5. Substances in the Surroundings –Their States and Properties

Change of state of substances

Take pieces of wax in a bowl and heat them on a candle or spirit lamp.
1. How do the pieces of wax change ?
2. What was the initial state of wax?
3. What did it get converted into?
Now keep the same bowl in cold water. What happens ?

Read this list of substances : Spirit, camphor, petrol, ghee, coconut oil, naphthalene balls, ammonium chloride (navsagar).
1. Which ones freeze in winter ?
2. Which liquid have you seen change into a vapour ?
3. Which solids directly change into the gaseous state ?

In the past…

In the 19th century, the scientist J. Willard Gibbs showed that the characteristic properties of a substance depend on its physical state and the arrangement of particles in it.

Heat and change of physical state

You have learnt that change in the physical state of a substance is an effect of the amount of heat in it. On gaining heat the substance changes from solid to liquid and liquid to gas. On the other hand, when the substance cools, or loses heat, it changes from gaseous to liquid and liquid to solid state.

Does water change into vapour the moment we place the vessel on a stove ? Does water kept in fridge change at once into ice ?

A specific amount of heat must be gained or lost before the state of a substance can change. The change in physical state is determined by how hot the substance becomes on gaining heat, or how cold, on losing it. How do we tell how hot or cold a substance is ?

The temperature and a thermometer

When a substance gets heat, it becomes warm and then hot. We put our hand or finger in the water to judge how hot it is, but that is not an accurate measure. Besides, if the substance is very hot, we could get scalded.

 A thermometer is used to measure temperature. Degrees Celsius (o C) is the unit of measuring temperature. There are several types of thermometers available. Nowadays digital thermometers are frequently used.

Take a thermometer from the laboratory. The bulb at its lower end is filled with mercury. The mercury rises to a certain level in the capillary tube above the bulb. You will see a scale next to the mercury column. Reading the figure near the level of the mercury tells us the temperature of air around the bulb of the thermometer. Hold the thermometer in water so that the bulb is completely immersed in the water and read the temperature of the water. Repeat the activity taking some hot water in one vessel, and cold water or ice in another. Note the temperatures.


Water is continuously evaporating. We know that water spilled on the floor dries up slowly on its own. This evaporation occurs from the surface of the water. What happens when water boils? As the water gets heated, its temperature increases and it evaporates at a faster and faster rate. When water kept on a stove attains a particular temperature or level of heat, then evaporation takes place in all parts of the body of water. Then we see water bubbles rising at faster and faster rates to the surface and steam mixing in the air. This is called boiling of water or ebullition. At sea-level, pure water boils at 1000 C. This is the boiling point of water. When water vapour cools, it is converted into water again. This process is called condensation. Condensation of steam also takes place at 1000 C. It means that the boiling point and condensation point of water are one and the same.

Take some water in a beaker and place a thermometer in it. Heat the beaker on a spirit lamp. Note the boiling point of water. Repeat the activity, adding salt or sugar to the water and note the boiling point again. What do you inferfrom it?


The water kept in a fridge or on ice becomes cooler and cooler, that is, its temperature falls. At a certain temperature, water does not get any cooler, but starts freezing and forms ice. The temperature at which this happens, is called the freezing point of water.

The temperature of a substance can fall below 00 C, e.g., the temperature of air in the freezer of a refrigerator is around -180 C. It is read as ‘minus 18 degrees Celsius’.

 When ice gets heat, it starts melting or changing into the liquid state again. Ice melts at 00 C. It means that the freezing point and melting point of water are one and the same. Each substance has a specific boiling point which is also its condensation point. Each substance has a specific melting point which is the same as its freezing point.

Various uses of changes in physical state
1. Candles are made by melting paraffin wax.
2. Solid carbon dioxide (dry ice) is used to make ice cream and to keep it frozen.
3. Liquid nitrogen is used in animal husbandry.
4. Sand (silica) is melted to make glass.
5. Metals like gold and silver are melted to make ornaments.
6. Iron is melted to make tools.


