Science5 閲覧数·更新日 Jun 16, 2026·5 ページ

The Ultimate Guide to the pH Scale

The pH scale is your key to understanding how acidic...

1 / 5

of 5

# The pH Scale

What is the pH scale?

The pH scale is a way to measure how acidic or alkaline a substance is. It's a
numerical scale that g

Understanding the pH Scale Basics

Ever wondered why lemons taste so sour or why soap feels slippery? It's all about pH - which stands for 'potential of Hydrogen' and measures how acidic or alkaline something is.

The pH scale runs from 0 to 14, and it's dead simple once you know the rules. Acids have a pH less than 7 and taste sour (think vinegar or citrus fruits), whilst alkalis have a pH greater than 7 and feel soapy to touch. Right in the middle at pH 7 is neutral - like pure water.

Here's what makes this tricky: the scale is logarithmic. This means each whole number represents a 10-times change in strength. So battery acid at pH 1 isn't just a bit stronger than lemon juice at pH 2 - it's actually 10 times stronger!

Key Insight: All alkalis are bases, but not all bases are alkalis. Alkalis are simply bases that can dissolve in water.

of 5

How Strong is Strong?

The pH scale breaks down into three clear zones that you need to master. Acidic substances $pH 0-6.9$ get stronger as the numbers get smaller - so pH 1 battery acid will definitely ruin your day more than pH 6 milk will.

At exactly pH 7, you've got neutral territory. Pure water sits here perfectly balanced, neither acidic nor alkaline. It's your reference point for everything else.

Alkaline substances $pH 7.1-14$ work the opposite way - higher numbers mean stronger alkalis. pH 8 egg whites are pretty mild, but pH 14 oven cleaner is seriously dangerous stuff that'll burn through skin.

Remember This: Each step down the pH scale means 10 times more acidic. So pH 3 is 100 times more acidic than pH 5!

of 5

Testing pH with Indicators

You can't just eyeball a substance and guess its pH - you need indicators, which are special chemicals that change colour based on acidity. There are two main types you'll use in the lab.

Litmus paper is the simple option that just tells you acid or alkali. Blue litmus turns red in acid (remember: "Acid turns litmus Red"), whilst red litmus turns blue in alkalis. It's handy for quick tests but won't tell you how strong the acid or alkali is.

Universal indicator is where things get interesting. This clever chemical gives you a rainbow of colours that match specific pH ranges. Red means strong acid $pH 0-2$ , orange and yellow show weaker acids, green is neutral, and blue through to purple indicates increasingly strong alkalis.

Lab Tip: Universal indicator comes as both liquid drops and test strips - both work the same way and give you an actual pH reading.

of 5

Real-World pH Examples

Let's say you're testing household vinegar with universal indicator paper. Dip it in, and you'll see it turn orange, telling you the vinegar is acidic with a pH around 3-4. That's exactly what you'd expect from something that tastes so sour!

Here's a trickier scenario: you've got an unknown clear liquid that keeps blue litmus blue but turns red litmus blue. The first test rules out acid (since blue litmus didn't change), and the second test confirms it's definitely alkaline. You'd need universal indicator to find out exactly how strong.

Neutralisation happens everywhere in real life. When you take an antacid for heartburn, you're adding a mild alkali to neutralise excess stomach acid. The reaction brings your stomach pH back closer to normal, which stops the burning sensation.

Safety First: Never taste chemicals to test if they're acidic! Always use indicators - your tongue isn't worth the risk.

of 5

Key Points for Success

Remember that pure water is neutral at pH 7, but tap water might be slightly different due to dissolved minerals. Rainwater is actually slightly acidic (pH 5.6) because it picks up carbon dioxide from the atmosphere.

For your exams, focus on the indicator colour changes: acids turn blue litmus red and make universal indicator red/orange/yellow, whilst alkalis turn red litmus blue and make universal indicator blue/purple. Neutral substances keep blue litmus blue, leave red litmus red, and turn universal indicator green.

The most common mistake? Mixing up litmus colours. Just remember the simple rule: acid makes litmus red, alkali makes litmus blue. Strong acids and alkalis are both corrosive and dangerous - always wear safety goggles when handling them.

Exam Success: Know your examples! Lemon juice (pH 2), vinegar (pH 3), pure water (pH 7), baking soda (pH 9), and household bleach (pH 13).

