Maharashtra Board Class 6 Science Chapter 5 Substances in the Surroundings Their States and Properties PDF Download

Substances in the Surroundings – Their States and Properties

Can You Recall?

Name the solid, liquid and gaseous states of water.

Change of State of Substances

Let's Try This

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?

When a substance changes from one state to another, the process is called change of state of the substance.

Teacher's Note

When you heat wax on a candle, it melts into liquid. When you cool it, it becomes solid again. This is like how ghee melts when hot and becomes hard when cold in your kitchen.

Exam Trick

Remember: Heat makes solids into liquids, and liquids into gases. Cold does the opposite. Think of ice melting into water when you add heat.

Points to Remember

Change of state happens when we heat or cool things.
Solids, liquids, and gases are three states of matter.
Wax is solid when cold and liquid when hot.
The same substance can change between different states.

Can You Tell?

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?

What Do We Learn From This?

The state of a substance changes if it is heated or cooled. Every substance in our surroundings is found in either the solid, liquid, or gaseous state.

Teacher's Note

In winter, oil and ghee become thick or freeze because the temperature drops. Camphor disappears from a box because it evaporates into gas.

Exam Trick

Remember: Naphthalene balls in your cupboard slowly disappear. This is sublimation – solid changing directly to gas without becoming liquid first.

Points to Remember

Different substances have different freezing points.
Some liquids turn into vapour at room temperature.
Some solids can change directly into gas.
Temperature controls the state of substances.

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.

Section 5.2: Various Substances

The three states of water are ice (solid), water (liquid), and steam or water vapour (gas).

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.

Teacher's Note

When you heat ice, it becomes water. When you heat water, it becomes steam. This happens because heat gives energy to the particles to move more freely.

Exam Trick

Remember the pattern: Solid to Liquid is melting. Liquid to Gas is boiling. When cooling happens, it goes backwards – Gas to Liquid is condensation, Liquid to Solid is freezing.

Points to Remember

Heat causes solids to melt into liquids.
Heat causes liquids to boil into gases.
Cold causes gases to condense into liquids.
Cold causes liquids to freeze into solids.
Temperature measures how hot or cold something 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 (°C) is the unit of measuring temperature. There are several types of thermometers available. Nowadays digital thermometers are frequently used.

Can You Tell?

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?

We use a thermometer to measure temperature accurately.

Teacher's Note

A thermometer has a bulb filled with mercury that rises as it gets hot. In India, we use Celsius scale where water freezes at 0°C and boils at 100°C.

Exam Trick

Remember: Solids have fixed shape and volume. Liquids have fixed volume but not fixed shape. Gases have neither fixed shape nor volume – they spread everywhere.

Points to Remember

Thermometer measures temperature in degrees Celsius.
Mercury rises when temperature increases.
Mercury falls when temperature decreases.
Solid has definite shape and volume.
Liquid takes shape of container but has fixed volume.

Section 5.4: Recording the Temperature

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.

Teacher's Note

In India, our normal body temperature is about 37°C. When you have a fever, the thermometer shows more than 37°C. When it is very cold in winter, the temperature can drop below 0°C.

Exam Trick

Remember key temperatures: Water freezes at 0°C, water boils at 100°C, and your body is 37°C. Always immerse the bulb completely in the substance you are measuring.

Points to Remember

Mercury rises in thermometer when heat increases.
Thermometer bulb must be fully immersed in liquid.
Read temperature at the level of mercury column.
Celsius (°C) is the unit used in India.
Different substances have different hot and cold temperatures.

Section 5.5: Boiling

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 100°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 100°C. It means that the boiling point and condensation point of water are one and the same.

Teacher's Note

When water boils, it makes bubbles and turns into steam. Boiling is faster than evaporation. In India, water boils at 100°C at sea level, but in mountains it boils at lower temperatures because there is less air pressure.

Exam Trick

Remember: Boiling and condensation both happen at 100°C for water. Boiling = Liquid to Gas. Condensation = Gas back to Liquid. They are reverse processes at the same temperature.

Points to Remember

Boiling is faster evaporation from the whole body of liquid.
Water boils at 100°C at sea level.
Steam is the gas form of water.
Condensation is when steam cools and becomes water again.
Bubbles form throughout the liquid during boiling.

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