makes 1 mole of water. 1 mole of a compound is the mass of that substance in grams equal in magnitude to its molecular mass. The SI unit is mol.

Avogadro’s Number
The number of molecules in one mole of any substance is constant. The Italian scientist Avogadro did pioneering work in this context. Therefore this number is called Avogadro’s number and is denoted by the symbol NA. Later scientists demonstrated experimentally that the value of Avogadro’s number is 6.022 × 10²³. A mole of any substance stands for 6.022 × 10²³ molecules. Just as a dozen has 12 items, a century has 100 or a gross has 144, a mole means 6.022 × 10²³. For example, a mole of water, that is, 18g of water contains 6.022 × 10²³ molecules of water.
How many molecules are there in 66 g of CO₂?
Method: The molecular mass of CO₂ is 44

∴ n = 1.5 mol
∴ 1 mol of CO₂ contains 6.022 × 10²³ molecules.
∴ 1.5 mol CO₂ contains 1.5 × 6.022 × 10²³ molecules = 9.033 × 10²³ molecules

The number of molecules in a given quantity of a substance is determined by its molecular mass. The number of molecules in equal masses of different substances is different. One mole quantities of different substances have different masses measured in grams.

Valency
The capacity of an element to combine is called its valency. The valency of an element is indicated by a specific number. It is the number of chemical bonds formed by one atom of that element with other atoms. In the 18th and 19th centuries, the laws of chemical combination were used to find out the valencies of elements. In the 20th century, the relationship of the valency of an element with its electronic configuration was recognized.

A sodium atom gives away 1e^– and a cation of sodium is formed, hence, the valency of sodium is one. A chlorine atom takes up 1e^– and forms an anion of chlorine (chloride), and thus, the valency of chlorine is one. After the give and take of electrons is over, the electronic configuration of both the resulting ions has a complete octet. Due to the attraction between the unit but opposite charges on the two ions, one chemical bond is formed between Na⁺ and Cl^–, and the compound NaCl is formed. Thus, a sodium atom can give away 1e^– while a chlorine atom can take up 1e^–. This means that the valency of both the elements sodium and chlorine is one. From this, the electronic definition of valency is as follows: “The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency of that element.”

The National Chemical Laboratory (NCL), a unit of the CSIR, was established in 1950. Its objectives are to conduct research in the various branches of chemistry, to aid industry, and to develop new technology to make profitable use of the country’s natural resources. The Laboratory conducts research in fields such as biotechnology, nanotechnology, and polymer science.

Variable Valency
Under different conditions, the atoms of some elements give away or take up different numbers of electrons. In such cases, those elements exhibit more than one valency. This property of elements is called variable valency.

Iron (Fe) exhibits the variable valencies 2 and 3. Therefore iron forms two compounds with chlorine FeCl₂ and FeCl₃.

Radicals
Compounds with ionic bonds have two constituents. These are a cation (positively charged ion) and anion (negatively charged ion). They take part independently in chemical reactions, and are, therefore, called radicals. It is seen from the above chart that different bases such as NaOH, and KOH are formed when various cationic radicals are paired with the anionic radical, hydroxide. Hence the cationic radicals are also called basic radicals. Different bases are distinguished from each other by the basic radicals in them. On the other hand, different acids, such as HCl, and HBr are formed when various anionic radicals are paired with the cationic radical H⁺. Therefore, the anionic radicals are called acidic radicals. The difference in the composition of various acids becomes clear by the acidic radicals in them.

Which are the basic radicals and which are the acidic radicals among the following?
Ag⁺, Cu²⁺, Cl^–, I^–, SO^2-₄, Fe³⁺, Ca²⁺, NO^–₃, S₂^–-, NH⁺₄, K⁺, MnO^–₄, Na⁺
Generally, basic radicals are formed by the removal of electrons from the atoms of metals, such as Na⁺, and Cu²⁺. However, there are some exceptions, such as NH⁺₄. Similarly, the acidic radicals are formed by adding electrons to the atoms of non-metals, such as Cl^–, and S₂^–. But there are some exceptions like MnO^–₄.

Classify the following radicals into two types. While doing this use a criterion other than the criteria used above.
Ag⁺, Mg²⁺, Cl^–, SO^2-₄, Fe²⁺, ClO^–₃, NH⁺₄, Br^–, NO^–₃
Monoatomic radicals such as Na⁺, Cu⁺, and Cl^– are called simple radicals. When a radical is a group of atoms carrying charge, such as SO^2-₄, and NH⁺₄, it is called composite radical. The magnitude of the charge on any radical is its valency.

Chemical Formulae of Compounds: Recapitulation
The characteristic of a compound formed by ionic bonds is that its molecule has two parts. These are a cation and an anion, that is, a basic radical and an acidic radical. These two parts are oppositely charged. The force of attraction between them constitutes the ionic bond. The name of an ionic compound has two words. The first word is the name of the cation and the second is the name of the anion. For example, while writing the formula of a compound such as sodium chloride the symbol of the cation is written on the left, and adjoined to it on the right is the symbol of the anion. The charges are not shown though the number of the ions is written as a subscript on the right of the symbol of the ion. This number is obtained easily by the method of cross-multiplication of the valencies. The steps for writing a chemical formula are shown below.

Step 1: To write the symbols of the radicals. (Basic radical on the left.)
Na SO₄
Step 2: Write the valency below the respective radical.
Na SO₄
1 2
Step 3: Cross-multiply as shown by the arrows the number of radicals.

Step 4: Write down the chemical formula of the compound.
Na₂SO₄
To write the chemical formulae of compounds, it is necessary to know the valency of the various radicals. The names and symbols along with the charge of common radicals are given in the chart below.

Detailed Maharashtra State Board Class 9 Science Notes Measurement of Matter is particularly useful for answering essay questions.