そんなこと聞いてくれるのを待ってたよ...

KnowunityのAIコンパニオンとは？

KnowunityのAIコンパニオンは学生向けに設計されたAIツールで、単なる答えを提供するだけではありません。数百万のKnowunityリソースを基に構築され、関連する情報、個別の学習プラン、クイズ、コンテンツをチャット内で直接提供し、あなたの個別の学習過程に適応します。

Knowunityアプリはどこでダウンロードできますか？

Google Play StoreとApple App Storeからアプリをダウンロードできます。

Knowunityは本当に無料ですか？

その通り！学習コンテンツへの無料アクセス、仲間の学生とのつながり、そして即座のサポートを手のひらで楽しもう。

Scienceの人気コンテンツ

Science

Science/Physics notes - Speed, density, work

Speed density work formulas

3rd Year81

Science

Atomic Structure and the Periodic Table

Students will learn about the structure of atoms, including protons, neutrons, and electrons, and how elements are organised and classified in the Periodic Table based on their electronic configuration and chemical properties.

5th Year41

Science

Human Body Systems

An introduction to major human body systems such as the digestive, circulatory, respiratory, and nervous systems, understanding their basic functions.

2nd Year131

Science

Junior Cert Science Notes: The Cell

Notes on the Cell

3rd Year261

Science

Cells

Junior cert science cells summary

1st Year611

Science

Human Reproductive System

A basic overview of the male and female reproductive systems, including their main structures and functions.

TY191

Science

Light and Optics

Exploring the properties of light, including how it travels, reflects, and refracts, and how we perceive colours.

2nd Year50

探しているものが見つからない？他の教科も見てみよう。

生徒たちが愛用中 — あなたもきっと気に入るはず。

4.6/5App Store

4.7/5Google Play

このアプリはとても使いやすくて、デザインも良いです。今のところ探していたものは全て見つかったし、プレゼン資料からもたくさん学べました！絶対に課題でも使いたいと思います！もちろん、アイデアを得るのにもすごく役立ちます。

Stefan SiOSユーザー

このアプリは本当に素晴らしいです。学習ノートやサポート資料がとても豊富で[...]。例えば、私の苦手科目はフランス語なんですが、このアプリにはサポートオプションがたくさんあります。このアプリのおかげでフランス語が上達しました。誰にでもおすすめしたいです。

Samantha KlichAndroidユーザー

すごい、本当に驚いた。広告で何度も見かけたからアプリを試してみたら、めちゃくちゃ感動した。このアプリは学校で欲しかった「まさにこれ！」って感じのサポートで、特に練習問題や要点まとめみたいな機能がたくさんあって、個人的にすごく助かってる。

AnnaiOSユーザー

Science5 閲覧数·更新日 Jun 16, 2026·5 ページ

The Ultimate Guide to the pH Scale

The pH scale is your key to understanding how acidic or alkaline substances are around you - from the lemon juice in your kitchen to the soap you wash with. It's a simple 0-14 scale that scientists use everywhere, and...

of 5

Understanding the pH Scale Basics

Ever wondered why lemons taste so sour or why soap feels slippery? It's all about pH - which stands for 'potential of Hydrogen' and measures how acidic or alkaline something is.

Key Insight: All alkalis are bases, but not all bases are alkalis. Alkalis are simply bases that can dissolve in water.

of 5

How Strong is Strong?

At exactly pH 7, you've got neutral territory. Pure water sits here perfectly balanced, neither acidic nor alkaline. It's your reference point for everything else.

Remember This: Each step down the pH scale means 10 times more acidic. So pH 3 is 100 times more acidic than pH 5!

of 5

Testing pH with Indicators

You can't just eyeball a substance and guess its pH - you need indicators, which are special chemicals that change colour based on acidity. There are two main types you'll use in the lab.

Lab Tip: Universal indicator comes as both liquid drops and test strips - both work the same way and give you an actual pH reading.

of 5

Real-World pH Examples

Safety First: Never taste chemicals to test if they're acidic! Always use indicators - your tongue isn't worth the risk.

of 5

Key Points for Success

Exam Success: Know your examples! Lemon juice (pH 2), vinegar (pH 3), pure water (pH 7), baking soda (pH 9), and household bleach (pH 13).