Changes Around Us - Class 6 Notes & Olympiad Questions

Changes Around Us

a) Changes in substances refer to the different ways that materials can transform or alter.
b) These changes can involve various characteristics of the substances, such as physical properties (e.g., colour, shape, size, state of matter, temperature) or chemical properties (e.g., reactivity, composition, stability).
c) Changes in substances can occur through a variety of processes or factors, including:

Heating: When you heat something, like a piece of ice, it can change from a solid to a liquid (water). Similarly, if you heat water, it can change from a liquid to a gas (steam). So, heating can make substances change their form.

Applying force: When you apply force to something, like squeezing a sponge, it changes its shape. The sponge gets compressed or squished. This is an example of a physical change.

Mixing with other substances: When you mix different substances together, they can react and create new substances. For instance, when you mix vinegar and baking soda, they create a fizzing reaction and release carbon dioxide gas. This is a chemical change because new substances (carbon dioxide and water) are formed.

Environmental factors: Substances can also change due to environmental factors. For example, when iron is exposed to moisture and air for a long time, it can rust and turn reddish-brown. This is a chemical change caused by the reaction of iron with oxygen and water.

Different Types of Changes

Reversible and Irreversible Changes

Physical and Chemical Changes

Periodic and Non-periodic Change

Exothermic and Endothermic Change

Change in Size due to Expansion and Contraction

a) When we heat a substance, its particles start moving faster and spread out. This makes the substance expand or become larger.
b) When we cool a substance, its particles slow down and come closer together. This makes the substance contract or become smaller.
c) Different materials can expand or contract by different amounts when heated or cooled.
d) Example: When the temperature increases, the liquid mercury inside a thermometer expands. This causes the mercury level to rise and gives a higher reading. When the temperature decreases, the mercury contracts and the level falls, giving a lower reading on the thermometer.
e) It's important to note that liquids also expand when heated, similar to solids.

Understanding how substances expand or contract with changes in temperature helps us explain various phenomena, such as the functioning of thermometers and the behaviour of different materials when heated or cooled.

Change of State on Heating

When we heat different substances, they can undergo changes in their state (solid, liquid, or gas). Some different processes involved are:

Melting:When a solid is heated, it can change into a liquid state. This process is called melting.
Example: When you heat ice, it melts and turns into liquid water.

Evaporation: When a liquid is heated, it can change into a gas state. This process is called evaporation.
Example: When you heat water, it evaporates and turns into water vapour (an invisible gas).

Condensation: When a gas is cooled, it can change into a liquid state. This process is called condensation.
Example: When water vapour in the air cools down, it condenses and forms water droplets, leading to the formation of clouds or dew.

Sublimation: Some solids can directly change into a gas state without going through the liquid state. This process is called sublimation.
Example: When solid dry ice (frozen carbon dioxide) is heated, it directly changes into carbon dioxide gas without becoming a liquid.

Anomalous Expansion of Water

a) When most substances are cooled, they contract and become smaller in volume. However, water behaves differently.
b) When water is cooled from higher temperatures, it follows normal behaviour and contracts, just like other substances.
c) However, when it gets colder than 4°C, something different happens. Instead of contracting, water starts to expand and takes up more space.
d) This means that when water freezes and turns into ice, it actually becomes less dense than liquid water. That's why ice can float on top of the water.
e) Scientists call this unusual behaviour the "anomalous expansion of water."
f) The anomalous expansion of water is important for life in rivers and lakes. When it gets really cold, the top layer of water freezes and forms ice. This ice acts as a protective layer, keeping the water underneath warmer and allowing plants and animals to survive.