Melting and Boiling

                                Melting and Boiling

As we learned in solids, liquids, and gases all matter exists in certain states or phases. Water can be liquid water, solid ice, or gas vapor. It's still all water, however, and made up of molecules of 2 hydrogen atoms and 1 oxygen atom (H2O). 

 
Lava is melted or liquid rock

Melting and Freezing 

When a solid turns into a liquid it is called melting. There is a temperature at which this happens called the melting point. As the energy in the molecules increases from a rise in temperature, the molecules start moving faster. Soon they have enough energy to break free of their rigid structure and start moving around more easily. The matter becomes a liquid. The melting point for water is 0 degrees C (32 degrees F). 

When the opposite happens and a liquid turns into a solid, it is called freezing. 

Boiling and Condensation 

When a liquid becomes a gas it is called boiling or vaporization. Again, at a certain temperature called the boiling point, the molecules will gain enough energy to break free and become a gas. The boiling point for water is 100 degrees C (212 degrees F).

 
Hot gas from steam engine condensing

When the opposite occurs and a gas becomes a liquid, it is called condensation. 

Evaporation 

Evaporation is liquid becoming a gas that happens only on the surface of a liquid. Evaporation doesn't always need a high temperature to occur. Even though the overall energy and temperature of a liquid may be low, the molecules on the surface that are in contact with the air and gases around them, can be high energy. These molecules on the surface will slowly become gases through evaporation. You can see evaporation when water on your skin dries or a puddle in the street slowly goes away. 

Standard State 

Scientist use the term "standard state" to describe the state an element or substance is in at "room conditions" of 25 degrees C and one atmosphere of air pressure. Most of the elements, like gold and iron, are solids in their standard state. Only two elements are liquid in their standard states: mercury and bromine. Some of the elements that are gases in their natural state include hydrogen, oxygen, nitrogen, and the noble gases. 

Fun facts about Melting and Boiling

• When rocks get really hot they turn into a liquid called magma or lava.
• Gas can be turned into a liquid through pressure. By squeezing all the gas molecules tightly together a gas can become liquid.
• We use natural gas in our homes in its gas state, but when it's shipped in ocean tankers it's shipped in a liquid state to save on space.
• Mercury has the interesting properties of being both a metal and a liquid in its standard state.

Pure Substances and Mixtures

                     Pure Substances and Mixtures

What is matter?

Matter can be defined as any substance that has inertia, occupies space and has mass.

How is matter classified?

Scientists of the world classify matter as solid, liquid or gas, but there is one more interesting way to classify it. Matter can also be classified as pure substances and mixtures.

What is a pure substance?

A pure substance is a type of matter which exists in its most basic or purest form and cannot be broken down further. Examples of pure substances include water, gases like carbon dioxide, oxygen and metals like platinum, gold and silver.

Each pure substance has its own set of unique chemical and physical properties which helps us in identifying it.

Examples of pure substances

Water has a freezing and melting point of 0°C and a boiling point of 100°C. It is colourless, tasteless and odourless.
Gold is considered pure at 24 karat. It is yellow in color, solid at room temperature and is regarded as a good conductor of electricity. It is also malleable and ductile in nature.

Types of pure substances

Pure substances can be divided into two categories – elements and compounds.
Elements are made up of the same types of atoms. The known elements listed in the periodic table can be considered pure substances. Examples of elements include hydrogen, oxygen, gold, silver
Compounds are made up of different types of atoms joined together by chemical bonds. Examples of compounds include water, glucose, salt and carbon dioxide.

What is a mixture?

Mixture is a combination of two or more pure substances where each substance keeps its own identity upon mixing. Mixtures are present almost everywhere on Earth. Look at rocks, the ocean, rivers or even the atmosphere. All of them are mixtures! In other words, anything that you can mix together is a mixture. Even the foods you eat.

Why is it called a mixture?

It means the fundamental chemical structure of the components in a mixture does not change upon mixing.

Examples of mixtures

Although water is a pure substance, if you put sand into a glass of water, it would turn into a mixture. Each of the components of a mixture can be separated from one another. You can always separate the sand from water by filtering it.

If you take a mixture of salt and water, you can separate it by evaporating the water, to get salt in the container. Air, too, is a mixture of different gases such as carbon dioxide, oxygen, nitrogen and water vapour etc. Blood is a mixture made up of different types of blood cells and plasma.

Types of mixtures

1. Homogeneous mixture – The components of a homogeneous mixture have a uniform composition, and cannot be seen separately. The prefix ‘homo’ means same and it tells us that when two substances combine extremely well with one another, they form a uniform mixture. For example, sugar and water do not chemically react and form another compound although the water does turn sweet!
2. Heterogeneous mixture – The components of a heterogeneous mixture do not have a uniform composition and can be viewed separately without losing their identity. For example, if you mix sulfur powder with iron dust, you can easily see the two separately. You can even separate the iron dust by using a magnet.

How do we differentiate between pure substances and mixtures?

• A Pure Substance is matter which cannot be separated into its basic components by using a physical or a chemical process. The physical and chemical properties of pure substances are non-changing, if it is on its own without disturbing.
• A Mixture is made up of a combination of two or more substances that are not united using a chemical reaction. The physical and chemical properties of mixtures vary.