 Iodine crystals do not melt on heating but change directly to the gaseous state. When the fumes of iodine hit the walls of the funnel they cool to form solid crystals of iodine and stick to the funnel walls. Thus, on heating, iodine does not melt and change to a liquid but directly changes to the gaseous state. The change of a solid substance directly into a gas or vapour without first changing into a liquid is called sublimation.

How will you identify the following?
l A glass : Is it made of plastic, steel or glass ?
l A rod : Iron or aluminium?
l A door :Wooden or glass ?
l A white powder : Salt or chalk powder ?

To answer the above questions, you considered their properties, e.g., their transparency, hardness, weight, colour, the sound produced from it, solubility in water, etc. Substances can be identified by studying their properties. They can be put to use according to their properties. Let’s study the properties of substances in greater detail.

Properties of substances.

l What will happen if pressure is applied on substances like chalk, brick, alum, glass or a rajgira wadi ? These substances break into small pieces or particles. Such substances are said to be brittle. This property of substances is called brittleness.

l Take an iron nail. Try to pierce a cardboard sheet, wet mud and a piece of wood using the nail. What happens ? The nail easily pierces wet mud, but not the piece of wood. It can pierce the cardboard sheet with some effort. Why does this happen ? The hardness of a substance is determined by how much resistance it offers to the substances being pushed through it. Which is the hardest known substance ?

 l Stretch a rubber band and let it go or apply pressure on a piece of sponge and release it. What do you see ? The rubber band and the sponge go back to their original shape. Some substances change their shape when a force is applied on them but return to their original shape and size when the force is removed. This property is called elasticity.

l Take a flat metal sheet of the size of a notebook. Holding it at an angle, put a drop of water, honey and gum at different places on the board. Observe how they flow down the slope. Liquids flow downward on a sloping surface. This property is called fluidity. The fluidity of any liquid is determined by how easily it flows.

l If two blocks of the same size, one wooden and the other of iron, are weighed in a balance, how would they compare ? The mass of different substances having the same volume can be different. This difference is because of the difference in their densities. Between substances of the same volume, the ones with greater density are heavier than the those of lesser density.

l Take a glass of water. Add some salt, fine sand and sugar to it and try to dissolve them. Repeat this, replacing water with kerosene. What do you observe ? Some solid substances dissolve in a particular liquid. If a solid does not dissolve in a liquid, it is said to be insoluble in that liquid e.g. salt is soluble in water, but insoluble in kerosene. You know of many beverages, made by using water and soluble substances. The property of a substance of getting dissolved, is called its solubility.

l When we can look through a substance and see things on the other side, then that substance is said to be transparent. This property of the substance is called transparency. Glass, some types of plastic, clean water and air are transparent substances.

 Identify the objects shown in figure 5.14. From which substances are they made ? What are these substances called as a group?

Metals : Substances like copper, gold, iron, aluminium are called metals. Metals are found in the form of minerals deep inside the earth. Minerals from the earth’s crust are processed to obtain the metal. In daily life, metals have various important uses. Let us study some common properties of metals.

Properties of metals

Take a piece of copper or aluminium wire or a small nail. Hammer it repeatedly. What do you observe ? On hammering repeatedly, the wire becomes flat, i.e., it forms a thin sheet. Metals can be converted into sheets by hammering. This property of metals is called malleability.

Hot iron is hammered and made into thin sheets. Visit a blacksmith’s shop to observe how this hammering causes it to stretch. Iron bars made to revolve continuously while being hammered become longer. The iron can be drawn into a wire. Metals can be stretched and drawn into wires. This property of metals is called ductility. Metals like silver, gold, copper, platinum can be drawn into fine wires.

Electricity flows through metals. All metals are conductors of electricity to a greater or lesser extent. This property is called electrical conductivity. Even when a piece of a metal is heated at one place, the whole of it becomes hot. It shows that metals allow heat to flow through them. This property is called thermal conductivity. Metals have a typical shine or lustre. Every metal has a characteristic colour by which it can be identified.

  1. Pluck the string of a musical instrument like a tanpura or veena, ring a bell or hit a steel box with a metal spoon.
  2. Strike a wooden table or a marble floor with a wooden stick. Note the difference in the sounds produced in the two cases.

Metals produce a ringing sound. This property is called the sonority of metals.